THE IMPACT OF DROUGHT IN THE LIGHT OF CHANGING SOIL PROPERTIES

In: Droughts: Causes, Effects and Predictions ISBN 978-1-60456-285-9 Editor: Javier M. Sánchez © 2008 Nova Science Publishers, Inc.

Chapter 2

THE IMPACT OF DROUGHT IN THE LIGHT OF CHANGING SOIL PROPERTIES

Bernhard Lucke1, Iourii Nikolskii2, Michael Schmidt3, Rupert Bäumler4, Norbert Nowaczyk5 and Ziad al-Saad5

1 Brandenburg University of Technology Cottbus, Chair of Environmental Planning P.O. Box 101344, 03013 Cottbus, Germany; [email protected]

2Colegio de Postgraduados, Campus Montecillo, Mexico 3 Brandenburg University of Technology Cottbus, Chair of Environmental Planning

P.O. Box 101344, 03013 Cottbus, Germany 4 Friedrich-Alexander University Erlangen-Nürnberg, Institute of Geography

Kochstr. 4/4, 91054 Erlangen, Germany 5 GeoForschungsZentrum Potsdam, Section 3.3, Telegrafenberg, C 321

14473 Potsdam, Germany 6 Faculty of Archaeology, Al-Hussein Bin Talal University

P. O. Box 358, Umm Al-Summaq-Amman, Jordan

ABSTRACT

It is expected that global warming will materialize primarily as lack of rain in semi-arid countries, leading to drought and failure of crops. But apart from the direct impact on harvests, changing temperatures also affect soil properties. A sustained drought or different rainfall pattern may lead to a reduction of organic matter, loss of aggregate stability, and long-term degradation of soil structure and nutrient availability. In this context, it is often assumed that historic land use initiated such a negative development and was connected with accelerated soil erosion, while the impacts of climate variations were aggravated due to a negative feedback loop of precipitation reduction with the overexploitation of the land. However, an investigation of paleosols in the Decapolis region in northern Jordan indicates that it is mainly climate which governs soil development. The paleosol pattern points to increasing aridity since the end of the last Ice Age, and at least two prehistoric phases of instability and dramatic landscape change. The importance of climate variations for changing soil properties can also be modeled using the geographic law of soil zonality, and be

No part of this digital document may be reproduced, stored in a retrieval system or transmitted in any form orby any means. The publisher has taken reasonable care in the preparation of this digital document, but makes noexpressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. Noliability is assumed for incidental or consequential damages in connection with or arising out of informationcontained herein. This digital document is sold with the clear understanding that the publisher is not engaged inrendering legal, medical or any other professional services.

Bernhard Lucke, Iourii Nikolskii, Michael Schmidt et al. 70

expressed in quantitative form. This approach was used in Mexico in order to predict future fertility and crop productivity: preliminary results show that the contribution of a climatically provoked soil fertility deterioration to diminishing wheat harvests can reach 30%. Combining the above mentioned approaches allows improved assessment of past and future fertility changes, and could be used to validate climate reconstructions. As it seems, the main threat for soils in arid and semi-arid areas is not deterioration due to reduced vegetation cover or erosion, but calcification. As well, climate change and drought may not only be a matter of reduced precipitation, but also of more frequent heavy rainstorms. The impact of the latter could be most important for all land use systems, but is only rarely considered. In this context, future land use strategies should focus on adaptation to extreme events. Since the paleosols indicate how landscapes changed rapidly despite the absence of agriculture, it seems questionable whether measures like soil stabilization or reforestation can be successful if climate change continues.

INTRODUCTION It is assumed that global warming may lead to more sustained droughts in semi-arid areas

and the desert belts, possibly connected with a climatic pattern of less but heavier rainfall events (UN, 2007). If this scenario materializes, it could lead to more frequent crop failures, accelerated desertification, and loss of arable land, ultimately forcing people to abandon the desert belts. The following migrations may become a challenge for more developed countries as well. In order to cope with such a development, or to prevent it, it is discussed whether a reduction of human pressure can compensate for the increased climatic forcing, and how land use can best adapt to changing conditions. Agencies combating desertification usually focus on soil protection, since land degradation due to soil erosion is understood to be mainly man-induced, and seen as the ultimate, irreversible outcome of desertification (UN, 2007). As long as an intact soil cover is present, rehabilitation of an area is possible.

Recommendations for combating desertification usually mention reforestation, reduction of grazing pressure and other means to increase vegetation cover, and construction of soil-retaining devices. These activities shall fix soils by means of roots and organic matter, create wind barriers, and increase infiltration so that the impacts of wind and water as main erosive forces are reduced. It is also discussed whether a large-scale increase of vegetation may create a local climate which would support the further establishment of plants due to a positive feedback with vegetation cover. But all the above mentioned measures of combating desertification are based on the assumption that soils will remain as they are today if not eroded by wind and water. However, sustained climate change may not only affect the vegetation, but also soil properties. It is well known that rising temperatures may for example reduce aggregate stability (Imeson and Lavee, 1998), change the composition of soil microorganisms, and affect nutrient availability and organic matter content (Bens et al., 2006). Therefore a soil's capacity to support vegetation can change: without knowing how land suitability will look like under continued warming, some of the above mentioned activities could be doomed to fail. The impact of coming droughts has to be considered in the light of changing soil properties, which may make it necessary to completely revise risk assessments.

Paleosols deliver a long-term record of soil development and are a key for dealing with global warming, since they inform us how soils and landscapes changed under past land use

The Impact of Drought in the Light of Changing Soil Properties 71

and climate variations, considering that what already happened may happen again. In recent years important progress was achieved regarding paleosols in semi-arid regions of Jordan and the Levant, mostly limestone soils which are often obscured by calcareous material (Cordova et al., 2005, Maher, 2005, Lucke, 2007). The look into the past allows better distinguishing between human and natural forcing, since the paleosol record covers periods with and without agriculture. Gvirtzman and Wieder (2001) showed that paleosols are archives that are suited to document the crossing of thresholds in landscape dynamics, which makes them best suited to investigate desertification in the light of climatic variations.

Soil development in relation to climate can also be modeled using the geographic law of soil zonality. It is based on the establishment of a quantitative relationship between Budyko’s radiative index of dryness (I) and modal values of soil properties (f) (Nikolskii et al., 2006). Existing long-term climate change forecasts can be used to calculate the values of I. The quantitative relationships between a regional integral index of soil fertility (F) and climate conditions (I) at the present time can be determined for geomorphologically homogeneous groups of soils located in different climatic zones. The relationships F(I) can be obtained on the basis of dependencies of regional modal values consisting mainly of organic matter contents, plant-available P and K contents, and pH values. The relationships F(I) can be applied to assess agricultural soil degradation in rain-fed areas, which was successfully demonstrated by Nikolskii et al. (2006) for the case of Mexico. The same relationships can be used to reconstruct past climates using known mean statistical regional values of paleosol properties.

The purpose of this paper is threefold: In the first part, paleosols in the Decapolis region in northern Jordan are discussed as indicators for the impacts of climate and man on the environment in the light of changing soil properties. In the second part, a study focusing on the prediction of climatically induced soil changes in Mexico during the 21st century is presented. The areas, methods and time-frames of investigation are different: while the paleosol research in the Decapolis region uses indicators based on a genetic model of long-term soil changes, climatically induced relatively short-term soil alterations in Mexico are predicted using a mathematical model, based only on the geographic law of soil zonality, and expressed in quantitative form. Although based on different parameters, these two approaches and research areas inform each other well. While paleosol investigations usually deliver little information about fertility changes, especially regarding the short time frames decisive for agriculture, the mathematical modeling of soil fertility delivers few insight into the involved time frames, the reversibility of the process, and long-term consequences. So last but not least, it will be suggested how the two approaches could be combined in order to predict future land suitability under given climatic scenarios. In order to achieve this, some methodologies are discussed in detail in order to illuminate the constraints and potentials of soil research.


PART I: PALEOSOL INVESTIGATION IN NORTHERN JORDAN

The Study Area: The Decapolis Region The Decapolis region is situated in Northern Jordan (Figure 1). Its name relates to the

ruins of the Decapolis cities, which are famous due to the well-preserved monumental architecture from the Roman and Byzantine period. Poor development of the region during the 19th century was often attributed to land degradation: rain-fed agriculture formed the economic basis of the ancient Decapolis cities, and the ruins make clear that agriculture in this region must have exceeded subsistence economy. Several authors postulated that historic mismanagement and erosion were responsible for the abandonment of ancient sites (Lowdermilk, 1944; Dregne, 1983; Hillel, 1991), although it was never proven that the erosion is man-made and not natural (Geraty and LaBianca, 1986). The area contains three basic landscape units: the alluvial plain of the Jordan valley, a heavily dissected limestone plateau east of the Jordan valley, and a level plain further east which merges with the Arabian Desert. The present study presents a selection of soils and paleosols which were investigated in the course of a larger project on historic desertification in Jordan (Lucke, 2007). The first is located on the limestone plateau close to the village Yarmouk (Figure 2, Table 1), the second in the bottom of Wadi Ziqlab (Figure 3, Table 2), and the third in the Jordan valley (Figure 4, Table 3). The discussed soils are derived from limestone formations and chalk and show either red color (Terra Rossa according to Reifenberg (1947), or are greyish-white (Rendzina according to Reifenberg (1947). In the present paper, soil classifications according to the newly published World Reference Base for Soil Resources (WRB, 2006) are presented with the tables, but for the sake of simplicity the terms suggested by Reifenberg (1947) will be used in the text.

Figure 1. Approximate location of the Decapolis region in Northern Jordan (big box) and the investigation region (small box). The presented sampling places are marked: JV = Jordan Valley, WZ = Wadi Ziqlab, Y = Village of Yarmouk.


The Terrae Rossae in Jordan is considered to be degraded (Horowitz, 1979). In this context, deep soils like the one near the village of Yarmouk have a uniform appearance, but a closer look and lab analyses reveal variations with depth which indicate environmental changes (see Table 4). This applies also to topsoils: a case study near the site of Abila found slight differences which seemingly follow the remains of field borders (Lucke, 2002, Lucke et al., 2005). Excavations in Wadi Ziqlab revealed a complex pattern of colluvial deposits in the Wadi, which stems from landslides and slope collapses. The Jordan valley acts as a sediment trap, and the soil profile shows considerable differences of sedimentation and soil formation.

The soil on the limestone plateau near the village of Yarmouk was sampled at a street cut showing the strongly varying soil depth, which seems related to the karst weathering and changing hardness of the limestone, and led to preservation of a paleosol in the karst pocket (N 32° 42' 54.0'', EO 35° 52' 28.8'', 381 m above sea level, 400 mm mean annual precipitation). The visual impression suggested that the deepest part of the profile might be a paleosol characterised by slickensides (AB 3-7), that was eroded so that huge boulders from the neighbouring elevated areas were exposed (between Yar 3 and 4). A brighter soil developed on top of these (Yar 3), that was covered by a darker red one but which contains many big stones (Yar 2). The very top consists of a brighter and coarser soil (Yar 1, 1a). The 14C-age of soil organic matter of sample AB 3 was determined as 2320 BP +- 60 (lab. no. Erl-N-Jordanien 09/06-3, δ 13C -26,3). However due to the presence of roots, this is only a minimum age. Quartz-based OSL-dating found that AB 3 saw light the last time 6880 BC +- 700 (Lab. no. RF2007 Jord AB3). This date might correspond to the regional "Yarmoukian landslide"-event, which buried various sites in Jordan and the Mediterranean around 6200 BC (Weniger et al., 2005).

Figure 2. Sampling the profile on the limestone plateau near the village of Yarmouk.


Table 1. Sampling the Vertic Calcisol (Clayic, Eutric, Chromic) (Thapto-Cambisolic) near the village of Yarmouk

Sample No.

Depth [cm]

Description/Comment Color (dry) Age

AB 1 0-50 Moderate humous, extremely calcareous, clay (C), subangular blocky structure, dry extremely hard, moist very friable, moderately sticky, moderately plastic, many fine roots, 30% angular limestone detritus

7.5 YR 5/3 dull brown

AB 2 50-100 Slightly humous, extremely calcareous, clay (C), subangular blocky structure, dry extremely hard, moist very friable, moderately sticky, moderately plastic, few fine roots, 20% angular limestone detritus


AB 3 100- 150

Moderate to very slightly humous-decreasing with depth, strongly calcareous, clay (C), angular blocky structure breaking to wedges, slickensides, dry extremely hard, moist very friable, very sticky, very plastic, few olive roots, 10% angular limestone detritus

5 YR 3/4 dark reddish brown

6880 BC or 386 BC

AB 4 150- 200

As above 5 YR 4/4 dull reddish brown

AB 5 200- 250

Slightly humous, calcareous, clay (C), angular blocky structure breaking to wedges, slickensides, dry extremely hard, moist very friable, very sticky, very plastic

5 YR 4/4 dull reddish brown

AB 6 250- 300

As above 7.5 YR 4/4 brown

AB 7 300- 350

As above 10 YR 3/4 dark brown

AB 8 350+ Soft limestone Yar 1 0-30 Moderate humous, extremely calcareous, clay (C),

subangular blocky structure, dry extremely hard, moist very friable, moderately sticky, moderately plastic, many fine roots, 30% angular limestone detritus


Yar 2 30-80 As above, many big stones, seems to refer to AB 1 and AB 2


Yar 3 80-120 As above, over band of angular limestone cobbles 7.5 YR 5/4 dull brown

Yar 4 120-160 slightly humous, calcareous, clay (C), angular blocky structure breaking to wedges, slickensides, dry extremely hard, moist very friable, very sticky, very plastic

10 YR 5/3 dull yellowish brown

Yar 5 160+ Hard limestone Yar 1a 0-30 As Yar 1 2.5 YR 6/4

dull orange

Yar Mitte 200 In the deep profile, directly under color change like at Yar 3/4. As Yar 4

10 YR 4/4 dark yellowish brown

Yar Mitte unten

350 As Yar 4, over saprolite 10 YR 4/4 dark yellowish brown


Wadi Ziqlab is a deeply incised valley draining towards the Jordan. The wadi must have been much flatter and gently sloping during the Early Pleistocene, since substantial archaeological sites from the Paleolithic are present only on the plateau and upper terraces. These show remains of huge sinkholes and slope failures, which indicate how the wadi was incised over time. As the Jordan rift valley deepened, local base levels dropped and incision was enhanced. Sampling in Wadi Ziqlab could take advantage of ongoing excavations by Lisa Maher and Edward Banning, and samples were taken from the wadi bottom where a Kebaran prehistoric settlement had been located (N 32° 30' 25.7", EO 35° 42' 15.6", 214 m above sea level, 300 mm mean annual precipitation). While the wadi slopes are today covered by greyish-white Rendzinas, the excavation trench revealed a red paleosol in 77 cm depth, which was dug out until 152 cm without reaching its bottom. Its surface could be dated to 11,000 BC according to artefacts (pers. comm., Lisa Maher, July 2005). The upper 55 cm of the profile consist of grey colluvium, covering remains of a Byzantine field stone wall on a still red subsoil.

Figure 3. Sampling Wadi Ziqlab.


Table 2. Sampling the Colluvic Calcisol (Calcaric, Eutric) (Thapto-Chromic) in Wadi Ziqlab

Sample No.

Depth [cm]


Ziq 1 0-20 moderate humous, extremely calcareous, silt, dry friable, wet moderately plastic, many fine roots

7.5 YR 7/2 light brownish grey

Ziq 2 20-55 As above, associated with the remains of a Byzantine field stone wall

2.5 Y 8/3 pale yellow

Ziq 3 55-77 Reddish soil directly under Byzantine field stone wall: slightly humous, extremely calcareous, clay (C), subangular blocky structure, dry very hard, moist very friable, moderately sticky, moderately plastic, few fine roots, 20% angular limestone detritus


Byzantine

Ziq 4 77-? As above, and associated with a Keberan site (old surface exposed at 77 cm depth)

7.5YR 6/3 dull brown

11,000 BC

Table 3. Sampling the Calcaric Fluvisol (Siltic, Eutric) (Thapto-Skeletic)

in the Jordan Valley

Sample No.

Depth [cm]


Jor 1 0-35 Strongly humous, extremely calcareous, silt, dry friable, wet moderately plastic, many fine roots

7.5 YR 2/1 black

Jor 2 35-70 as above - Jor 3 70-95 60% angular limestone detritus, many big stones,

slightly humous, extremely calcareous, silt, dry friable, wet moderately plastic, few roots

10 YR 3/4 dark brown

Jor 4 95-105 Slightly humous, extremely calcareous, silt, dry very hard, wet moderately plastic, few roots

-

Jor 5 105-120 As above, and many small holes (re-dissolved concretions or root paths?), sticky

5 YR 5/8 bright reddish brown

Jor 6 120-140 As above - Jor 7 140-160 As above, but small holes are not present any more. 5 YR 5/6

bright reddish brown

Jor 8 160-180 As above Jor 9 180-200 Slightly humous, extremely calcareous, clay (C), dry

very hard, moist very friable, moderately sticky, moderately plastic, a few stones (the only ones below Jor 3)

-

Jor 10 200-220 As above, and free of detritus 7.5 YR 4/4 brown

Jor 11 220-240 Extremely calcareous, silt, dry very hard, moist friable, moderately sticky, moderately plastic

-

Jor 12 240-? Extremely calcareous, sand,, moist friable, moderately sticky, weakly plastic, unfortunately only 10 cm exposed

7.5 YR 5/8 bright brown


In the Jordan valley, the sampling place is located in the delta of Wadi Ziqlab near the ancient Tell Arba'in (N 32° 30' 59.9", EO 35° 35' 37.8", -225 m below sea level, 250 mm mean annual precipitation). The Jordan rift valley is hot and humid, and characterized by silty, greyish-black soils. The relief is mostly flat or with slight slopes, and the profile lies in the irrigated area between the King-Abdullah-canal and the Jordan River. It was hoped to find buried surfaces with traces of ancient land use connected with the tell, but unfortunately no archaeological material could be noted. Excavation did not reach groundwater, but had to be stopped in 250 cm depth due to technical limits.

Figure 4. Sampling the Jordan Valley.


In order to compare the soils with the chalk bedrock (which is sometimes so soft that it can be plowed and cultivated), and to assess the importance of chalk additions in the course of soil development, sample TZ 54 was taken from a prominent rock outcrop with erosion marks close to Wadi Ziqlab (geologically part of the Belqua-group B.3 according to Bender (1974).

Methods: Paleosol Investigation Soils and paleosols in the Decapolis region were analyzed regarding Redness Rating

(RR), pH, electrical conductivity, organic matter, CaCO3-content, soil development indices based on iron and manganese oxides, magnetic susceptibility, and texture. These belong to the most widely used methods in paleosol research and allow comparison of soil development from a genetic point of view (Lucke, 2007).

Color is a prominent parameter for soil description and sample selection, and might reflect different periods and intensities of land use since color changes sometimes seemingly follow field borders (Lucke et al., 2005). In order to make it statistically comparable, colors have to be translated into numbers which is achieved with the Redness Rating. The pH delivers information about the chemical "milieu" of the soil, for example about leaching processes and base accumulation. The electrical conductivity allows an assessment of the degree of base accumulation, i.e. soil salinity. Organic matter provides information about the humus content and may indicate buried land surfaces. CaCO3 is a weathering indicator since it is leached from limestone-derived soils with increasing age or moisture. Also, it is important for soil structure and affects fertility: if present if huge amounts, it might lead to fixation of plant available phosphorus, but if missing, fertility might be reduced due to a loss of bases in a more acid milieu. The content of pedogenic oxides and hydroxides of iron and manganese usually rises with increasing weathering intensity and age of soils. In combination with other parameters, it is suited for a characterization of development stage and relative age of soils. The specific magnetic susceptibility χ (referring to the soil mass) quantifies the ease with which a material can be magnetised in a weak field, and reflects the sum of its magnetic properties (dia-, para-, superpara-, ferro, antiferri- and ferrimagnetism). Finally, the texture is a criterion that on the one hand delivers a relative age estimation, since soils should become finer when older. Also, the texture has to be known if the basic behavior of the soils and suitability for cultivation shall be evaluated.

For collection of soil samples, freezer bags were filled from a ca. 5 cm thick strip in the middle of the identified layers/horizons. The samples were air-dried for 48 hours at 40° C and then sieved by 2 mm. The fraction > 2 mm (skeleton content) was determined and archived, and all further analyses conducted with the fraction < 2 mm. Color was determined in the laboratory under standardized light, using the Munsell soil color chart. The Redness Rating (RR) was computed with the dry samples according to Hurst (1977). It is calculated according the the formula: RR=H*Brightness/Chroma, where H is: 5R = 5, 7.5 R = 7.5, 10 R = 10, 2.5 YR = 12.5, 5 YR = 15, 7.5 YR = 17.5, 10 YR = 20 (the index becomes the smaller the redder the soil). For example, 10 YR 8/1 = 160, and 2.5 YR 4/8 = 6,3. The calculation of RR as proposed by Buntley and Westin (1965) and modified by Cordova (2000), or Torrent et al. (1980) produced results which were less consistent than the original index of Hurst.


The pH was determined with a glass electrode (pH-meter 530 by WTW, with electrode InLab 423 by Mettler-Toledo) in distilled water, with a soil: water solution of 1:2,5. Electrical conductivity was measured with a GMH 3410 conductivity meter by Greisinger electronic in a soil:water solution of 1:5 (according to Schlichting et al., 1995). Contents of N, CaCO3 and Corg were determined using a Leco TrueSpec C/N-analyser. Samples were finely ground and examined in doubles in the C/N-analyser, which delivered the total C- and N-contents. Additionally, ignition loss was determined according to Schlichting et al. (1995): the samples were heated for two hours to 430 °C, which is supposed to eliminate organic matter, but not to expel CO2 from calcium carbonate. These samples were then analyzed in the C/N-analyser, too, so that Corg could be calculated from the difference after correcting the second C-content by the weight loss caused by ignition. This allows to also calculate the CaCO3-content under the assumption that the remaining inorganic carbonates are fixed in form of CaCO3 (which is indicated by thin sections and SEM-analyses, as there was no evidence for the presence of dolomite).

Pedogenic oxides were extracted with sodium dithionite at room temperature according to Holmgren (Schlichting et al., 1995), and the iron and manganese contents measured with an atomic absorption spectrometer (M-series by Unicam, thermo-flame-spectrometer). Weakly crystallized pedogenic oxides were extracted in the dark using puffered (pH 3.25) oxalate-solution according to Schwertmann (Schlichting et al., 1995). Their iron and manganese contents were determined with the atomic absorption spectrometer (AAS), too. Total element contents were examined with X-ray fluorescence analysis (XRF). A wavelength-dispersive sequency spectrometer type PW 2400 was used, and samples prepared according to DIN 51001 (2003). The specific susceptibility χ was determined using a Kappabridge KLY-3S magnetic susceptibility meter (875 Hz), using the sieved but otherwise untreated fine soil < 2 mm in plastic containers.

Particle sizes were analyzed after removing CaCO3 with 10% hydrochloric acid (until no visible reaction occurred any more). Hydrochloric acid (HCl) was washed out again and those samples with Corg > 2% treated with 30% hydrogen peroxide (H2O2) in the warm waterbath (~50 °C), until no visible reaction occurred any more. Hydrogen peroxide was evaporated and samples dispersed with sodium hexametaphosphate (Na4P2O4). Wet sieving determined the sand fraction (according to DIN 19683, 1973), while the smaller particles were analyzed with the sedimentation analysis according to Köhn (Schlichting et al., 1995).

Results: Paleosols in the Decapolis Region The three presented soils are located along a climatic and erosional gradient: soil

development and erosion should be most pronounced in the Yarmouk soil, while the Jordan valley should act as a final sediment trap. The Jordan valley soil indeed indicates alternating periods of landscape stability and instability. Stability was seemingly connected with soil development and is indicated by more reddish layers, while periods of increased sedimentation were connected with stronger admixtures of gravel and chalk. Although there are so far no dates available yet, the Jordan valley profile may correspond to the evidence found in Wadi Ziqlab. There, archaeological dating and a detailed analysis of the soil pattern were conducted by the excavations of Banning and Maher.


In the bottom of Wadi Ziqlab, the lowest exposed colluvium consists of cobbles overlain by fine sands. The deposition of the earlier reflects flooding events which were connected with erosional processes during the Upper Paleolithic, probably reflecting increased aridity at the onset of the last glacial (Maher, 2005). The sand indicates a shift from high to low energy transport during the later Early Epipaleolithic (Maher, 2005). By then, a massive red colluvium covered the slopes, and organic remains visible in thin sections point to continuing pedogenesis (Maher, 2005). The landscape was stable until the Middle Epipaleolithic, while the valley still had a profile that was shallower and more longitudinal than today, and the main channel was flanked by broad, soil covered terraces (Maher, 2005). With the drying of Lake Lisan some time between 13,000 and 11,000 BC, large-scale erosion and incision resumed, while the base level energy raised again due to the lower levels of the lake. Erosion was again characterized by high energy mass transports since all sites in the younger, brighter colluvium covering the paleosol are associated with angular cobble. Beginning in the Neolithic around 6000 BC, this brighter colluvium was deposited with little post-depositional pedogenesis. Ongoing deposition is evident as all Neolithic sites in Wadi Ziqlab were covered by greyish-yellow soil of strongly varying thickness, and incision continued, too, indicated by the absence of these sites on the lowest wadi terraces (Maher, 2005). Compared with the huge amount of colluvium deposited during the onset of the last glaciation, erosion since the Neolithic seems however small. In average, Byzantine material was only found in the upper 20 cm of the bright colluvium. That most of the younger sediments were also deposited by high intensity mass movement events is indicated by a high density of angular cobbles, and interruption of soil surfaces (Maher, 2005). As well, colluvia vary locally, and were affected by re-deposition. For example, very strong erosion is present in the central part of Wadi Ziqlab, where older terraces are completely missing. As well, Natufian and PPNA sites are apparently missing, which indicates they were removed by massive erosion, which might also explain the sharp color change to the brighter colluvium (Maher, 2005).

The profile presented here contains the red colluvium with signs of ongoing soil genesis after it deposition (samples Ziq 3 and 4), and the brighter calcareous colluvium deposited since the Neolithic (samples Ziq 1 and 2). As can be seen from the profile, deposition of the latter did not take place simultaneously all over the wadi, since the red soil was still exposed at this location when the Byzantine field wall was constructed.

The soil near the village of Yarmouk also suggests that a very significant erosional phase took place around 6880 BC, according to the OSL-date for sample AB 3. It seems likely that the slightly brighter topsoil was moved there from the more elevated parts of the limestone plateau and corresponds to the brighter colluvium in Wadi Ziqlab which was deposited after 6000 BC. Although the Yarmouk-profile is very homogeneous apart from the slight color change between samples AB 2 and 3, physical and chemical analyses provide further insight (Table 4).

Tab

le 4

. Res

ults

of l

abor

ator

y an

alys

es. T

he fe

rtili

ty F

was

cal

cula

ted

with

ref

erri

ng to

the

pres

ente

d sa

mpl

es o

nly,

usi

ng O

Mm

ax, K

max

an

d P m

ax a

s in

Jor

1, Z

iq 1

and

TZ

54.

Sam

ple

No.

R

R

(dry

) C

org

%

CaC

O3

%

Fed

[mg/

g]

Feo/

Fed

Fed/

Fet

Fed/

(Fe t/

C

aCO

3)

Mn d

/M

n t

χ (1/k

g)

*E-3

pH

C

ondu

c-tiv

ity

(μS/

cm)

Cla

y %

Si

lt %

Sa

nd

%

Skel

e-to

n %

K

%

P %

N

%

Ferti

lity

F

Jor 1

70

1.

8 51

6.

0 0.

05

0.30

16

0.

54

140

8.3

124

44

19

37

33

0.73

0.

66

- 0.

68

Jor 3

40

0.

6 64

4.

4 0.

12

0.31

20

0.

38

75

8.7

120

39

16

45

61

0.40

0.

50

0.06

0.

28

Jor 5

26

0.

2 61

7.

2 0.

04

0.38

23

0.

61

121

8.9

140

53

32

15

3 0.

48

0.58

-

0.20

Jo

r 7

41

0.3

62

4.7

0.10

0.

27

17

0.30

81

8.

9 14

4 58

23

19

1

0.43

0.

52

0.03

0.

21

Jor 1

0 26

0.

6 41

11

.2

0.06

0.

37

15

0.57

21

4 8.

1 35

8 67

23

10

0

0.84

0.

42

0.04

0.

35

Jor 1

2 18

0.

04

65

3.9

0.07

0.

40

26

0.23

59

8.

8 12

0 23

13

64

13

0.

27

0.47

-

0.12

Zi

q 1

61

1.4

65

10.0

0.

02

0.16

11

0.

05

36

8.2

94

69

21

10

9 1.

61

0.15

0.

15

0.55

Zi

q 2

- 0.

6 63

15

.0

0.03

0.

60

38

0.07

50

8.

2 11

7 71

25

4

5 0.

54

1.11

0.

08

0.39

Zi

q 3

35

0.8

50

10.8

0.

03

0.65

33

0.

26

215

8.3

155

75

22

3 24

0.

76

0.86

0.

05

0.43

Zi

q 4

35

0.9

45

10.8

0.

02

0.38

17

0.

63

288

8.1

197

74

22

4 22

0.

66

1.15

0.

10

0.48

A

B 1

29

1.

2 44

10

.3

0.05

0.

32

14

0.76

20

0 8.

2 16

0 69

22

9

30

0.48

0.

15

0.09

0.

45

AB

2

29

1.0

48

7.5

0.08

0.

24

12

0.51

8.1

194

68

21

11

18

0.45

0.

15

0.08

0.

38

AB

3

11

1.5

32

13.3

0.

04

0.34

11

0.

80

262

8.2

129

77

21

2 10

0.

62

0.15

0.

25

0.53

A

B 4

15

0.

8 30

9.

3 0.

07

0.23

7

0.62

8.3

133

77

21

2 12

0.

69

0.13

0.

03

0.35

A

B 5

15

0.

5 22

8.

8 0.

09

0.18

4

0.50

8.4

154

76

23

1 10

0.

81

0.13

0.

01

0.25

A

B 6

18

0.

7 12

13

.6

0.06

0.

26

3 0.

77

351

8.3

247

77

19

4 5

0.95

0.

16

0.03

0.

33

AB

7

15

0.7

11

13.4

0.

07

0.64

7

2.15

8.3

236

78

19

3 2

0.85

0.

12

0.01

0.

32

Tab

le 4

. (co

ntin

ued)

Sam

ple

No.

R

R

(dry

) C

org

%

CaC

O3

%

Fed

[mg/

g]

Feo/

Fed

Fed/

Fet

Fed/

(Fe t/

C

aCO

3)

Mn d

/M

n t

χ (1/k

g)

*E-3

pH

C

ondu

c-tiv

ity

(μS/

cm)

Cla

y %

Si

lt %

Sa

nd

%

Skel

e-to

n %

K

%

P %

N

%

Ferti

lity

F

Yar

1

29

1.3

52

7.6

0.06

8.

1 13

3 63

22

15

27

0.

07

Y

ar 2

13

1.

1 46

10

.1

0.04

8.

2 12

5 84

14

2

6

0.

06

Y

ar 3

13

0.

9 49

10

.2

0.04

8.

3 13

2 75

19

6

29

0.02

Yar

4

33

0.9

19

18.5

0.

04

8.2

140

75

24

1 12

0.

02

Y

ar 1

a 19

1.

2 55

9.

8 0.

15

8.2

129

65

22

13

27

0.12

Yar

1 M

itte

20

0.4

12

11.6

0.

08

8.3

349

67

31

2 25

0.

05

Y

ar1

Mitt

e un

ten

20

0.3

37

7.1

0.07

8.

9 27

4 68

27

5

17

0.08

TZ 5

4 -

1.0

74

5.6

0.03

0.

51

37

0.02

16

8.

3 77

0 64

25

11

0.

5 0.

36

1.4

0.27

0.

46


pH-Values and Conductivity In all samples, the pH ranges between 7 and 9, connected with slightly alkaline

conditions. This indicates that a semi-arid climate prevailed at all locations and periods, leading to an accumulation of bases and reflected by enrichment in CaCO3. Conductivity shows greater variations, although none of the samples is saline. To bear relevance for crop production, conductivity should be higher than 2 mS. The low values indicate that winter rains always washed salts out, and that irrigation did not lead to salt accumulation. A detailed soil survey in the Jordan valley showed that results from the site sampled in the framework of this thesis are representative for the greatest part of the valley (KfW, 2005). It can be concluded that neither dry periods nor historical land use did cause salinity, or that salts were later washed out.

It is however interesting to note that conductivity values rise in the paleosols. Samples Jor 10, Ziq 4, Ab 6, AB 7, Yar Mitte and Yar Mitte unten are all characterized by slightly higher conductivity. This might point to an accumulation of bases in clay minerals and aeolian deposition (Lucke, 2007), or indicate that those soils were exposed to drier conditions before being buried by new material. They might be taken as support for the thesis that landscape instabilities due to extreme precipitation events are connected with or preceded by increased aridity.

Organic Matter

Although mostly low, organic matter content seems to well reflect old land surfaces, and is elevated in present surfaces. In the Jordan valley profile, there is an increase in Jor 10 while the horizons above are characterized by very low Corg contents. As well, the absence of skeleton content in Jor 10 points to a very stable landscape and few flash floods with little impact. Thus the Jordan valley profile may well reflect dry periods with reduced plant cover, but only few and intense periods of landscape change due to its relative remoteness from gravel and sediment sources. Sample Jor 3 seems to reflect an exceptional period of heavy sedimentation, which probably took place on a regional scale. Unfortunately, no date is available so far.

Corg increases sharply in AB 3 in the Yarmouk profile, supporting that it is a relic land surface that was covered quickly. A slight increase in AB 6 might point to another relic land surface. It can be concluded that the soil organic matter which accumulates in the area seems persistent, and a good indicator of buried surfaces.

Calcium Carbonate

The first aspect which attracts attention is the decrease of CaCO3-content with depth in the soils, which supports field observations that the upper parts are younger and less developed. In the Yarmouk profile, the lowest paleosol has about 10% CaCO3, the middle part 20-30%, and the upper part deposited after 6880 BC about 50% CaCO3. Samples Yar 1-4 indicate that the general tendency observed with the AB-samples might be attributed to events which led to deposition of new material and thus breaks in the profile. As already suggested by the field evidence, sample Yar 4 seems to correspond to a paleosol also represented by AB 5, which was eroded so that huge stones were exposed on the surface. Then a brighter soil was deposited (Yar 3), and some fluvial activity is indicated by the rising skeleton content. Yar 3 and Yar 4 are very similar, except that Yar 3 shows a sharply increasing CaCO3-


content. We think this can best be explained by an invasion of calcareous dust, and that the much brighter appearance of Yar 3 in the field is related to the sharply rising CaCO3-content.

When carefully sampling the Yarmouk profile, it was noted that a thin band of saprolite was visible over bedrock, and the soil directly above is represented by sample Yar Mitte unten. It is also characterized by strongly raising CaCO3-content, but not as strongly as at Yar 3. If Yar 3 would represent the eroded subsoil of the neighbouring areas, a gravel band or stones such as in Yar Mitte unten should be visible (which are not represented by the skeleton content, since the sampling had to avoid bigger stones as these did not fit into freezer bags, and would have made it impossible to restrict sampling to a 5 cm-strip). Yar Mitte unten indicates that the lower parts of the soils are derived from the bedrock, and as it seems these horizons were eroded at various, more elevated positions while the rock was covered again by other soil material.

Moved subsoil from the vicinity seems represented by Yar 2 which contains a thick gravel band and has the same calcium carbonate content as Yar 3, indicating that re-calcification affected the whole area before soils were moved again. As suggested by the OSL-date from AB 3, the re-calcification took place before 6880 BC.

The trend of re-calcification is also represented by the colluvium in Wadi Ziqlab. The bright colluvium is characterized by high calcium carbonate contents of more than 60%. It seems in fact very similar to the chalk sample TZ 54, pointing to the soft rocks along the wadi slopes possible source of calcareous material. A look into the Jordan valley indicates that mainly soft, easily erodible chalk from the steep slopes was deposited there too, since CaCO3-contents are very high throughout the profile. Samples such as Ziq 4 (the Epipaleolithic paleosol in Wadi Ziqlab) and Jor 10 might represent mixtures of formerly de-calcified Terra Rossa with chalk on the one hand, and ongoing leaching processes due to higher precipitation on the other hand (as indicated by thin sections, see Maher, 2005). This is also supported by the color, which is brighter than the plateau soil at the Yarmouk profile (see subchapter iron oxides and color). Taking the colluvia together, they point to increasing deposition of calcareous dust or steadily reduced leaching processes since the end of the Ice Age. Only sample Jor 1 falls out, but can be explained by leaching as a consequence of irrigation since the Jordan valley is intensively cultivated. This would also imply that only Jor 1 experienced cultivation, and explain why no archaeological material could be found in the profile.

Iron Oxides and Color

The color differences observed in soils of the Decapolis region were supposed to be connected with changing sets of iron oxides or organic matter (Lucke et al., 2005). But there is no trend between RR and Corg. Soil toposequences, characterised by color variations related to different goethite/hematite ratios, are well-known from the tropics (Cornell and Schwertmann, 2003). Redder soils form on faster drying crests, while more yellowish soils are present in valleys. Singer et al. (1998) describe slight differences in moisture retention of soils in Israel that might already lead to different ratios of iron oxides and thus color variation. Higher moisture tensions due to faster drying and warming of the soil were supposedly connected with stronger hematite formation – and according to Singer et al. (1998), the studied Terrae Rossae dried faster than the CaCO3-rich Rendzinas.

However field observations gave the impression that the color differences might be related to CaCO3 (as a white powder) and skeleton contents. It seems possible that the Rendzina investigated by Singer et al. (1998) is in fact a Terra Rossa that was later enriched


with chalk, which would also explain why the differences in water-holding capacity to the other examined Terra Rossa were so small. To test this hypothesis, a sample from a Terra Rossa free of calcium carbonate was mixed with pure CaCO3 and with chalk powder in a laboratory experiment. The redness rating decreased steadily with increasing calcareous additions (with a R² of 0.97 and 0.99), which makes is safe to say that the observed color differences are caused by CaCO3-enrichment (Lucke, 2007). To which degree the different types of iron oxides impact the color cannot be discussed, since the respective contents of hematite and goethite were not analyzed, but they seem rather irrelevant in the light of the strong negative correlation of redness and CaCO3-content.

From the extracted pedogenic oxides, the Feo/Fed activity index was calculated, representing the content of weakly crystalline iron oxides extracted by oxalate (Feo) and the total content of pedogenic iron oxides extracted by dithionite (Fed), which allows evaluating how well crystallized the pedogenic iron is. As well, the comparison of Fed with the total iron content (Fet) informs about the state of primary iron dissolution (Fed/Fet: weathering index). The more weathered and older the soil, the higher the Fed/Fet ratio should become, which is applicable to soil catenas and chronosequences. In contrast, the Feo/Fed ratio should decrease with stronger weathering, but is not suited for chronosequences.

For the presented soils, the weathering index Fed/Fet is not in agreement with field observations. The problem can best be illustrated and explained with sample TZ 54. Although it represents in-situ chalk in the Wadi el-Arab, the Fed/Fet ratio is higher than in most soil samples. This points to the presence of pre-weathered iron in the sedimentary rocks, but the content of which could strongly vary throughout the rocks. The expectation from the field is that additions of calcareous dust would reduce maturity of the soils. Therefore the indices were modified by the CaCO3-content. Although division by calcium carbonate content does not allow distinguishing between those samples where pre-weathered iron from calcareous dust is present and those where it is not, it refers to the part without CaCO3 and should make it possible to neutralize the impact of calcium carbonate enrichment. This modification changes the character of the Fed/t values: now the lower numbers express stronger weathering.

This modified weathering index seems to work well: according to the index, the Epipaleolithic soil in Wadi Ziqlab (Ziq) 4 is developed similarly to the red paleosol in the Jordan valley (Jor 10) and the present surface soil of the Yarmouk-profile (AB 1), but there seems to be an analytical error in the total element content of sample Ziq 1 (the sum of total elements of the available analysis is 137%). The modified weathering index supports the conclusion from CaCO3-contents that the Yarmouk-soil was less affected by re-calcification, pointing to deposition of less calcareous material due to the situation on a high plateau.

Feo in general is very low, which indicates good development of the soils: according to Torrent et al. (1980), a Feo/Fed ratio < 0.1 can be considered as very small. Although there is some increase in those layers of the Jordan valley soil which seem connected with phases of instability, the activity index and does not deliver further information. Modifying the index by division with the CaCO3-content led to more pronounced numbers, but did not change the pattern, which indicates that the pre-weathered iron from the rock is extracted to a lower degree by oxalate.

Manganese Oxides and Magnetic Susceptibility

Manganese is extracted with sodium dithionite and oxalate, too, but not as well as iron (Bascomb, 1968, McKenzie, 1989). Although manganese oxides are less investigated than


iron oxides, and although the extraction with dithionite might not be optimal for manganese, a weathering index formed by the Mnd/Mnt ratio should deliver similar information as the Fe-weathering index, and performs in fact much better than the iron index for the investigated soils. The Mn-weathering index turns once >1, which might be connected with another analytical error represented by a total element sum of 88%. As it seems, manganese oxides were not bound in the calcareous sediments, or not to the degree of iron oxides. The manganese weathering index was also modified with CaCO3, but which produced no meaningful results. However no weakly crystallized manganese oxides are known that would preferably be extracted by oxalate, which does dissolve already a great part of the oxides. Therefore calculation of an activity index as with iron makes no sense.

Apart from supporting the soil maturity degree suggested by the modified iron weathering index, the Mnd/t ratio indicates that the strongly rising CaCO3-content in the upper Yarmouk-profile is more connected with calcareous dust and secondary calcium carbonate (which was already present in the moved soil), and not with weakly weathered subsoil or chalk. If chalky material would have been added to a greater degree, the Mnd/Mnt index should be lower, as for example in the Jordan valley, or in Wadi Ziqlab. It should be mentioned that the formation of manganese oxides is closely connected with seasonal waterlogging in soils where relative high concentrations of iron oxides already exist (McKenzie, 1989). The strongly elevated Mnd/Mnt-ratios in the paleosols may therefore indicate that the area once experienced significantly moister conditions than today. Interestingly, the redness rating correlates best with the Mnd/Mnt ratio after removing the impossible value >1, even better than with Fed or the Fed/(Fet/CaCO3)-ratio. This is surprising since manganese oxides are amorphous or form black spots when well-crystallized. Soil color in the Decapolis region seems a matter of general soil maturity, which is probably the reason why the redness rating of Hurst (1977) performs best, since it is based on the total amount of iron oxides and not the hematite/goethite ratio.

The analysis of the magnetic susceptibility provided further insights: the best developed soils show the highest specific susceptibility χ, which also correlates strongly and positively with Mnd/t. For example, the Jordan valley profile has susceptibilities which mirror the field impression of alternating periods of soil development. The total susceptibility seems related to absolute soil development intensity, and, like expected from field observations, is most pronounced in Jor 10. The Jordan valley data indicate that the magnetic susceptibility is in fact the best parameter to assess soil development intensity. As discussed in the previous chapters, calcium carbonate content and iron oxides show similar tendencies, but are more difficult to interpret due to the presence of chalk admixtures. But how can the higher susceptibilities be explained? "Magnetic enhancement" is common in topsoils (Thompson and Oldfield, 1986) and could be attributed to a variety of reasons (see Evans and Heller, 2003):

1. Atmospheric fallout of ash or other deposition of magnetite-bearing material. 2. Natural fires that caused thermal transformation of weakly magnetic iron oxides

(especially hydroxides) to magnetite or maghemite in the presence of organic matter (Cornell and Schwertmann, 2003).

3. Inorganic formation of ultra-fine grained magnetites by oxidation of Fe2+ to ferrihydrite and magnetite at room temperature and near-neutral pH, due to transfer of electrons from decomposing organic matter under highly variable redox conditions and strongly changing soil moisture (Maher and Taylor, 1988).


4. Bacterial metabolism producing extracellular ultrafine magnetite grains under anaerobic conditions in periodically waterlogged soils, or intracellular magnetosome formation by magnetotactic bacteria under more oxygenated conditions in topsoils (Faßbinder et al., 1990).

5. Weathering of iron-bearing minerals due to wetting and drying cycles with ferrihydrite formation under presence of organic matter. Iron-reducing bacteria utilize ferrihydrite as terminal electron acceptor in their fermentation mechanism, so that partial dehydration and reduction of ferrihydrite to magnetite will take place in the presence of excess Fe2+. Magnetite might later be oxidized to maghemite (Dearing et al., 1996).

All possible processes except addition of foreign material are connected with organic

matter, and – apart from fires – with release of Fe2+ and ferrihydrite formation. The last point seems the most important for soils of the Decapolis region, as it is essentially the same process that leads to rubefication. Hydrolysis and dissolution of Fe2+, as well as the presence of organic matter, are closely connected with precipitation. The production of new magnetite and maghemite during pedogenesis may thus semi-quantitatively reflect precipitation. In fact, Heller et al. (1993) and Maher et al. (1994) used susceptibility and 10Be-content for reconstructions of paleorainfall on the Chinese loess plateau. In the case of the Decapolis region, it is very remarkable that from all investigated soil parameters, the Mnd/Mnt-ratio shows the strongest positive correlation (R²=0.72) to the specific susceptibility χ (after removing the impossible Mnd/Mnt values >1). As with color, manganese has no relation to magnetic properties, which supports that the high susceptibilities of the paleosols are inherited from moister periods of stronger pedogenesis and vegetation cover. The correlation to manganese oxides lets not only process 5 (weathering of iron), but also processes 3 and 4 (organic and inorganic magnetite formation) seem possible, as they are connected with periodic waterlogging which might be related to high extractable manganese.

The next strongest correlation of the specific susceptibility χ is a negative one to calcium carbonate content (R²=0.55). Increasing calcium carbonate contents are always associated with diminishing susceptibilities. As sample TZ 54 illustrates, the chalk is characterized by a very low susceptibility. Susceptibility shows only a weaker positive correlation to pedogenic iron Fed (R2=0.41). Since susceptibility is connected with iron-bearing minerals (Evans and Heller, 2003), this weak relationship might come as surprise – but in the case of the soils in the Decapolis region, it only confirms the impression from the iron oxide analysis that varying inheritance of pre-weathered iron from the sedimentary rocks, as well as deposition of foreign iron-bearing clay minerals, skew the measured contents of iron minerals. The magnetic analyses very clearly support the conclusions from the previous chapters: pedogenesis was stronger in the paleosols, and soil maturity later reduced by chalk additions.

Texture

The soils are extremely clay-rich. This confirms field observations: the Terra Rossa forms cracks and huge, very hard lumps when drying, pointing to a strong shrink-swell behaviour connected with a lot of clay. In this context, the limited organic matter seems very important for fertility since it preserves a granular soil structure. The Rendzinas from the Jordan valley and Wadi Ziqlab seem rich in clay, too, but which could be due to the residue of dissolved chalk additions which form a great part of the soil substance. The chalk sample TZ


54 indicates that already the rock's residue might consist of more than 60% clay. Nevertheless, clay contents of 70-80% could represent a weathering climax. While weathering processes might initially lead to a quick increase of fine material, texture will change less and less the more strongly the soil is weathered. Despite high clay content and the presence of slickensides, the analytical data suggest that mixing processes are limited and that even the lower part of the Yarmouk-profile is layered despite its uniform appearance. A closer look at the Yarmouk-profile reveals that the silt content remains the same throughout the profile, while the drop of clay is associated with an increase of sand, probably representing fluvially transported limestone particles. This points to a basically unchanged composition of the soil substrate, but increased movement and less weathering in the upper layers.

Field observations suggest that the weakly developed calcareous soils consist mainly of silt, since they are easily deflated and show no cracks when dry. Decisive for the soils' behaviour should be the form in which CaCO3 is present: as secondary precipitate of drying soils, or primary deposit coming from the rocks. Since even the deepest part of the Yarmouk-profile still contains CaCO3, deposition of calcareous dust there may never have ceased. If true, the rise of CaCO3 in the upper part of the soils may not so much be connected with increasing dust deposition, but decreasing leaching. This would mean that soil development might reflect a general tendency of increasing aridity accompanied by short periods of heavy erosion and soil movement, possibly connected with post-glacial warming. Deflation of the Terra Rossa is however not indicated due to the tendency of the clay-rich soil to form hard lumps when dry, and because aeolian activity should be seen in the landscape, but which shows no soil accumulations in wind-protected areas, and no concentration of gravel and coarse material at wind-exposed sites.

Discussion: Paleosols in Northern Jordan The example of the Decapolis region shows that paleosols can be widespread in semi-arid

areas, but are sometimes difficult to recognize since the color differences may be slight. For the Terra Rossa in northern Jordan, these color differences however match different phases of soil development and refer to soil maturity and calcium carbonate content. The paleosol pattern found in the Decapolis region seems to resemble the climatically governed soil degradation scheme proposed by Fedoroff and Courty (1999). The Terrae Rossae, even if exposed at the surface (as on most plateaus), are relic soils that formed before the Neolithic, and now show different stages of degradation. Nowell et al. (2003) estimated their age according to artifacts to 125,000 years and older.

According to Fedoroff and Courty (1999), cracking clay-rich soils that are not completely hydrated any more come to a standstill in their formation. Consequently allochtonous material, mostly sand or calcareous deposits, is filling the cracks, leading to an overall decrease of soil maturity. When the cracks are not operating any more, an important soil feature is missing that could drain excess water from heavy rainstorms. As well, organic matter content and biological activity are reduced under aridity, which has a negative impact on soil fabric and nutrient availability. According to Bens et al. (2006), a deteriorating soil fabric may cause soils to repel rainwater when dry, leading to very high runoff and an increasing erosion risk, and to a decrease of root penetration depth and vegetation cover.


Lower rainfall and higher evaporation are also connected with fewer leaching and enrichment of calcium carbonate and other bases. According to Imeson and Lavee (1998), higher temperatures in the Judean mountains led to an increasing concentration of soluble salts and did therefore directly reduce aggregate stability. Combined with the overall diminished vegetation, aridity thus degrades soils directly and indirectly by lowering their structural integrity and aggravating the impact of floods and wind erosion.

The presented profiles show that several phases of landscape change in northern Jordan were recorded by the soils. The available dates suggest that these changes took place since the end of the last Ice Age, and can be described as:

• an overall tendency to reduced soil maturity, either by decreased leaching or

increasing deposition of calcareous material, and likely connected with diminished precipitation during post-glacial warming

• the occurrence of at least 2 extreme erosion phases which interrupted soil development, and led to deposition of new material until landscape stability resumed

• the landscape seems stable since the invention of agriculture, and erosion due to man's activities negligible - but humans may have contributed to rising CaCO3-contents in the soils by releasing calcareous dust. Although evident for all soils presented here, this is however a locally different phenomenon in the Decapolis region and seems related to the proximity of chalk exposures (Lucke, 2007).

According to the dates which are available so far, the periods connected with heavy

erosion may correspond to the Younger Dryas (11,000 BC) and the Yarmoukian landslides (~6200 BC) (Lucke, 2007). It seems possible that heavy flooding repeated during historical periods, indicated by colluvia in Byzantine and Mamluk ruins (Lucke, 2007). However no Terra Rossa was eroded any more, either because the later events did not reach the magnitude of prehistoric erosional phases, or because the area is already in the stable state of completed erosion (NSM&LUP, 1993).

There is no evidence that human activity in the Decapolis dominated soil degradation: even the re-calcification seems to have taken place mainly before 6000 BC. While human activity may locally have contributed to deterioration, the visible development pattern can clearly be attributed to climate, and the last occurence of global warming at the end of the Ice Age seemingly had a detrimental impact. However, from a nutrients point of view the calcareous Rendzinas seem not inferior to the Terrae Rossae (although it was not yet evaluated to which degree phosphorus is fixed by calcium carbonate). Does this mean that the visible soil degradation had no effect for fertility? As already indicated by the high pH-values, the chemical milieu of soils is dominated by base accumulation and not leaching, and the chalk sample TZ 54 indicates that nutrients can be bound in significant amounts in calcareous sediments.

Due to this, and because of the strong variation of annual precipitation, manure with artificial fertilizer is not recommended in Jordan (NSM&LUP, 1993). However, soils show a general N-deficiency, which is connected with the low levels of organic matter, and can be attributed to the semi-arid climate: the highest Corg-content measured in the Decapolis region under oak forest numbered no more than 3.6%. Organic matter also plays a key role for water supply, since it increases the water-holding capacity and has a positive impact on soil


structure. Determining the plant-available water is closely connected with the texture, but as discussed above, the texture of calcareous soils is difficult to ascertain as calcium carbonate has to be removed, and it is not possible to distinguish between primary and secondary carbonates. A solution might be the year-round weighting of soil blocks in the field as conducted by Singer et al. (1998). Their results indicate that secondary carbonate in Mediterranean Red Soils has little impact on the moisture holding capacity. Things are different with primary carbonates: according to Purschke (1990), the average density of Terrae Rossae is 1.6 g/cm3, and their plant-available moisture capacity ~23%, which is low and due to the high clay content. But the average plant-available moisture capacity of Rendzinas numbers only ~13%, and their density 1.2 g/cm3, which is marginal for permanent crops.

In this context, it should be mentioned that the BET-procedure may deliver additional information about soil structure. This method calculates the specific surface of samples by their nitrogen adsorption. The specific surface is usually strongly correlated to the clay content; however tests yielded different results for soils in the Decapolis (Lucke, 2007): the better developed soils were characterized by a higher porosity, which seems related to lower contents of primary carbonates. Last but not least, we can also utilize plants as indicators for water and fertility. As discussed by Ackermann et al. (2004), Sarcopoterium spinosum continuously occupied a berm representing the chalk spoil of an ancient siege ditch close to the site Tell es-Safi. As it seems, primary carbonates have a bad water-holding capacity since the berm is nearly exclusively occupied by Sarcopoterium spinosum, which is better adapted to such conditions than other plants in the area. Terrae Rossae and Rendzinas seem to represent the soil types described in Jesus' parable of the sower (Matthew 13:3-23): Seeds will sprout quickly on Rendzinas as they warm up faster, but are more endangered by the summer drought than on the deep and clay-rich Terrae Rossae.

The importance of nutrient levels seems generally overruled by the soil's water-holding capacity and rainfall, which may also directly be connected with nutrients supply: according to Singer et al. (1992), dust brought with the rains to Mount Carmel provides up to 100% of the nutrients requirement of pine forest. This is further supported by an evaluation of harvest data in Jordan, which showed that the main agricultural areas focus on deep, well-developed Terrae Rossae, and that yields in the semi-arid zones with 200-600 mm annual precipitation strongly depend on rainfall (Lucke, 2007).

We can conclude that the main process reducing fertility in Jordan is not erosion of topsoil, but calcareous additions, and loss of organic matter due to reduced vegetation. The climatic forcing of this process seems reflected by the distribution of soils in the landscape: for example, Wadi Ziqlab illustrates how temperature and soil type may be related to changing fertility. While the mean annual precipitation is basically stable over the whole course of the wadi, temperatures vary strongly and follow an altitude gradient of 1300 m over a valley length of 20 km (Fisher et al., 1966). The soil and vegetation pattern gives a strong impression of increasing aridity with lower altitude. Precipitation cannot explain this trend: the contrasting character of the landscape is due to temperature, evaporation, and changing soil properties (Fisher et al., 1966).The increasing likeliness for deposition of calcareous material in the lower course of the wadi may be connected with this pattern, and the soil and vegetation distribution in Wadi Ziqlab could to some degree allow description of thresholds for permanent soil and vegetation change with regard to temperature, evaporation, and calcification. Considering the water balance and vegetation, the direct impact of temperature


and evaporation may not be as relevant as the indirect role of changing soil properties and local microclimatic controls. This question was illuminated by a study focusing on the geographic law of soil zonality in Mexico, which is presented in the next subchapter.

PART II: MODELING SOIL DEVELOPMENT IN MEXICO

The Study Area: Mexico In order to predict the alteration of Mexican authomorphic rainfed agricultural soils (with

deep water table) as well as virgin soils (not used in agriculture) due to global warming, the relationships between some virgin soil properties (f) and Budyko’s radiative index of dryness were established for relatively flat lands (with slopes of less than 3%) (Nikolskii et al., 2006). Data on cation exchange capacity, base saturation, pH, organic matter content, exchangeable bases, available P and K, and the content of clay in the 0-20 cm surface layer were taken from digital soil maps (scales 1 : 50,000 and 1 : 250,000) of whole Mexico. The sampling points were located at altitudes ranging from 0 to 2500 m, covered by virgin vegetation, and characterized by similar geomorphologic conditions and subsoil properties. They were connected with sharp climatic contrasts (from an arid climate with less than 200 mm annual precipitation to a humid tropical climate with precipitation exceeding 2500 mm, and a range of mean annual temperatures from 5 to 24° C) and diverse vegetation communities. About 1800 sampling points formed a network, and data of nearby weather stations were used to calculate the radiative index of dryness (I).

Methods: Mathematical Modeling of Soil Change It is known that the geographical distribution of the world's main soils types correlates

with mean annual values of net radiation (R) and precipitation (Pr), or with Budyko’s radiative index of dryness (I) (Volobuev, 1974; Grigor’ev, 1954):

I = R/(L x Pr) (1)

where (R) and (Pr) are expressed in kJ/m2 and in mm, respectively, and (L) is the latent heat of evaporation equal to 2.51 kJ/m2 per mm. The surface water runoff (S) is considered only for humid tropical zones with an annual precipitation of more than 2000 mm. In such conditions it is necessary to use (Pr – S) instead of Pr.

For our study, the mean annual net radiation (R) was calculated from the albedo of natural vegetation in different climatic zones of Mexico (Tejeda et al., 1999; Torres, 1995), global annual radiation, and the annual balance of long wave radiation. Global annual radiation was determined using an empirical relationship based on known values of solar radiation (Sellers, 1965). The annual balance of long wave radiation was calculated using known data of mean monthly air temperatures (Sellers, 1965). The comparison of calculated net radiation values with measured data showed a strong similarity (with a correlation coefficient > 0.9).


The relationships between particular soil properties (f) and (I) values were established calculating (I) values for the sampling points after an interpolation of climatic data (air temperature and precipitation) obtained from adjacent meteorological stations. The absolute altitude of particular plots played no role for the character of these relationships, though it affected the radiative index of dryness. Predicted scenarios of climate change in Mexico for a doubling of the CO2-content by the end of the 21st century were used for calculation of I in the year 2100, as they contain monthly average data on expected air temperatures, precipitation, and global radiation (Gay, 2003). We assumed that the albedo of surfaces covered by natural vegetation will not change significantly.

Statistical treatment of the averaged data of each soil property (f) allowed us to obtain regional dependencies on the climatic index (I). In this treatment, the total range of (I) values (from 0.3 to 12) was subdivided into several intervals. Within these intervals, the random variation of soil properties due to (I) can be ignored. The size of these intervals depended on the particular soil properties and the absolute values of (I). For example, an interval of a radiative index of dryness (I) equal to 0.5 was applied for pH, cation exchange capacity, and the content of Ca, P, and Mg for a range of (I) from 0 to 4; for the rest of the studied soil properties, the same interval was applied within a range of (I) from 0.3 to 3. The size of the intervals was increased to 1.0 for values of (I) from 4 to 6 and from 3 to 6, respectively. The range of (I) from 6 to 12 was subdivided into two intervals (equal to 3) for all soil properties.

This approach allowed us to consider data on soil properties within every particular interval of (I) as statistically homogeneous samples. The particular soil properties normalized by their mean arithmetic values were determined in each of these intervals. The statistical distribution of the normalized data on every particular soil property was then analysed, and modal values of normalized data were obtained along with modal absolute values of particular soil properties (f) for every particular interval of (I) values.

The resulting quantitative relationship of a modal regional value of a soil properties (f) and the mean radiative index of dryness (I) schematically shown in Figure 5 can be used to predict soil development under climate change.

Figure 5. A schematic dependence between a soil property (f) and the climatic index (I). (f2000max) is the

maximal value of the property (f) observed in the analysed soils; (I20000) and (f2000

0) are the (I) index and (f) property values at one of the studied sites today. (I2100

1), (f21001) and (I2100

2), (f21002) are possible

scenarios for the alteration of (I) and (f) at the end of the 21st century.


It is important to remember that the f(I) relationship does not depend on climate change, but reflects the geographic law of soil zonality. It is assumed that soil properties come to equilibrium when climate change is rather slow. According to Arnold et al. (1990), this applies to organic matter, mineral nutrients contents, and pH. The direct impact of rising CO2-contents on soil organic matter is rather small and was ignored taking into account the estimations made by Bazzaz et al (1996), Reichstein et al. (2005), and Knorr at al. (2005).

If the dryness index (I2000) is known for a selected geographical point at the beginning of the 21st century, the regional soil property (i.e., the averaged value according to the area) will be equal to (f2000

0), whereas the local property (i.e., the specific value for a certain point) at a particular point (f(loc)) will be (f(loc)2000

0), which can differ from the regional (f20000) value.

If the predicted value of the (I21001) or (I2100

2) index is known, we may obtain the corresponding new values of the regional property (f2100

1) or (f21002) from the graph f(I) and

assess the possible modification in the local property (f(loc)20000) to the (f(local)2100

1) or (f(local)2100

2) value: f(local)2100

1 = f(local)20000 + (f2100

1 – f20000) or f(local)2100

2 = f(local)20000 - (f2000

0 – f21002) (2)

To estimate changes of groups of soil properties, we may use an approach based on the

regional integral index of soil fertility (F) in a dimensionless form as suggested by Pegov and Khomyakov (1991):

F = 0.46maxOM

OM + 0.28

maxmax KK

PP

∗ + 0.26 2

26pH

e⎟⎠⎞

⎜⎝⎛ −

− (3)

where OM, P, and K are organic matter, phosphorus and potassium content, respectively, while OMmax, Pmax, and Kmax are the maximal values observed in the investigation area. The values of F are varying from 0 (for completely degraded soils) to 1 (for the maximal possible fertility in the considered area).

In order to predict the alteration of (F) and its meaning for agricultural land, we may calculate the regional values of virgin soil fertility (F2000) and local values (F(agr)2000) with equation (3). Then we can assess the difference (F2100 - F2000) using graphs F(I), as for f(I) and schematically shown in Figure 5. Considering agricultural land, it should be remembered that the climatic indices (I2000) and (I2100) may differ from virgin soils under natural vegetation. Apart from that, the calculation of fertility changes agricultural soil fertility changes was done in the following manner:

F(agr)2100 = F(agr)2000 + (F2100–F2000). (4) These results may be used for assessing the vulnerability of agricultural crops to climate

change. A simple approach to predict the agricultural crop productivity at the end of the 21st century (Y2100) according to FAO-IIASA (2000) and Pegov and Khomyakov (1991) is presented in equation (5):

Y2100 = Y2100

max x K2100water x F(agr)2100 (5)


where Y2100max is the maximum possible crop yield (kg ha-1) depending on the predicted

monthly values of temperature, global radiation, and biological plant properties; K2100water is a

coefficient based on of soil water availability and depending on predicted monthly values of temperature, global radiation and precipitation; and F(agr)2100 is the predicted agricultural soil fertility index. K2100

water and F(agr)2100 are dimensionless and vary between 0 and 1.

Results: Modeling Soil Development in Mexico Using the methodology described above, the graphs f(I) were obtained. Confidence

intervals (with a 95% probability) for changes of soil properties f numbered +-15% of the modal values of (f). The correlation coefficient (R²) between factual modal values and the approximating curves is high for all analysed soil properties: 0.94 < R² < 0.99. In general, the curves lie within confidence intervals of soil properties changes.

The curves illustrating changes in the contents of organic matter, exchangeable Ca, Mg, K, Na, available P; and clay particles, as well as for cation exchange capacity, have distinct peaks. They reach their maximum upon certain values of (I). Base saturation and pH values increase with the rise in (I). The analysis of modal values shows that the maximum values of soils properties are observed at the following values of (I): organic matter content (7%) at (I) = 1.2 (Pr = 1200-1800 mm); cation exchange capacity (23 meq/100 g soil) at (I) = 2-2.5 (Pr = 350-800 mm); clay content (35%) at (I) = 1.5-2.5 (Pr = 350-1000 mm); exchangeable Ca (27 meq/100 g soil) at (I) = 1.5-3 (Pr = 300-1000 mm); exchangeable Mg (4.5 meq/100 g soil) at (I) = 1 (Pr = 1500-2000 mm); exchangeable Na (0.7 meq/100 g soil) at (I) = 2 (Pr = 500-800 mm); exchangeable K (1.3 meq/l00 g soil) at (I) = 2.5-3.5 (Pr = 250-700 mm); and mobile P (4.5 meq/100 g soil) at (I) = 2.5-3.0 (Pr = 300-350 mm).

It should be noted that similar relationships f(I) were obtained for the European and Asian parts of Russia (Aydarov, 1985). The comparison of f(I)-graphs shows that the dependencies of organic matter content, cation exchange capacity, clay particle content and pH values on radiative index of dryness (I) are practically the same. This means that such relationships are probably universal and can be used as first approximation in different countries. The relationships f(I) published by Aydarov (1985) considered also the content of water stable aggregates and the ratio between humic and fulvic forms of organic matter, which were not investigated in Mexico. Unfortunately, no f(I)-relationships were yet established for Jordan and the Levant.

The F(I)-relationship reflects a certain dependence of soil fertility on the climatic index. The fertility index (F) ranges from 0.23–0.33 in the arid regions (with a dryness index (I) exceeding 4) to 0.79 in the regions with a moderate climate (with (I) averaging 1.5). The fertility index decreases to 0.37–0.6 in areas with wet tropical climate and an (I) of less than 1. Nikolskii et al. (2006) found that the index varies most noticeably in the interval 0 < I < 3. This means that the soils which are most “susceptible” to climate change are located in zones where the dryness index lies within 0 < I < 3, i.e., the humid tropical zone (I ≤ 1), the semi-humid zone (1 < I ≤ 2), and the semi-arid zone (2 < I ≤ 3). The potential impact of climate change may obviously be estimated mathematically as an absolute value of the partial derivative (δF/δI), meaning the (F) variation per unit change of (I).

Depending on the dryness index, either an increase or a decrease of soil fertility is expected, ranging from +25 to –35%. If one assumes that a change of the fertility index (F)


within 20% is low, and a change above 20% is substantial, we may conclude that soil fertility will decrease substantially in a number of Mexican regions (predominantly, those with wet tropical and arid climates), if the CO2 content doubles by the end of the 21st century (Figures 6 and 7, Table 5).

Figure 6. Prediction of changes of the radiative index of dryness (I) in Mexico during for a doubling of the CO2-content in the atmosphere according to Gay (2003).

Table 5. Examples of the fertility index alteration for virgin soils in Mexico as assessed for a doubling of the CO2-content in the atmosphere

I F State Pr2000

(mm) 2000 2100

ΔI (%) 2000 2100

ΔF (%)

Tabasco 2890 0.73 0.51 - 30 0.50 0.37 - 26 Chiapas 1923 0.91 0.72 - 21 0.51 0.40 - 22 Veracruz 1546 1.30 1.05 - 19 0.77 0.67 - 3 Campeche 1265 1.43 1.52 + 6 0.77 0.78 + 3 Zacatecas 1191 1.56 1.60 + 3 0.77 0.77 0 Michoacan 1031 1.50 1.39 - 7 0.79 0.76 - 4 Nayarit 976 1.70 1.57 - 8 0.76 0.78 + 3 Puebla 816 1.90 1.68 - 12 0.70 0.77 + 10 Oaxaca 730 1.90 1.55 - 18 0.70 0.79 + 13 Chihuahua 604 2.20 2.85 30 0.59 0.46 - 22 Coahuila 535 3.00 3.95 32 0.44 0.34 - 23 Sonora 365 3.90 2.76 - 29 0.32 0.39 + 22 Nuevo Leon 315 5.18 5.55 + 7 0.26 0.25 - 4 Baja California 175 6.20 8.00 +29 0.24 0.23 - 4


Figure 7. Prediction of the future regional integral soil fertility index (F) in Mexico for a doubling of the CO2-content in the atmosphere according to Gay (2003). The changes of (I) and (F) were calculated according to the formula: ΔI = (I2100 – I2000)/I2000 and ΔF = (F2100 – F2000)/F2000 In order to qualitatively characterize the predicted changes of (I) and (F), the following terms were used: > 30% increase – very strong increase 20-30% increase – strong increase 10-20% increase – moderate increase 5-10% increase – low increase 0±5% change – no change 5-10% decrease – low decrease 10-20% decrease – moderate decrease 20-30% decrease – strong decrease

The results also show significant changes of agricultural productivity: the contribution of

soil fertility deterioration due to climate change is assessed to number up to 30% of the potential crop yield (Castillo-Alvalez et al., 2007).


Discussion: Modeling Soil Development in Mexico Global warming will probably have the following effects in Mexico: • a moderate and weak decrease of soil fertility in the semi-arid and arid central and

northern regions • a relatively significant (up to 35%) decrease of soil fertility in the semi-humid and

humid tropical regions • a relatively significant (up to 30%) deterioration of agricultural crop productivity in

the semi-humid and humid tropical regions due to reduced soil fertility • a rather insignificant contribution of climatically induced soil degradation to crop

productivity in the arid and semi-arid zones. The main factor influencing the vulnerability of agricultural crop productivity in these zones is precipitation

• with regard to the distribution of soils in Jordan, it is unlikely that the direct impact of temperature and evaporation is behind the strongly changing vegetation in Wadi Ziqlab. These parameters would primarily impact plant growth and organic matter content, but only minor changes of the fertility index F are found since organic matter contents are already low. In this context, a calculation of fertility according to equation (3) reveals little differences between paleosols and topsoils in the Decapolis region, although it cannot be taken into account whether organic matter contents were higher in the past.

CONCLUSION The impact of future droughts with regard to soil properties depends on the climate zone.

Regarding humid and semi-humid areas, soil deterioration may be responsible for up to 30% of future crop losses. This loss would mainly be connected with a reduction of organic matter, which might in turn contribute to further warming. However, no significant degradation is expected for the arid and semi-arid areas: organic matter levels in these regions are already low, and the impact of further warming might be less significant. In this context, soil degradation in the Decapolis region seems primarily related to calcareous additions, and only secondarily to organic matter. Once calcification took place, it seems irreversible under the present climatic conditions, and exercises an important influence for vegetation cover and soil structure. Unfortunately, the impact of CaCO3 is hard to quantify since its coagulating impact on soil particles requires its removal prior to particle size analysis, and because it is difficult to differentiate between primary and secondary carbonates. Future studies using advanced methods like gas adsorption (BET-procedure) might provide further insight.

Paleosol research suffers in this context from the difficulty to reconstruct past levels of organic matter. Future studies of soil magnetics might allow for improved assessments, since the "magnetic enhancement" of soils is closely connected with organic matter and precipitation. Combined with soil development indices based on pedogenic oxides of Fe and Mn, the magnetic susceptibilities suggest that the vegetation cover of the paleosols was much denser than today. If the profiles were dated more precisely, it might be possible to reconstruct past precipitation levels, and to better describe the role of time for soil formation.


There is no evidence that ancient land use degraded soils in the Decapolis region - it may have had a local negative effect due to soil movements and deposition of calcareous material, but may also have improved fertility temporarily at some places, since archaeological material points to intensive organic manure during the Roman and Byzantine period (Lucke et al., 2005, Schmidt et al., 2006). Erosion seems to have taken place mainly in form of land slides. These are usually triggered by heavy rainstorms, and as it seems at least 2 periods of massive erosion took place since the end of the last Ice Age.

Reviewing the size of colluvia deposited during these times, soil conservation measures seem desperate. This is supported by failures of development aid projects, who constructed check dams and terraces only to see them being washed away under the impact of a deluge following an exceptional heavy rainfall event (Grove and Rackham, 2003, Lucke, 2007). But although soils in the Decapolis region seem to exhibit an incipient stage of climatically governed degradation as suggested by Fedoroff and Courty (1999), which is likely connected with diminished precipitation under post-glacial warming, the greatest part of the area is still covered by relic soils. It seems well possible that the remaining Terrae Rossae are in fact erosion-resistant (Lucke, 2007), and that most soils in Jordan are in the stable state of completed erosion for the current climatic conditions since the beginning of the Neolithic (NSM&LUP, 1993). However, it is evident that the Terrae Rossae extended once into the areas which are now desert and steppe (Cordova, 2000, Khresat and Taimeh, 1998), and where most red soils were removed in processes as described by Fedoroff and Courty (1999). Investigating the remaining paleosols in the desert would deliver important clues about time-frames and intensities of soil degradation, and will make it possible to better assess the future fate of the red soils in the desert belts.

Man's option to deal with the possible consequences of global warming seem in this context very limited: should climate change lead to another period of massive erosion, the only possible answer might be evacuation of risk areas. For such a scenario, the impacts on agriculture would be quite unimportant compared with the consequences for water harvesting systems and infrastructure. With regard to agricultural productivity, a loss of organic matter is reversible and depends on vegetation cover. A strategy to address global warming might therefore be irrigation, since this may also prevent calcification. Issar et al. (2007) suggested to use fossil water for greening the deserts, in order to sequester carbon from the atmosphere, retain or improve soil fertility, and provide opportunities for people living in the desert belts who might otherwise be displaced. In the light of the evidence presented above, this might in fact be the most realistic approach to deal with global warming. In contrast, it is unlikely that soil conservation measures will succeed if plantations are not irrigated, and it seems unfeasible to strive for a sustainable exploitation of resources in the desert belt based on their current availability, since this is only possible if the natural water supply will not shrink. Reviewing possible scenarios of climate change, it seems possible that change may not be linear, but be connected with a switch to a different equilibrium, which makes it very difficult to say which areas will be affected, and how internal environmental feedbacks in those regions will look like (Lucke, 2007).

Choosing the right strategy for dealing with coming droughts depends to a large degree on the correct prediction of climate change. In this context, the investigation of paleosols and other environmental archives could provide further important insights. Soil fertility prediction as presented in this paper could also be used for modelling backwards, focusing on representative paleosols properties (f). Using a graph f(I) as schematically shown in fig. 5 for


modern soils, it would be possible to compare past and present radiative indexes of dryness in the light of climate models and reconstructions from other proxies in order to test their soundness.

ACKNOWLEDGMENTS We gratefully acknowledge the support of German Research Foundation (DFG, grant no.

SCHM 2107/2-1), the Jordan Valley Authority, and Yarmouk University, Irbid, Jordan. We would like to thank Lisa Maher and Edward Banning for their support, cooperation, and many fruitful discussions. We are also grateful to the Secretaria del Medio Ambiente y Recursos Naturales (SEMARNAT) and the Consejo Nacional de Ciencia y Tecnología (CONACYT) for supporting the Mexican research.

REFERENCES

Ackermann, O., Maeir, A. & Bruins, H. (2004). Unique human-made catenary changes and their effect on soil and vegetation in the semi-arid Mediterranean zone: a case study on Sarcopoterium spinosum distribution near Tell es-Safi/Gath, Israel. Catena, 57, 309-330.

Arnold, R.W., Szabolcs, I. & Targulian, V.O. (1990). Global soil change. (Eds.) Laxenburg: IIASA Publications.

Aydarov, I.P. (1985). Water, salt and nutrient soil regime management in arid zones. Moscow: Agropromizdat Publications (in Russian).

Bascomb C. (1968). Distribution of pyrophosphate-extractable iron and organic carbon in soils of various groups. Journal of Soil Science, 19, 250-268.

Bazzaz, F. & Sombroek, W. (1996). Global Climate Change and Agricultural Production: Direct and Indirect Effects of Changing Hydrological, Pedological and Plant Physiological Processes. Rome: FAO.

Bender, F. (1974). Geology of Jordan. Berlin: Gebrüder Bornträger. Bens, O., Buczko, U., Sieber, S., & Hüttl, R. (2006). Spatial variability of O layer thickness

and humus forms under different pine beech-forest transformation stages in NE Germany.Journal of Plant Nutrition and Soil Science, 169 (1), 5-15.

Buntley, G. & Westin, F. (1965). A comparative study of developmental color in a Chestnut-Chernozem-Brunizem soil climosequence. Soil Science of America Proceedings, 29, 579-582.

Castillo-Álvarez, M., Nikolskii-Gavrilov, I., Ortiz-Solorio, C.A., et al. (2007). Alteración de la fertilidad del suelo por el cambio climático y su efecto en la productividad agrícola. Interciencia, 32, 368-376.

Conde, C., Liverman, D., Flores, M., et al. (1998). Vulnerability of rainfed maize in Mexico to climate change. Climate Research, 9, 17-23.

Cordova, C.E. (2000). Geomorphological Evidence of Intense Prehistoric Soil Erosion in the Highlands of Central Jordan. Physical Geography, 21 (6), 538-567.


Cordova, C., Foley, C., Nowell, A., et al. (2005). Landforms, sediments, soil development and prehistoric site settings in the Madaba-Dhiban Plateau, Jordan. Geoarchaeology, 20 (1), 29-56.

Cornell, R. & Schwertmann, U. (2003). The Iron Oxides. Weinheim: Wiley VCH. Dearing, J., Hay, K., Baban, S., Huddleston, S., Wellington, E., Loveland, P. (1996).

Magnetic susceptibility of soil: An evaluation of conflicting theories using a national data set. Geophysical Journal International 127, 728-734.

DIN 19683, Deutsches Institut für Normung (1973). Blatt 1-3, Physikalische Laboruntersuchungen, Bestimmung der Korngrößenzusammensetzung nach Vorbehandlung mit Natriumpyrophosphat. Berlin: Beuth Verlag.

DIN 51001, Deutsches Institut für Normung (2003). Allgemeine Arbeitsgrundlagen zur Röntgenfluoreszenz-Analyse (RFA) sowie Beiblatt 1 (Uebersicht stoffgruppenbezogener Aufschlussverfahren zur Herstellung von Proben fuer die RFA). Berlin: Beuth Verlag.

Dregne, H. (1983). Desertification of Arid Lands. Advances in Desert and Arid Land Technology and Development, Vol. 3. London: Harwood.

Evans, M. & Heller, F. (2003). Environmental Magnetism. Principles and Applications of Enviromagentics. Amsterdam: Academic Press.

FAO-IIASA (2000). Global agroecological zones. Methodology and results of the Global Agro-Ecological Zones Model. CD-ROM, Version 1.0, URL: www.fao.org.

Faßbinder, J., Stanjek, H. & Vali, H. (1990). Occurence of magnetic bacteria in soil. Nature, 343, 161-163.

Fedoroff, N, & Courty, M. (1999). Soil and Soil forming processes under increasing aridity. In Singhvi, A. & Derbyshire, E. (Eds.), Paleoenvironmental Reconstruction in Arid Lands (73-108).Rotterdam: A.A. Balkema.

Fisher, W., Atkinson, K., Beaumont, P., et al. (1966). Soil Survey of Wadi Ziqlab. Durham: University of Durham.

Gay, C. (Ed.) (2003). Una visión hacia el siglo XXI. El cambio climático en México. Mexico City: UNAM.

Geraty, L.T. & LaBianca, Ø. (Eds.) (1986). Hesban 2. Environmental Foundations. Berrrien Springs: Andrews University Press.

Grigor'ev, A.A. (1954). Some regularities of geographical zonality. Izv. Akad. Nauk SSSR, Ser. Geogr., 5, 5-17, (in Russian).

Grove, A.T. & Rackham, O. (2003). The Nature of Mediterranean Europe: an Ecological History. New Haven: Yale University Press.

Gvirtzman, G. & Wieder, M. (2001). Climate of the Last 53,000 Years in the eastern Mediterranean, based on soil-sequence Stratigraphy in the coastal plain of Israel. Quaternary Science Reviews, 20, 1827-1849.

Heller, F., Shen, C., Beer, J., Liu, M., Bronger, A., Suter, M., & Bonani, G. (1993). Quantitative estimates and paleoclimatic implications of pedogenic ferromagnetic mineral

formation in Chinese loess. Earth and Planetary Science Letters, 114, 385-390. Hillel, D. (1991). Out of the Earth. Civilization and the life of the soil. New York: The free

press. Horowitz, A. (1979). The Quaternary of Israel. New York: Academic Press. Hurst, V.J. (1977). Visual estimation of iron in saprolite. Geological Society of America

Bulletin, 88, 174-176.


Imeson, A. & Lavee, H. (1998). Soil erosion and climate change: the transect approach and the influence of scale. Geomorphology, 23, 219-227.

Issar, A., Arias, M., Berliner, P., Bogardi, J., Faure, H., Lezine, M., Prinz, D., Rognon, P., Riedaker, A., Tanaka, T., Tsiourtis, T., Liu, T., Williams, J. (2007). A CALL FOR GLOBAL ACTION - Replant the deserts! Personal communication, call issued by the members of the steering committee for a project envisioned in the framework of World Water Vision and The International Hydrological Program (IHP), UNESCO.

KfW (2005). Feasibility study for the re-use of treated wastewater in irrigated agriculture in the Jordan valley. Semi-detailed soil survey, Vol. 1. Amman: Kreditanstalt für Wiederaufbau.

Khresat, S. & Taimeh, A. (1998). Properties and characterization of vertisols developed on limestone in a semi-arid environment. Journal of Arid Environments, 40, 235-244. Knorr W., Prentice I.C. & Holland, E.A. (2005). Long-term sensitivity of soil carbon turnover

to warming. Nature, 433, 298-301. Lowdermilk, W. (1944). Palestine – land of promise. London: Victor Gollancz. Lucke, B. (2002). Abila's Abandonment. Anthology of Scientific Publications & Research

Papers, RPR 14. Cottbus: Chair of Environmental Planning, BTU Cottbus. Lucke, B., Schmidt, M., al-Saad, Z., et al. (2005). The Abandonment of the Decapolis Region

in Northern Jordan – Forced by Environmental Change? Quaternary International, 135, 65-81.

Lucke, B. (2007). Demise of the Decapolis. Past and Present Desertification in the Context of Soil Development, Land Use, and Climate. Dissertation at University of Cottbus, URL: http://nbn-resolving.de/urn/resolver.pl?urn=urn:nbn:de:kobv:co1-opus-3431.

Maher, B. & Taylor, R. (1988). Formation of ultrafine-grained magnetite in soils. Nature 336, 368-370.

Maher, B., Thompson, R. & Zhou, L.-P. (1994). Spatial and temporal reconstructions of changes in the Asian paleomonsoon: A new mineral magnetic approach. Earth and Planetary Science Letters, 125, 462-471.

Maher, L. (2005). The Epipaleolithic in context: paleolandscapes and prehistoric occupation in Wadi Ziqlab, Northern Jordan. Toronto: Dissertation, Department of Anthropology, University of Toronto.

McKenzie, R. (1989). Manganese Oxides and Hydroxides. In Dixon, J., Weed, S., Dinauer, R. (Eds.), Minerals in Soil Environments (439-466). Madison: Soil Science Society of America.

Nikolskii, I., Bakhlaeva, O.S., Contreraz-Benitez, A., & Ordaz-Chaparro, V. (2002). Assessment of Changes in Soil Properties as Dependent on Hydrothermic Conditions of Plowlands (by the Example of Mexico). Eurasian Soil Science, 35, 1031-1036.

Nikolskii, I., Castillo-Alvares, M., Bakhlaeva, O.S., Roman-Calleros, X.A., Maslov. B.S. (2006). The Influence of the Possible Global Climate Change on the Properties of Mexican Soils. Eurasian Soil Science, 39 (11), 1164-1169.

Nowell, A., Bisson, M., Cordova, C. (2003). Wadi al-Koum survey. American Journal of Archaeology, 107, 453–455.

NSM&LUP (1993). Hashemite Kingdom of Jordan, Ministry of Agriculture, Hunting Technical Services Ltd., Soil Survey and Land Resources Centre, Cranfield University, UK. National Soil Map and Land Use Project. Amman: Ministry of Agriculture.


Pegov, S.A. & Khomyakov, P.M. (1991). Simulation of Ecological Systems Alteration. Leningrad: Publ. Gidrometeoizdat (in Russian).

Purschke, D. (1990). Das nordostjordanische Hochland. Analyse und Bewertung des Naturraumes als agrares Nutzpotential. Eichstätt: Dissertation, Mathematical-Geographical Faculty, University of Eichstätt.

Reichstein, M., Kätterer, T., Andren, O., et al. (2005). Does the temperature sensitivity of descomposition vary with soil organic matter quality? Biogeosciences Discussions, 2, 738-747.

Reifenberg, A. (1947). The soils of Palestine. London: T. Murby. Schlichting, E., Blume, H.P. & Stahr, K. (1995). Bodenkundliches Praktikum (2nd Ed.).

Berlin: Blackwell. Schmidt, M., Lucke, B., Bäumler, R., et al. (2006). The Decapolis region (Northern Jordan)

as historical example of desertification? Evidence from soil development and distribution. Quaternary International, 151, 74-86.

Sellers, W.D. (1965). Physical Climatology. Chicago: University of Ontario Press. Singer, A., Danin, A. & Zöttl, H. (1992). Atmospheric Dust and Aerosols as Sources of Nutirents in a Mediterranean Ecosystem of Israel. Bonn: Bundesministerium für Forschung

und Technologie. Singer, A., Schwertmann, U. & Friedl, J. (1998). Iron oxide mineralogy of Terre Rosse and

Rendzinas in relation to their moisture and temperature regime. European Journal of Soil Science, 49, 385-395.

Tejeda, M.A., Vazquez, I.L. & Rivas, D.A. (1999). Algoritmos simples para estimar datos mensuales de irradiacion solar y horarios termohigrometricos en la Republica Mexicana. Vera Cruz: Publ. Univ. Veracruzana.

Thompson, R. & Oldfield, F. (1986). Environmental Magnetism. London: Allen & Unwin. Torrent, J., Schwertmann, U. & Schulze, D.G. (1980). Iron oxide mineralogy of of some soils

of two river terrace sequences in Spain. Geoderma, 23, 191-208. Torres, R.E. (1995). Agrometeorologia. Mexico City. UN (2007). Re-thinking Policies to Cope with Desertification. 2007-10-15, URL:

http://news.bbc.co.uk/2/hi/africa/6247802.stm. Volobuev, V.R. (1974). Introduction into the Energy of Pedogenesis. Moscow: Publ. Nauka,

(in Russian). WRB (2006). World reference base for soil resources 2006. World Soil Resources Reports

103. Rome: FAO.

THE IMPACT OF DROUGHT IN THE LIGHT OF CHANGING SOIL PROPERTIES

Documents

Transcript of THE IMPACT OF DROUGHT IN THE LIGHT OF CHANGING SOIL PROPERTIES