An appraisal of soil organic C content in Mediterranean agricultural soils

An appraisal of soil organic C content in Mediterraneanagricultural soils

J . Romanya1 & P. Rovira

1Departament de Productes Naturals, Biologia Vegetal i Edafologia, Universitat de Barcelona, Av. Joan XXIII s ⁄ n, BarcelonaE-08028, Spain, and 2Centre Tecnologic Forestal de Catalunya (CTFC), Ctra. de St. Llorenc de Morunys, Km 2 (direccio Port del

Comte), Solsona E-25280, Spain

Abstract

Low soil organic carbon (SOC) levels in dry areas can affect soil functions and may thus indicate soil

degradation. This study assesses the significance of SOC content in Mediterranean arable soils based on

the analysis of a broad data set of 2613 soils sampled from Mediterranean grasslands and agricultural

land. The distribution in values of SOC, pH, clay and carbonates was analysed according to different

climatic areas (semi-arid, Mediterranean temperate, Mediterranean continental and Atlantic) and with

respect to six different land uses (grassland, cereal crops, olives and nuts, vineyards, fruit trees and

vegetable gardens). The general trend was for low SOC in arable land and decreased with aridity. In

wet areas (Atlantic and Mediterranean continental), acidic soils had a higher SOC content than did

calcareous soils, whereas in the Mediterranean temperate area SOC had little relationship to soil pH. In

low SOC arable soils, the SOC content was related to clay content. In calcareous arable soils of the

Mediterranean temperate zone, SOC content was more closely related to carbonates than to clay. In

contrast to the Atlantic area, Mediterranean grassland soils had much lower amounts of SOC than

forest soils. Mediterranean calcareous and temperate acidic soils under grassland had SOC-to-clay

ratios similar to or only slightly greater than that under a crop regime. In contrast, Mediterranean

continental acidic soils under grassland had a much higher SOC-to-clay ratio than arable soils. This

suggests a low resilience of the Mediterranean temperate and calcareous arable soils in terms of SOC

recovery after the secession of ploughing, which may be a result of intensive use of these soils over

many centuries. Consequently, we hypothesize that the Mediterranean calcareous soils have undergone

significant changes that are not readily reversed after ploughing ceases. Such changes may be related to

alterations in soil aggregation and porosity which, in turn, are associated with soil carbonate dynamics.

Decarbonation processes (the depletion of active carbonates) may therefore be relevant to the

reclamation of highly calcareous arable soils through fostering soil re-aggregation. The article

concludes by discussing the suitability of zero tillage, manuring or the introduction of woody species to

increase SOC in calcareous arable soils that are highly depleted of organic matter.

Keywords: Degradation threshold, carbonates, organic C stability, soil resilience, soil structure, zero

tillage

Introduction

The Mediterranean climate can be considered as a

transitional one between temperate and dry subtropical

climates. It is characterized by summer drought, variable

rainfall, and mild or moderately cold winters. Mediterranean

climate is present on the five continents, mainly in the west

and at latitudes between 35� and 42� in areas under the

periodic influence of subtropical anticyclones. The areas of

the world that experience Mediterranean climate are normally

confined to narrow strips of land, thereby highlighting the

transitional nature of this climate. This climatic type reaches

its maximum extent in the western Mediterranean, and the

Iberian Peninsula is probably one of the largest areas in the

world to indicate that it can be taken as representative of this

climate. In this area it grades from a temperate Atlantic to a

semi-arid one (Allue-Andrade, 1990).

Human activity has a long history in the Mediterranean

basin with land cultivation having been continuouslyCorrespondence: J. Romanya. E-mail: jromanya@ub.edu

Received July 2010; accepted after revision March 2011

Soil Use and Management, September 2011, 27, 321–332 doi: 10.1111/j.1475-2743.2011.00346.x

ª 2011 The Authors. Journal compilation ª 2011 British Society of Soil Science 321

SoilUseandManagement

practised for more than 5000 years (Yaalon, 1997). In recent

decades, the abandonment of low-intensity agricultural land

around the northern rim of the Mediterranean sea has

contributed to an increase in forest cover (Mazzoleni et al.,

2004). At the same time, the remaining agriculture has been

intensified through the general use of machinery, mineral

fertilizers and other chemicals.

Agricultural soils typically have lower soil organic carbon

(SOC) contents than natural soils. This is especially so in

cultivated soils as ploughing may increase the rate of

decomposition of soil C. In the Mediterranean area, the soil

organic matter content in surface soils is generally low (Jones

et al., 2005). Soil organic matter affects chemical, biological

and physical processes, and therefore plays an essential role in

soil functions. For this reason, several authors have proposed

soil organic matter as an appropriate indicator of soil quality

(Karlen et al., 1997; Lal, 1998; Stenberg, 1998; Sands &

Podmore, 2000; Arshad & Martin, 2002; Nortcliff, 2002). The

benefits associated with soil organic matter are because of its

capacity to retain nutrients. Moreover, soil organic matter

may favour soil aeration and aggregation (Waters & Oades,

1991; Beare et al., 1994), thus reducing soil compaction and

favouring water infiltration, water retention and plant water

use (Quejereta et al., 2000; Barzegar et al., 2002).

As a low SOC content may have negative impacts on soil

physical properties and on nutrient cycling, efforts have been

made to define soil degradation thresholds based on SOC

(Loveland & Webb, 2003). Based on this it is estimated that

16% of cultivated land in Europe is vulnerable to

desertification (Holland, 2004), although this proportion may

be even higher in areas with harsh climates such as in the

Mediterranean, which experiences frequent summer droughts.

Studies on changes in soil organic matter often detect these

changes in light or coarse fractions; the more stable fractions,

often associated with clay, remain unchanged (Steffens et al.,

2009). The SOC associated with the light and coarse fractions

is positively correlated with soil respiration (Janzen et al.,

1992; Rovira et al., 2010) whereas that associated with fine

particles is not.

Soil carbonates have traditionally been regarded as

stabilizers of SOC (Duchafour, 1982) and there is evidence

for their role in preserving highly humified organic matter

(Olk et al., 1995). However, there is a lack of data on the

protective capacity of carbonates, but soil organic matter

stabilization may be enhanced in clay-rich and perhaps also

in carbonate-rich soils.

Through using an extensive database on Spanish grassland

and agricultural soils (Lopez Arias & Grau Corbı, 2005a,b)

the present study aims to determine the variability in the

SOC of those agricultural soils of Spain that are associated

with cropping systems and to compare them to grassland and

natural soils in each climatic zone. Soils in Spain range from

temperate, Atlantic to semi-arid soils to thus include a wide

range of Mediterranean soils. This study evaluates SOC

variability in terms of both climatic conditions and soil use

(cropping system) to thus provide greater insight into the

significance of the low organic matter levels of Mediterranean

arable soils.

Material and methods

Database

The variability in SOC across different climates and cropping

systems was analysed by means of the database of surface (0–

25 cm) arable and grassland soils of Peninsular Spain (Lopez

Arias & Grau Corbı, 2005a,b). The database contains data

from 2613 plots evenly distributed across all the grassland

and cropped areas of Peninsular Spain. This is based on a

regular 8 · 8 km grid with sampling plots at 50 · 50 m. The

centre of each plot coincides with a pan-European network

node, which also coincides with the Level I node of the ICP

forest database (Montoya & Lopez Arias, 1997). Around the

centre of each sampling plot, three sampling areas were

defined from which six to seven soil samples (0–25 cm) were

collected and bulked to a single soil sample of 19–21

subsamples. All soils were sampled between 2001 and 2003.

The data consist of 24 groups of cropping systems and

grassland, with only a few forest plantations, non-productive

land and scrubland. Because these cases were not sufficiently

replicated, their data were not included in the statistical

analysis. To obtain a representative number of replicates and

to aid data analysis, the remaining land uses were merged

into six general cropping systems, namely grasslands, cereal

crops (occasionally legumes), olives and nuts (hereinafter

referred to as olives), vineyards, vegetable gardens and fruit

trees. In the Mediterranean areas, vegetables and fruit trees

are generally irrigated. The cereal crops, olives and vineyards

are only irrigated in a few cases. Our approach enabled us to

make general assessments regarding cropping systems, but we

were not been able to assess the effect of management

practices within each cropping system.

Climate

The phytoclimatic map of Spain by Allue-Andrade (1990)

was used to assign a climatic type to 2613 plots

corresponding to the six selected cropping systems that were

analysed. The original 20 climatic classes of Allue-Andrade

were grouped into six broad climatic types: (i) semi-arid,

which corresponds to potential thorn scrubby vegetation of

Rhamnus lycioides and Quercus coccifera; (ii) Mediterranean

temperate, corresponding to the potential area of macchia

shrublands with Q. coccifera, Olea europaea ssp. sylvestris

and Pistacia lentiscus, as well as forests with Q. ilex ssp.

rotundifolia vegetation; (iii) Mediterranean continental,

corresponding to the potential area of Mediterranean

marcesent ⁄ semi-deciduous and deciduous oaks (Q. faginea,

322 J. Romanya & P. Rovira

ª 2011 The Authors. Journal compilation ª 2011 British Society of Soil Science, Soil Use and Management, 27, 321–332

Q. pyrenaica, Q. humilis) and to the area of Q. ilex ssp. ilex

forests; (iv) Atlantic, which corresponds to the potential area

of humid deciduous forests of Q. robur, Q. petraea and Fagus

sylvatica; (v) mountain, which corresponds to subalpine

forests of Pinus mugo ssp. uncinata and, sometimes

P. sylvestris, as found in the mountainous areas such as the

Pyrenees, the Picos de Europa or Guadarrama; and (vi)

mountain summits, which coincides with the potential area of

alpine grassland and shrubs above the tree line. Figure 1

shows the distribution of these climatic types in Peninsular

Spain and the Balearic Islands. Of the six climatic types, data

are only available for the first four (semi-arid, Mediterranean

temperate, Mediterranean continental and Atlantic), and

henceforth the discussion is limited to these.

Soil analyses

All soils were sieved £2 mm prior to analyses. Soil pH was

measured in a soil suspension at a 10 ⁄ 25 ratio. Lower soil

organic carbon was measured by chromic oxidation in soils

with pH>6.5 and by total C determination via dry

combustion in soils with pH<6.5. The soil texture was

determined by sieving and sedimentation in accordance with

ISO ⁄DIS 1277. The soil carbonates were measured using the

Bernard calcimeter (Association Francaise de Normalization;

AFNOR · 31-105).

The soil bulk density was calculated from the following

equation, obtained from a series of 25 Spanish agricultural

soils:

Bd ðg/cm3Þ ¼ 10ð0:1856þ 0:00106 Clayð%Þ�0:08546 OCð%ÞÞ R2 ¼ 0:385:

Statistical analysis

Differences between climates, land use and soil pH were

tested by one-way, two-way and factorial ANOVA. Because

of the unbalanced nature of the data these differences were

only tested for grassland and cereal crops excluding the

semi-arid zone. All variables expressed as proportions were

normalized by an arcsine of square root transformation

(Neter et al., 1990). Spearman correlations were performed

with soil clay, carbonates and SOC. To separate the effects

of climate and land use from the effects of soil clay

content on SOC accumulation, we calculated for all soils

the ratio between SOC and clay content (SOC-to-clay

ratio). Linear or exponential regression fits were performed

between soil pH and SOC-to-clay ratios. Differences

between the best-fit curves for the grasslands and arable

land in each climatic area were tested using analysis of

covariance (ANCOVA).

Results

Effects of cropping systems and climate

Surface SOC (Figure 2 and Table 1) was lowest in the semi-

arid and temperate Mediterranean soils. Arable soils, mainly

those associated with extensive crops (cereals, olives and

vineyards), had very low values, often below the 1%

degradation threshold (Loveland & Webb, 2003). Vineyards

had the lowest levels in both the Mediterranean temperate

and Mediterranean continental soils. Lower SOC was

higher in the continental Mediterranean rather than in

Mediterranean temperate soils for all studied land use classes,

AtlanticMountain summitsMountainMed. continentalMed. temperateArid

Climatic types

Figure 1 Distribution of climatic types in

Peninsular Spain.

SOC in Mediterranean agricultural soils 323

although the greatest differences were found in non-arable

soils (grasslands). The Atlantic soils had the highest values.

The smallest increase was observed for cereals in alkaline

soils, which showed an increase of ca. 80% with respect to

the continental Mediterranean soils.

In the temperate and continental Mediterranean areas,

soils under crops that are usually irrigated (fruit trees and

gardens) had the highest SOC levels, except for the alkaline

garden soils.

Effects of pH

Although the proportion of alkaline soils was much lower

under Atlantic climatic conditions (13.5% vs. 80.5% for

Mediterranean temperate

Semi-arid

2.5 pH > 6.5 pH < 6.5

Mediterranean continental

Atlantic

Grassl. Cereals Olives Vineyards Fruit trees Gardens0.00.51.01.52.02.53.03.54.04.55.05.5

122255 35

Figure 2 Surface soil organic C in grassland

and agricultural soils of Peninsular Spain.

Dashed line indicates the 1% threshold for

temperate agricultural soils (Loveland &

Webb, 2003). Figures on the bars refer to

the number of observations in each case.

Mediterranean conditions), the ranges in pH in all studied

soils were similar for all climates, except for the semi-arid

soils, all of which were alkaline (Figures 2 and 4). The effect

of pH on SOC levels is related to climate (Table 1). In the

Mediterranean temperate grassland soils, higher SOC

contents are associated with high pH, whereas the continental

Mediterranean and Atlantic soils showed the opposite trend

(Figure 2). In arable soils, the effect of pH was more complex

because of interaction with both climate and land use. In the

temperate Mediterranean areas, cereal fields had the highest

SOC content when soil was alkaline, whereas under olive

trees SOC showed the opposite trend and there were no

differences for vineyards. In the continental Mediterranean

area, SOC under cereals did not vary with soil pH.

Differences between alkaline and acid garden soils were not

significant. In the Atlantic area, acidic soils had the highest

levels of organic C in all studied land uses.

Distribution of clay and soil carbonates

In all cases, alkaline soils had consistently higher levels of

clay than acidic soils (Tables 1 and 2). Differences in the clay

content across land uses in the temperate and continental

Mediterranean areas were only found in alkaline soils,

whereas in the Atlantic area there were large differences in

clay content between grassland and cereals for both acidic

and alkaline soils.

As expected, the carbonate content was greatest in semi-

arid soils and lowest in Atlantic soils (Tables 1 and 2). The

carbonate content in grassland soils steadily decreased as the

climate became wetter. In all cases, the soil carbonates had

higher values in agricultural soils than in the grassland ones,

and the differences between the grassland and arable land

increased as the climate became wetter. The Atlantic

agricultural soils had the lowest carbonate content for arable

soils, and the temperate and continental Mediterranean

agricultural areas had rather similar carbonate content with

the highest levels in the semi-arid soils. The soils under

cereals from the continental Mediterranean area had

significantly higher amounts of carbonate than those in the

temperate Mediterranean area. The clay and carbonate

contents were related to SOC content in some cases (Table 3).

Grassland soils only showed a strong relationship with clay

Table 1 Clay and carbonate content

pH Grassland Cereals Olives Vineyards Fruit trees Gardens

Semi-arid

Clay (%) <6.5 – – – – – –

Clay (%) >6.5 – 28.79 ± 3.9510 21.45 ± 1.7825 – 23.34 ± 1.7322 23.34 ± 2.7115

Carbonates (%) >6.5 – 38.50 ± 4.1110 42.28 ± 1.7825 – 36.68 ± 3.2122 38.56 ± 3.9916

Clay (%) <6.5 14.21a ± 0.7591 17.03b ± 0.93103 17.41 ± 1.5832 14.55 ± 1.9413 – –

Clay (%) >6.5 20.51a ± 1.4850 24.72b ± 0.51521 23.38 ± 0.72235 17.86 ± 0.63141 23.21 ± 1.1862 19.70 ± 1.9213

Carbonates (%) >6.5 18.88a ± 3.0650 23.77b ± 0.79534 30.91 ± 1.36243 29.41 ± 1.57143 26.15 ± 2.1860 20.62 ± 4.5113

Clay (%) <6.5 13.30a ± 0.8452 13.54a ± 0.7466 – – – 13.86 ± 2.769

Clay (%) >6.5 19.62a ± 1.5028 23.90b ± 0.41571 17.63 ± 0.9441 19.42 ± 0.9844 – 23.51 ± 2.7312

Carbonates (%) >6.5 12.07a ± 2.4629 25.18b ± 0.71562 33.97 ± 3.3250 25.31 ± 2.3243 – 15.42 ± 4.6012

Atlantic

Clay (%) <6.5 19.08a ± 0.83167 12.59b ± 0.8727 – – – 11.51 ± 0.9115

Clay (%) >6.5 25.80a ± 2.0129 16.65b ± 2.9612 – – – –

Carbonates (%) >6.5 8.31a ± 2.3329 19.33b ± 6.2712 – – – –

Mean, standard error and number of cases. Superscript letters indicate significant differences between grasslands and grassy crops. Vegetable

gardens are not included in the statistical analyses because of the low number of observations. Experimental conditions with <20 observations

have not been computed in the statistical analyses.

Table 2 ANOVA P-values showing significant differences in SOC,

SOC-to-clay ratio and clay content between climates, land use and

soil pH

Surface

SOC (%)

SOC-to-clay

ratio (g C ⁄ kgof clay)

Climate <0.0001 <0.0001 ns 0.0410

Land use <0.0001 <0.0001 ns <0.0001

pH <0.0001 <0.0001 <0.0001 –

Cl · LU <0.0001 0.0070 <0.0001 ns

Cl · pH <0.0001 <0.0001 ns

LU · pH ns ns ns

Cl · LU · pH 0.0021 ns ns

ANOVA P-values showing the significant differences in CaCO3

content between climates and land uses in the alkaline soils. ns, not

significant; 0.000 refers to P-values <0.0005. Cl, climate; LU, land

use; SOC, soil organic carbon.

for acidic soils in the temperate Mediterranean area. In

contrast, in Mediterranean arable soils with low levels of soil

organic matter, the clay content was strongly related to SOC

and mainly in alkaline soils. In the Mediterranean (temperate

and continental), the soil carbonate content had significant

correlation with SOC. However, soils of the temperate

Mediterranean area were the only ones to show stronger

relationships between carbonates and SOC than between clay

and SOC.

SOC-to-clay ratio

The SOC-to-clay ratio (calculated as g of SOC per kg of clay)

decreased with water deficit (Table 1, Figure 3). Under the

continental Mediterranean and Atlantic conditions, the SOC-

to-clay ratio was much higher in acidic than in alkaline soils,

whereas in the temperate Mediterranean soils, differences

between acidic and alkaline soils were either not found or

were rather small.

Effects of pH and land use

In the grassland soils, the SOC-to-clay ratio decreased with

pH in all climates, although in the temperate Mediterranean

area this decrease was small (Figure 4). The SOC-to-clay

ratio of soils under cereals in the Mediterranean temperate

area showed no effect of pH, while the continental

Mediterranean and Atlantic soils both had decreases in the

SOC-to-clay ratio with soil pH. The differences between the

grassland and cereal soils in the temperate Mediterranean

and continental areas demonstrated a clear decreasing trend

in SOC per unit of clay with increasing soil pH, and this was

more marked in the continental Mediterranean area. In the

temperate Mediterranean area, the differences between

grassland and cereal soils could be as high as 38 g SOC ⁄kg of

clay in acid soils (Figure 4), but decreased to less than

10 g SOC ⁄kg of clay in alkaline soils (pH = 8.5). In the

continental Mediterranean area, these differences ranged

from 170 g SOC ⁄kg of clay in acid soils to less than

10 g SOC ⁄kg of clay in soils of pH = 8.5. Finally, soils in

the Atlantic area showed significant differences between

grassland and cereal soils (Figure 4). These differences were

only significant in alkaline soils and amounted to about

60 g SOC ⁄kg of clay.

Effects of irrigation

The effects of irrigation were only studied for the temperate

Mediterranean area using the data that indicated whether

irrigation was applied or not. By including only alkaline soils,

there were increases in soil C in irrigated soils under cereals

compared with non-irrigated ones with a 34% increase in

SOC, equivalent to an increase of 0.8 kg C ⁄m2 for the top

25 cm of mineral soil (Table 4). Irrigation of olive groves did

not significantly change SOC content. In the semi-arid area,

irrigated soils growing vegetables and fruit trees soils did not

show any significant increase in SOC relative to non-irrigated

soils (Figure 2). However, in the temperate Mediterranean

area, the soils under fruit trees increased by 20% in SOC

compared with the soils under cereals. In the continental

Mediterranean area this increase was found for the garden

soils where there was a 33% increase for alkaline soils and a

95% increase for acidic ones.

Discussion

Distribution of SOC

The distribution of SOC in agricultural soils across different

climate zones indicates an increase as the Mediterranean

influence diminishes. Previous studies of Spanish soils under

Table 3 Spearman correlations between soil clay, carbonates and

soil organic carbon content

Organic C

Grassland Cereals Olives Vineyards

Clay <6.5 0.324 0.207 ns

0.002 0.036 –

91 103 32

Clay >6.5 ns 0.162 0.269 0.232

<0.001 <0.001 0.006

50 520 235 141

Carbonates >6.5 ns 0.431 0.269 0.256

<0.001 <0.001 0.002

43 530 243 143

Clay <6.5 ns ns

– –

Clay >6.5 ns 0.364 0.419 0.563

<0.001 0.006 <0.001

28 517 41 44

Carbonates >6.5 ns 0.271 0.301 0.302

<0.001 0.045 0.044

25 561 41 45

Atlantic

Clay <6.5 0.185 ns

0.017 – –

167 27

Clay >6.5 ns

29 – – –

Carbonates >6.5 ns

28 – – –

Significance (P-values) and the number of cases analysed are

indicated. ns, not significant. Experimental conditions with <20

observations have not been computed.

different land use have found a strong positive correlation

between SOC and mean annual rainfall (Hontoria et al.,

1999; Rodriguez-Murillo, 2001). Irrigation of cereals in

alkaline soils of the Mediterranean temperate area increased

the SOC levels and the SOC-to-clay ratio to similar levels

of those under grassland in the same climatic zone. The

differences in SOC between grasslands and arable land were

greater in wet climates than in Mediterranean temperate

conditions (see Table 1 and Figure 2). The results also show

that the acidic soils of Atlantic and Mediterranean

continental grasslands have a much higher SOC content and

SOC-to-clay ratio than do alkaline soils from the same area.

Atlantic

Grassl. Cereals Olives Vineyards Fruit trees Gardens0

Semi-arid

250 pH > 6.5 pH < 6.5

10 25 22 15

520 103 23532

Figure 3 Surface soil lower soil organic

carbon-to-clay ratio in grassland and

agricultural soils of Peninsular Spain.

Figures on the bars refer to the number of

observations in each case.

By contrast, acidic soils of the temperate Mediterranean

grasslands have lower levels of SOC compared with alkaline

soils. Decreased SOC values in acidic soils were also found in

the forest soils of the temperate Mediterranean area

(Table 5). In this climatic area, the alkaline forest soils have

unexpectedly high values of SOC.

4 5 6 7 8 90

4 5 6 7 8 9

4 5 6 7 8 9 4 5 6 7 8 9

Soil pH

Grasslands Cereals

Atlantic

Mediterranean temperate Mediterranean temperate

1200Atlantic

R2 = 0.034P = 0.023n = 135

R2 = 0.136P < 0.0001n = 78

R2 = 0.181P < 0.0001n = 196

R2 = 0.428P < 0.0001n = 38

R2 = 0.103P < 0.0001n = 577

n = 614

Mediterranean continentalMediterranean continental

Figure 4 Relationship between soil pH and surface lower soil organic carbon-to-clay ratio in grasslands and in cereal soils. n refers to the

number of observations. Analysis of covariance (ANCOVA) significance is indicated.

Table 4 Effects of irrigation on soil organic

C in Mediterranean alkaline soils (pH>

6.5). ns, not significant. P-values of ANOVA

are indicated when P < 0.05

Land use Rain-fed Irrigated ANOVA

Cereals

Organic C% 0.99 ± 0.03 n=491 1.33 ± 0.09 n=65 <0.001

Organic Cg kg of clay 51.62 ± 3.09 n=456 66.22 ± 6.38 n=64 ns

Organic Ckg ⁄m2 3.17 ± 0.06 n=457 3.97 ± 0.16 n=64 <0.001

Olives and nuts

Organic C% 0.93 ± 0.04 n=210 0.91 ± 0.08 n=45 ns

Organic Cg kg of clay 50.56 ± 4.25 n=199 53.14 ± 10.28 n=36 ns

Organic Ckg ⁄m2 3.00 ± 0.09 n=199 2.95 ± 0.21 n=36 ns

Distribution of clay and carbonates

The data from this study show that the clay and carbonates

do not vary independently in the Mediterranean area. The

soils with high clay content often show both large carbonate

content and high pH. High clay content occurred in the

Mediterranean and continental areas and coincides with

alkaline soils. In the Iberian Peninsula, the soil carbonate

contents are greatest in drier climates and in arable soils. In

the Mediterranean temperate and continental areas, irrigated

garden soils sustaining high productivities have the lowest

carbonate content. The accumulation of soil carbonates

depends on dissolution and precipitation processes. Research

based on a data set of forest soils from the eastern part of the

Iberian Peninsula established that decarbonation processes

(the depletion of active carbonates) are mainly driven by the

amount of infiltration, and this is influenced by climate, slope

position and topography (Rubio & Escudero, 2005).

Analysis of the database in the present study shows that

carbonate sensitivity to climate is greater in grassland soils

than in arable soils. Agricultural practices in the

Mediterranean area reduce both SOC and total porosity

(mainly macroporosity) (Sanchez-Maranon et al., 2002). Such

changes will affect water infiltration and storage capacity, as

well as producing soil compaction and increased erodibility.

Increased erodibility and reduced water infiltration may

explain the increases in soil carbonate in arable land relative

to grassland. As grassland soils are more sensitive to

decarbonation processes than arable ones, the differences in

carbonate content between arable and grassland soils are

greater in climates with the most positive water balances

(Mediterranean continental and Atlantic) and, consequently,

the greatest decarbonation capacity.

Stabilization of SOC

It is widely accepted that clay particles can physically protect

SOC against decomposition by stabilizing the organic

components into micro- and meso-aggregates (Golchin et al.,

1998). Chemical processes such as precipitation by Ca2+ or

Fe3+ may also contribute to the stabilization of organic C in

soils. Ca2+-mediated processes may be especially relevant in

soils with carbonates. Calcium carbonate can also favour soil

aggregation processes and thus increase the protection of

organic C contained in aggregates. It has been reported that

soil carbonates protect fresh organic matter from further

humification (Duchafour, 1982), and that this stabilization

process may also affect old, biochemically evolved organic

fractions. Therefore, the humic acids precipitated by either

Ca or carbonates, and which can be extracted by NaOH only

after treating the soil with HCl, have been shown to be older

and more humified than other humic acids, extractable by

NaOH without any HCl treatment (Olk et al., 1995, 2000).

This suggests a hypothetical, increased protective capacity

associated with the presence of CaCO3 in alkaline soils that

may also increase the protective capacity of soil fine particles.

In mollic horizons calcium content is often associated with

high organic carbon accumulation (Soil Survey Staff, 1990).

This increased protective capacity could account for the

large SOC increases found in calcareous forest soils of the

Mediterranean temperate area (Table 5). On the other hand,

these increases are minimal in organic matter from the

depleted arable soils in the same climatic zone (Figure 2) and

can mainly be attributed to changes in soil texture (Figure 3).

However, SOC levels in arable soils, but not in grasslands,

are strongly and positively related to carbonate content,

suggesting that in the Mediterranean temperate area, soil

carbonates may favour C accumulation in organic matter of

depleted soils (arable soils) but not in the grassland soils

showing moderate increases in SOC.

In wetter climates such as the continental or Atlantic

Mediterranean, where more intense decarbonation processes

occur (Rubio & Escudero, 2005), the protective capacity of

carbonates is less intense or absent. Under these conditions

therefore soil organic C per unit of clay is higher in acidic

than in alkaline soils (Figure 3). Thus, the protective capacity

of carbonates would be especially relevant in dry areas such

as the Mediterranean temperate area, whereas it is negligible

in soils that are slightly enriched with SOC such as in

grassland soils.

Table 5 Forest soil organic C for the first 25 cm of mineral soil (this does not include the organic layers); figures have been recalculated from

Rovira et al. (2007)

Mediterranean Continental Atlantic

pH>6.5 pH<6.5 pH>6.5 pH<6.5 pH>6.5 pH<6.5

SOC (%) 3.08 ± 0.16

n = 238

2.03 ± 0.18

n = 80

2.66 ± 0.16

n = 113

3.23 ± 0.17

n = 166

2.97 ± 0.41

n = 20

3.45 ± 0.17

n = 143

Forest – Grassland

(SOC %)

1.91 1.00 0.90 0.96 )0.86 )2.17

SOC, soil organic carbon.

Soil organic C and land degradation thresholds

In both temperate and continental Mediterranean areas, SOC

in most grassland and arable soils is much lower than 2%

(Figure 2). For the temperate regions, it is widely considered

that soil quality declines and soil erosion is favoured when

SOC is less than 2% (Loveland & Webb, 2003). These

authors also suggest that an organic carbon content of ca.

1% may not sustain yields because of a reduction in

mineralizable N, unless such soils are improved with

adequate amounts of mineral fertilizers. Even so, such

degraded soils may not fully recover their former productive

potential as a result of physical degradation. According to

these criteria, most Mediterranean agricultural soils can be

considered as degraded or on the verge of degradation. But

this conclusion is based on experimentation under temperate

conditions that may not be directly relevant to the wider

Mediterranean area where soil pH, temperature and the

presence of carbonates are different. The very low

concentrations of soil organic matter may be attributed to the

favourable conditions provided by Mediterranean climates as

regards SOC decomposition.

In the temperate Mediterranean area, the SOC-to-clay

ratio in calcareous grasslands is nearly as low as in the arable

soils (Figure 3). Figure 4 shows that in the temperate and

continental calcareous soils the difference between the

grassland and cereal calcareous soils may be almost zero,

while acidic soils show large differences for continental soils

and moderate differences for the temperate Mediterranean

soils. From this, we hypothesize that in Mediterranean and

continental calcareous soils the conversion of arable land to

grassland will result in none or only very small increases in

the SOC-to-clay ratio. In contrast this ratio will rise in acidic

soils, although the expected increases in the Mediterranean

temperate area will be rather low (Figure 4). Based on the

results shown in Figure 4, and assuming a mean clay content

of 22%, the expected increases in acidic soils can be estimated

at ca. 1.6 kg C ⁄m2 in the Mediterranean temperate area

and 8.9 kg C ⁄m2 in the Mediterranean continental area

corresponding to SOC increases of 59% and 275%,

respectively. In calcareous soils, the expected increases will be

as low as 0.1 kg C ⁄m2, a negligible SOC increase of about

2%. A medium-term study (15 yr) of zero tillage practices in

the calcareous Mediterranean temperate area revealed small

increases in the SOC that represented only a 14% increase

over conventional tillage, and which reached SOC levels of

only 0.8% for the first 30 cm (Hernanz et al., 2009). Zero

tillage in the Mediterranean temperate acidic soils has

resulted in greater long-term increases in SOC of up to 24%

(Hernanz et al., 2002) while in the Atlantic soils the increase

was much higher (ca. 38% after 10 yr) (Dıaz-Ravina et al.,

2005). It therefore appears likely that the cessation of

ploughing in the Mediterranean temperate area will not be

followed by large increases in SOC. Furthermore, these

increases are likely to be even lower in calcareous soils in

both the temperate and continental Mediterranean areas.

This is in contrast to the high SOC content in Mediterranean

temperate calcareous forest soils (Table 5; Rovira et al., 2007)

and may indicate the degradation of arable and grassland

soils in this area. The data set shows that the largest

carbonate concentration occurs in most arable calcareous

soils, especially in vineyard and olive grove soils, and that the

carbonate content of these soils is not sensitive to the climatic

differences between the temperate and continental

Mediterranean areas.

Soil resilience should be considered in the evaluation of

soil quality in managed ecosystems as it relates to the ability

of soil functions to recover as well as its physical form

(Schjønning et al., 2004). The small SOC variation between

arable and grassland alkaline soils that occurs in the

Mediterranean area suggests a slow recovery capacity

(resilience) for these soils, along with high stability. Acidic

soils in the same climatic regime have a similar or even

lower SOC-to-clay ratio (Figure 2) but a greater capacity to

recover. This suggests that alkaline soils have undergone

significant changes and that the degradation threshold will

not be the same for acidic and alkaline soils. These

degradation processes may relate to changes in soil

aggregation and porosity. It has been shown in the Atlantic

soils that under grassland, soil aggregation increases quickly,

while soil porosity increases more slowly (Haynes et al.,

1991). In alkaline soils, the increased levels of soil

carbonates in soil depleted of organic carbon may act as

cement between soil particles and aggregates, thereby

hindering the build-up of new aggregates when these soils

are converted into grasslands. Decarbonation processes may

therefore be relevant to the reclamation of these highly

calcareous soils, as they can favour soil re-aggregation. It

has been shown that decarbonation processes can be

promoted by adding organic manures to dryland arable soils

(Romanya & Rovira, 2007).

In the Mediterranean where there are acidic soils,

afforestation of former vineyards and cereal fields with Pinus

radiata has after a few decades resulted in soils with similar

or even higher levels of soil organic matter than in natural

forests (Romanya et al., 2000). In contrast in the

Mediterranean alkaline area, the abandonment of arable land

has, in some cases, led to low-productivity scrubby

ecosystems with low organic matter recovery and no long-

term decarbonation trend (Garcıa et al., 2007; Martı-Roura

et al., 2011). In these old fields, with no trees and low shrub

density, organic C levels are much lower than in mature

forest soils of the same area. Unlike grasslands of other

climates (Guo & Gifford, 2002), Mediterranean grasslands

have lower levels of SOC than forests (Table 5). As the

Mediterranean has been continuously occupied for more than

five millennia (Brueckner, 1986), soil degradation may have

commenced a long time ago and may also have affected

grassland soils that were previously cultivated or overgrazed.

Soils in these degraded grasslands have a potential to regain

their quality and to stabilize organic C if they are managed to

improve their structure. The introduction of woody plants

with robust rooting systems, or the addition of stabilized

organic matter (compost) would achieve this. However, the

establishment of leys, the cessation of ploughing, and the

establishment of herbaceous vegetation may not be sufficient

to produce large increases in SOC in degraded arable

calcareous soils.

Conclusions

The low SOC content of agricultural and grassland soils in

Mediterranean temperate and continental areas may indicate

soil degradation as a result of intensive use of these soils over

centuries. Soils that are left bare during most of the year,

such as with vineyards and rain-fed tree crops (mainly olives

and almond trees), have the lowest SOC concentrations. Soils

of drier areas (Mediterranean temperate) have a low capacity

to re-establish their original SOC-to-clay ratio after the

cessation of ploughing. Mediterranean calcareous arable soils

have increased levels of calcium carbonate compared with

grassland soils, along with virtually no capacity to recover

their original SOC-to-clay ratio after ploughing. This suggests

that soils enriched with calcium carbonate may have

undergone significant changes that will have negative effects

on soil functioning. In these soils the degradation threshold,

expressed by SOC per unit of clay, will be higher than in

carbonate-free acid soils. The management of soil organic

matter seems highly relevant for maintaining or improving

soil function in Mediterranean agricultural areas. In SOC-

depleted soils of the driest areas, and especially in

Mediterranean calcareous soils, the cessation of ploughing is

likely to trigger a small recovery in soil organic matter, and

in the case of calcareous soils the carbonate content will

remain virtually unchanged. In these circumstances, soil

management should aim to reinstate soil structure by

reducing the soil carbonate content. In Mediterranean

calcareous soils, the use of exogenous organic matter in

arable lands or the use of woody plants in set-aside land

should be tested to examine to what extent soil C stocks can

recover their soil functions.

Acknowledgements

We thank one of the anonymous referees for help in editing

the manuscript. This research was supported by the projects

Balangeis (SUM2006-0030-CO2-02), Agroeco II (CGL2009-

13497-CO2-02) and Graccie (CSD2007-00067) of the Spanish

Ministry of Science and Technology. This article was

presented at the COST 639 Meeting held in Florence in

March 2009 on soil organic matter in Mediterranean

ecosystems.

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An appraisal of soil organic C content in Mediterranean agricultural soils

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