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Soil profile improvement as a by-product of gully stabilization measures

Pavel Dlapa a,⁎, Katarína Chrenková a, Jorge Mataix-Solera b, Ivan Šimkovic a

a Department of Soil Science, Faculty of Natural Science, Comenius University, Mlynská dolina B-2, 842 15 Bratislava, Slovak Republicb GEA, Grupo de Edafología Ambiental, Department of Agrochemistry and Environment, University Miguel Hernández, Avda de la Universidad s/n, E-03202 Elche, Alicante, Spain

a b s t r a c ta r t i c l e i n f o

Article history:Received 7 July 2011Received in revised form 12 December 2011Accepted 13 December 2011Available online xxxx

Keywords:Gully erosionReforestationAggregate stabilitySoil organic matterSoil pHCarbonates

In this paper, we characterize the long-term effect of reforestation on the stabilization of soils affected bystrong gully erosion in Prietrž village in South-Western Slovakia. Permanent gullies there were reforested acentury ago with the black locust (Robinia pseudoacacia). Reforestation measures induced soil profile devel-opment on the gully slopes with distinct humus horizons. Soil-forming processes caused organic matteraccumulation, carbonate leaching and a decrease in pH in the surface horizons. One century after reforesta-tion, the key properties of the gully soils closely resemble those of the surrounding agricultural soils. Soilaggregate stability was used as a useful indicator which reflected the impact of reforestation on soil stabiliza-tion over the last hundred years. The overall effect of the observed changes is an increase in the aggregatestability of these reforested gully soils. Soil aggregate stability measured by the rainfall simulator methodpositively correlated with the organic carbon content, and it had negative correlations with carbonate con-tent and soil pH. The values determined for these gully soils do not differ significantly from those in the sur-rounding non-eroded agricultural soils. The extreme difference in aggregate stability between the topsoilsand the parent marl material is responsible for this landscape's susceptibility to severe gully erosion.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

In many landscapes under different climatic conditions and withdifferent land uses, the presence and dynamics of various types ofgully erosion can be observed (Poesen et al., 2003). Gully erosion isdefined as the erosion process whereby runoff-water accumulatesand often recurs in narrow channels, and over short periods itremoves the soil to considerable depths from this narrow area. Foragricultural land purposes, permanent gullies are often defined aschannels which are too deep to easily ameliorate with ordinaryfarm tillage equipment, and they typically range from 0.5 to asmuch as 25–30 m in depth (Soil Science Society of America, 2001).

It is recognized that gully erosion can considerably complicatepeoples' lives in some landscapes by decreasing the area of utilizablesoils and by disrupting transport routes. Although researchers notedthe necessity to address these problems and investigate the effective-ness of various soil stabilization measures in degraded landscapes(Burri et al., 2009), there still remains a lack of knowledge of thelong-term effect of these various stabilization measures.

The study site investigated in this paper is located in Central Europe,where gully erosion initiation and development was generally causedby land-use pressure and climatic fluctuations (Stankoviansky,2003a). Land-use pressure is associated with the deforestation which

occurred when the region was settled, while the climatic factor ismainly associated with colder and wetter fluctuations during “TheLittle Ice Age” in Central Europe. Both these factors overlapped inthe region between the mid-16th and the mid-19th centuries(Stankoviansky, 2003b).

Reforestation with black locust (Robinia pseudoacacia) was per-formed to stabilize steep walls of gullies in this region between themid-19th and early 20th centuries. Its strong spreading root systemmakes black locust a valuable tree for checking soil erosion. Thus, re-forestation either stabilized permanent gullies or at least restrictedtheir growth (Stankoviansky, 2003b). Permanent reforested gulliesform a characteristic landscape pattern as shown in Fig. 1.

The vegetation cover is often undervalued when concentratedflow erosion is considered. Traditional vegetative techniques to con-trol gully development relymainly on the effects of the above groundbiomass, whereas little attention has been paid to the role of below-ground biomass (De Baets et al., 2006). Increased resistance toerosion due to the presence of vegetation has beenmainly attributedto the impact of aboveground biomass on the overland energy flow-dissipation caused by increased hydraulic resistance (Poesen et al.,2003). Few studies have investigated the impact of naturalvegetation-roots on the resistance of topsoils in the concentratedflow erosion zones (e.g. De Baets et al., 2006; Li, 1995; Sidorchukand Grigorev, 1998). Pollen (2007) has shown considerable effectof mechanical root-reinforcement on soil stability of streambanks.Root networks also have effect on the hydrologic and hydraulic pro-cesses acting on steep streambanks (Pollen-Bankhead and Simon,

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⁎ Corresponding author. Tel.: +421 260296574; fax: +421 265429064.E-mail address: dlapa@fns.uniba.sk (P. Dlapa).

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2010). Overall, the impacts of vegetation and the effects of below-ground biomass on the resistance of topsoils to concentrated flowerosion remain under-researched areas (Poesen et al., 2003).

Some authors consider that soil aggregate stability is the mostappropriate indicator in protecting slopes from erosion and shallowmass movements (Barthes and Roose, 2002; Burri et al., 2009;Canton et al., 2009). Many authors consider soil aggregation as asynthetic parameter reflecting soil health, as it depends on chemical,physical and biological factors. In fact, it has been used as a pa-rameter indicative of soil system perturbation, and potential soilerosion since 1940 (Imeson, 1984). Most studies have been carriedout on agricultural soils (e.g. Chenu et al., 2000; Degens et al., 1994;Idowu, 2003; Milne and Haynes, 2004), and far fewer on soils onsteep slopes affected by erosion (Barthes and Roose, 2002; Cantonet al., 2009; Gros et al., 2004; Pohl et al., 2009). According to Shresthaet al. (2007), the knowledge of soil aggregate stability is useful in theevaluation of soil properties with regard to land-use systems. Unfortu-nately, as soil aggregation depends on many site-specific factors suchas geology and climate, the values gained in the case studies cannotbe used as a reference for other areas (Burri et al., 2009). Moreover, dif-ferent methods are used to determine this property and the resultsobtained with these different methodologies are not comparable sinceeachmethod uses different energies and/or sources to apply the energy.These include ultrasound, water immersion and sieving andwater-dropimpacts (Mataix-Solera et al., 2011).

The objective of this study is to determine whether aggregate sta-bility increased with soil and vegetation development on the slopinggully walls. This helped us to evaluate whether the measures againstgully erosion, such as reforestation at the end of the 19th century,were effective in the stabilization of gully soils. Therefore our goalwas to determine whether changes in soil management over long pe-riods had a significant impact on aggregate stability. Finally, we alsoanalyzed properties of the agricultural and gully soils in this regionto understand why this region is so susceptible to gully erosion.

2. Material and methods

2.1. Study area and soil sampling

The study site is located in the Prietrž village in South-WesternSlovakia, where an old gully system is located next to the village

church which was built in the first half of the 18th century. The gullysystem is about 450 m long and up to 20 m deep. The mean annualprecipitation ranges from 600 to 700 mm in this region, while themean annual temperature varies between 9 and 10 °C. The hottestmonth is Julywhich hasmean temperatures of 18–19 °Cwhile Januaryis the coldest month, with mean temperatures of −2 to −3 °C.

Following past strong gully erosion, the parent soil material wasexposed on the walls of this deep permanent gully system. Before re-forestation, the steep parts of the gully walls lacked tree vegetationand the gully walls were therefore extremely labile and subject toerosion. According to Stankoviansky (2003b), the gully walls herewere reforested with the black locust (R. pseudoacacia) at the end ofthe 19th century. This was carried out to avoid expected advancingerosion processes. It should be noted that the gully soils developedgradually from marl material which was originally calcareous with ahigh pH and almost completely lacking in organic carbon.

Two adjacent sites were compared for soil properties and aggre-gate stability: (i) sloping gully walls with biological stabilization mea-sures through reforestation dating back one century, and (ii) theadjacent agricultural soils unaffected by gully erosion. Samples fromdifferent soil horizons have also been compared in order to evaluatehow soil properties have changed over time. Gully soils in the C hori-zon were considered to be the reference point for the initial soil con-ditions existing before reforestation measures.

Samples were taken of 10 profiles of agricultural soils and 10 pro-files of gully soils (Fig. 2). Processes stabilizing soils in the middle andupper parts of the steep walls of gullies were preferred for this sam-pling and investigation. This choice eliminated the effects of accumu-lation processes on topsoil development at the foot of the walls andon the gully floors. A total of 50 soil samples were collected from ge-netic horizons and the various parameters which described changesin stabilized soils on the steep walls were then measured. Here, thechanges caused by partial soil-forming processes were especiallyaddressed. These included the soil organic carbon content, the soilpH, carbonate content, soil texture and the aggregate stability.

2.2. Analytical methods

The samples were air-dried and ground to pass through a 2 mmsieve before analysis. The determinations of soil pH, the total organiccarbon content and particle size distribution were achieved by the

Fig. 1. Satellite view of the region with gullies masked with recent forests.

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standard methods (Fiala, 1999). The particle size distribution wasdetermined by the amount of soil particles in suspension after varioussettling times, and the percentage of each size fraction was estab-lished by the pipette method. Soil pH values were identified using asoil/solution (water and KCl) ratio of 1:2.5 and the organic carbon(Corg) content was determined using oxidation with K2Cr2O7-H2SO4

and titration of non-reduced dichromate. The carbonate contentwas established by volumetric determination of CO2 released afterthe dissolution of soil carbonates by diluted HCl.

The aggregate stability was measured in triplicate for each soilsample using the rainfall simulator methodology described inMataix-Solera and Doerr (2004). This method examines the propor-tion of aggregates (4–0.25 mm) which remain intact after the soilsample is subjected to artificial rainfall with a specific intensity270 J m−2 for 1 min.

2.3. Statistical analysis

The differences in various properties of the gully and agriculturalsoils were statistically analyzed using the unpaired t-test. Theseresults are presented either as statistically significant statements in-cluded in the text or in the separate table containing the probabilityvalues of the t-test results. The significance statements denote theprobability values of two-tailed t-tests which were b0.05. For agiven set of variables, the correlation diagrams and correlationcoefficients were utilized to provide information on the relationshipsbetween the variables.

3. Results

Changes which took place in the degraded gully soils resultingfrom reforestation and soil profile development are apparent in theresults. Although Fig. 3 shows that these changes are macroscopicallyvisible in the field with distinct soil horizons, the laboratory analysisresults are more objective, and they make it possible to quantifyand statistically evaluate the observed changes.

Tables 1 and 2 show the comparison based on soil pH analysis. SoilpH values decrease towards the surface, therefore the gradual acidifi-cation of the surface horizons in the C and A/C horizons, from their

original pH above 8, by soil leaching and the accumulation of acidicorganic compounds in the topsoils, is quite remarkable. It is impor-tant to note that soil pH values in the A horizons of gully soils, withaverage pH/H2O=7.79 and pH/KCl=7.09, are comparable withthe agricultural soils which have an average pH/H2O=7.66 andpH/KCl=6.86. The observed differences between them are not sta-tistically significant.

Soil organic carbon contents in gully soils changed significantly, asclearly seen in Tables 1 and 2. Soil organic carbon rose from very lowcontent in the parent marl material (C horizons) to its actual content(average Corg content is 1.33% in A horizons). This is even higher thanin the agricultural soils (1.09% Corg on average) and this difference isstatistically significant at α=0.05 level. However, the A horizons ofgully soils are not ploughed and they are shallower at about 10–20 cm compared to 30 cm for the agricultural soils.

Agricultural soils contain a loamy A horizon with 27.1, 45.9, and27.0% of sand, silt and clay, respectively. Meanwhile, agricultural B

Fig. 2. View of the sampling sites of agricultural and gully soils at Prietrž village.

Fig. 3. Formation of humus horizon (about 15 cm thick) within the gully soil profile.

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horizon is loamy and contains 36.4, 40.2, and 23.4% of sand, silt andclay, respectively. Gully soils have an A horizon with a slightly highercontent of silt fraction which transforms this soil into a silty–loamtexture with 25.6% sand, 52.0% silt, and 22.4% clay. However, the30.9% sand, 47.6% silt and 21.5% clay in the gully soil A/C horizon pro-vide a loamy texture. The results of t-test for the difference in particlesize distribution showed that there is little difference in particle size

distributions between the gully and agricultural soils. However thegully soils contain a slightly lower clay content and a higher silt con-tent compared to the agricultural soils, and both these differences arestatistically significant at the α=0.05 level.

Average contents of carbonates in these soils are listed in Tables 1and 2. The results showed that carbonate leaching caused a gradualdecrease in the carbonate content of the gully soils. During this

Table 1Chemical and physical properties determined for samples from gully soils (profiles Gully 1–10).

Profile Horizon Depth (cm) Corg (%) CaCO3 (%) pH/H2O pH/KCl b0.002 mm (%) 0.05–0.002 mm (%) 2–0.05 mm (%) AS (%)

Gully 1 A 0–10 0.83 14.5 7.96 7.19 22.1 52.3 25.6 45.9Gully 2 A 0–10 1.54 2.4 7.75 7.00 24.2 52.9 22.9 75.5Gully 3 A 0–10 1.22 1.4 7.67 6.95 25.7 56.5 17.8 57.0Gully 4 A 0–10 1.08 4.1 7.87 7.12 23.4 51.6 25.0 57.0Gully 5 A 0–10 1.47 14.3 7.84 7.21 22.5 47.8 29.7 56.9Gully 6 A 0–10 1.39 4.5 7.79 7.20 22.4 51.8 25.8 51.3Gully 7 A 0–10 1.50 4.7 7.87 7.08 23.0 55.8 21.2 75.8Gully 8 A 0–10 1.57 0.0 7.36 6.51 22.8 56.5 20.7 77.3Gully 9 A 0–10 1.44 17.5 7.85 7.30 18.8 47.2 34.0 62.9Gully 10 A 0–10 1.28 18.7 7.89 7.31 18.7 48.1 33.2 56.7Average A 1.33 8.2 7.79 7.09 22.4 52.0 25.6 61.6Gully 1 A/C 20–30 0.32 20.5 8.17 7.25 23.5 49.2 27.3 20.9Gully 2 A/C 35–45 0.34 8.2 8.07 7.22 20.1 48.2 31.7 42.5Gully 3 A/C 35–45 0.49 8.5 8.06 7.20 20.6 46.7 32.7 50.1Gully 4 A/C 20–30 0.60 14.7 8.17 7.31 21.4 44.9 33.7 39.4Gully 5 A/C 20–30 0.64 14.6 8.04 7.24 22.7 48.1 29.2 48.2Gully 6 A/C 20–35 0.64 7.6 8.05 7.26 22.9 49.1 28.0 40.5Gully 7 A/C 20–30 0.24 5.4 8.17 7.18 23.6 52.5 23.9 38.0Gully 8 A/C 20–35 0.28 10.5 8.12 7.21 23.6 48.8 27.6 52.5Gully 9 A/C 20–30 0.45 21.0 8.01 7.37 20.6 44.8 34.6 39.4Gully 10 A/C 15–25 0.41 26.0 8.16 7.47 16.3 43.4 40.3 22.0Average A/C 0.44 13.7 8.10 7.27 21.5 47.6 30.9 39.4Gully 1 C 80–100 0.13 24.5 8.29 7.25 22.6 51.1 26.3 31.3Gully 2 C 80–90 0.13 13.7 8.23 7.30 16.7 37.3 46.0 31.6Gully 3 C 75–90 0.37 15.7 8.13 7.37 12.6 31.8 55.6 43.3Gully 4 C 60–75 0.15 21.6 8.34 7.43 18.8 48.8 32.4 23.6Gully 5 C 75–90 0.12 16.8 8.29 7.36 19.0 49.1 31.9 10.7Gully 6 C 70–85 0.43 22.6 8.22 7.48 15.6 43.5 40.9 19.9Gully 7 C 70–80 0.21 16.5 8.24 7.24 24.6 42.4 33.0 41.0Gully 8 C 80–95 0.28 8.5 8.18 7.24 20.2 44.1 35.7 44.4Gully 9 C 60–70 0.14 21.0 8.15 7.41 20.2 51.8 28.0 17.8Gully 10 C 55–65 0.13 30.0 8.27 7.56 12.4 35.9 51.7 24.4Average C 0.21 19.1 8.23 7.36 18.3 43.6 38.1 28.8

Abbreviations: depth — sampling depth; Corg — organic carbon content; CaCO3 — carbonate content; pH/H2O — soil pH determined in distilled water; pH/KCl — soil pH determinedin 1 M KCl; b0.002 mm – clay content; 0.05–0.002 mm – silt content; 2–0.05 mm – sand content; AS — aggregate stability.

Table 2Chemical and physical properties determined for samples from agricultural soils (profiles Agric 1–10).

Profile Horizon Depth (cm) Corg (%) CaCO3 (%) pH/H2O pH/KCl b0.002 mm (%) 0.05–0.002 mm (%) 2–0.05 mm (%) AS (%)

Agric1 A 0–10 1.29 1.6 7.76 6.97 30.0 49.1 20.9 81.5Agric 2 A 0–10 0.92 10.5 7.93 7.25 20.7 42.8 36.5 60.8Agric 3 A 0–10 1.12 8.0 7.88 7.20 22.3 44.9 32.8 65.4Agric 4 A 0–10 1.05 5.0 7.82 7.13 24.4 48.1 27.5 64.5Agric 5 A 0–10 1.04 9.5 7.93 7.15 26.2 46.9 26.9 56.9Agric 6 A 0–10 0.91 0.1 7.48 6.44 31.1 49.8 19.1 73.3Agric 7 A 0–10 1.15 0.4 7.71 6.87 34.4 49.6 16.0 68.9Agric 8 A 0–10 1.18 0.0 6.84 5.80 27.3 46.4 26.3 65.8Agric 9 A 0–10 1.23 0.2 7.40 6.52 30.5 42.8 26.7 63.6Agric 10 A 0–10 0.98 14.0 7.85 7.25 22.7 38.9 38.4 56.1Average A 1.09 4.9 7.66 6.86 27.0 45.9 27.1 65.7Agric1 B 40–50 0.17 0.1 7.67 6.25 29.7 54.0 16.3 52.4Agric 2 B 40–50 0.12 18.7 8.32 7.55 16.8 39.0 44.2 19.8Agric 3 B 40–50 0.23 14.0 8.18 7.33 22.0 43.7 34.3 37.5Agric 4 B 40–50 0.24 0.3 7.96 6.74 27.5 58.2 14.3 55.6Agric 5 B 40–50 0.13 1.0 8.06 6.92 35.1 32.7 32.2 42.2Agric 6 B 40–50 0.18 0.0 7.57 6.14 33.9 39.6 26.5 50.9Agric 7 B 40–50 0.33 1.1 8.00 6.87 25.9 53.2 20.9 48.0Agric 8 B 40–50 0.21 0.0 7.14 5.57 16.9 29.2 53.9 30.6Agric 9 B 40–50 0.21 7.7 8.18 7.33 13.2 22.3 64.5 29.3Agric 10 B 40–50 0.15 26.8 8.46 7.53 13.3 30.2 56.5 53.8Average B 0.20 7.0 7.95 6.82 23.4 40.2 36.4 42.0

Abbreviations: depth — sampling depth; Corg — organic carbon content; CaCO3 — carbonate content; pH/H2O — soil pH determined in distilled water; pH/KCl — soil pH determinedin 1 M KCl; b0.002 mm – clay content; 0.05–0.002 mm – silt content; 2–0.05 mm – sand content; AS — aggregate stability.

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gully soil development, the carbonate content decreased more thantwice in the A horizon (to 8.2%) compared to the parent marl material(with 19%). However, gully soil carbonate content still remains higherthan that in the agricultural soils, with 4.9% of carbonates in the Ahorizon.

The results of aggregate stability determination are listed inTables 1 and 2. It can be seen in Fig. 4 that soil-forming processes fol-lowing reforestation caused an increase in aggregate stability ingully soils (61.6% on average) close to that determined for agricul-tural soils (65.7%). Also, probability values for the t-test show thatthe difference between the A horizons of the gully and agriculturalsoils is not statistically significant (Table 3). But there is a statisticallysignificant difference at α=0.001 between the A horizon and sub-surface horizons of the gully soils. The very low aggregate stabilityin the C horizon of gully soils is quite notable at 28.8% when com-pared to that of the parent material. We assume that this materialwas exposed to erosion during gully development.

4. Discussion

Prior to reforestation, continuous gully erosion had a significanteffect on soil erodibility and the parent marl material exposed onthe soil surface possessed very low aggregate stability due to itsphysical, chemical and biological properties. This low aggregate sta-bility of the marls enabled strong soil erosion and the intensive ero-sion inhibited the development of stable soil profiles.

Reforestation measures affected the stabilization of degradedgully soils in several ways. Since a dense root network contributesdirectly to the development of a stable soil structure by enmeshingsoil particles and releasing glue-like root exudates (Degens et al.,1994; Tisdall and Oades, 1982), plants improve the aggregate stabil-ity of undeveloped soils within a very short time, usually within afew months (Burri et al., 2009; Gros et al., 2004).

The vegetation cover not only increases the soil cover's resistanceto erosion but it also accelerates soil-forming processes such as theaccumulation of soil organic matter, soil leaching, and biological pro-cesses. The overall effect of these beneficial changes in physical,chemical and biological properties and processes was observed inincreased aggregate stability of reforested topsoils. As a result, welldeveloped soil profiles with distinct humus horizons on the gullywalls were established (Fig. 3). Similar beneficial effects of vegeta-tion cover on the stabilization and recovery of degraded soils weredocumented by Gros et al. (2004), Moreno-de las Heras (2009) andBurri et al. (2009). Bork et al. (2001) reported rapid gully

development due to agricultural intensification and subsequentgully stabilization, as a consequence of reforestation by air-seeding.

The relationships between aggregate stability and other soilproperties are shown in the correlation diagrams in Figs. 5–7. It isevident that soil-forming processes caused the decrease in soil pH,the increase in organic carbon content and the decrease in carbonatecontent, and all these changes led to increased aggregate stability inthe gully soils. Although aggregate stability depends on clay andsand content, this is less influenced by pedogenesis and more bythe spatial variability within the parent material texture.

Such results seem apparently controversial compared to studieswhere authors reported beneficial effects of calcium carbonate onsoil aggregate stability (e.g. Ben-Hur et al., 1985; Le Bissonnais,1996). However, the aggregate stability of the studied soils increaseswith decreasing carbonate content, as shown in correlation diagramFig. 6. This can be explained by calcium carbonate's stabilizing effectdue to its solubility and release of Ca2+ ions. Although carbonatecontent was lowered more than twice as much than in the parentmarl material, the solubility of the preserved carbonates was stillsufficient to stabilize the aggregates.

Amezketa (1999) reported that the observed decrease in soil pHshould decrease negative charges and dispersion. This agrees withobserved increase in aggregate stability with decreasing soil pH dur-ing gully soil development, as shown in Fig. 7. However, this relation-ship can also be considered as an apparent correlation derived fromthe increase in organic carbon and decrease in carbonate contentsduring gully soil development.

It has been reported by many authors that the addition of organiccompounds to the soil increases dispersion of clay particles (e.g. Heiland Sposito, 1995; Piccolo andMbagwu, 1989). However, in contrast,positive correlations between soil organic matter and aggregatestability have also been frequently published (e.g. Chaney andSwift, 1984; Mbagwu and Piccolo, 1989). For example, Moreno-delas Heras (2009) regarded the accumulation of organic matter fromrestored vegetation as a key factor in soil aggregation on restoredmining slopes, andWick et al. (2009) reported that the accumulationof organic matter in reclaimed clay–loam soil was closely related to

020406080100

Gully A

Gully A/C

Gully C

Agric A

Agric B

Aggregate Stability (%)

Fig. 4. Comparison of aggregate stability determined for individual horizons of agricul-tural and gully soils. The boxes show the means and upper and lower quartiles of thedata, while the whiskers indicate the minimum and maximum values.

Table 3Probability values for t-test on differences in aggregate stabilities between horizons ofagricultural and gully soils.

Agric A Agric B Gully A Gully A/C Gully C

Agric A b0.001 0.349 b0.001 b0.001Agric B b0.001 0.001 0.610 0.023Gully A 0.349 0.001 b0.001 b0.001Gully A/C b0.001 0.610 b0.001 0.048Gully C b 0.001 0.023 b 0.001 0.048

0

10

20

30

40

50

60

70

80

90

0.0

Agg

rega

te S

tabi

lity

(%)

Corg (%)

r = 0.781

0.5 1.0 1.5 2.0

Fig. 5. Correlation diagram for relationship between aggregate stability and organiccarbon content in samples from the agricultural and gully soils.

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soil aggregation. Amezketa (1999) summarized the mechanismsof the stabilizing and dispersive effects of soil organic matter onsoil aggregate stability. Irrespective of these quoted controversial lit-erature reports, it can be concluded that the stabilizing effects in ourstudied soils exceed the dispersive effects of the soil organic matter.This is clearly apparent in correlation diagram, Fig. 5.

Themain reason for the susceptibility of this region's soils to gullydevelopment can be attributed to low aggregate stability values, asderived from our results for C horizon samples (Tables 1 and 2).The comparison of soil properties showed that the 28.8% aggregatestability in the parent soil material formed by marls is considerablylower than the 65.7% recorded in the ploughed A horizon of agricul-tural soils. This large difference in soil aggregate stability, whichexhibited statistical significance at α=0.001, is the source of insta-bility in the soil cover and susceptibility of the landscape to extremegully erosion.

The results also reflect the effectiveness of measures taken to sta-bilize these gully soils. After reforestation at the end of the 19th cen-tury, pedogenesis began to alter unsuitable properties present in themarl material in the gullies. The humus horizon started to developthrough partial soil-forming processes such as soil organic matter ac-cumulation and calcium carbonate leaching, which were accompa-nied by soil acidification, and therefore pH decrease. The resultingeffects were even so high that the current aggregate stability ofgully soils is statistically comparable with that of agricultural soils,and further development of these gullies has been discontinued.

5. Conclusions

Field research of reforested soils on steep walls of gullies showedthat prevailing climatic conditions, and the beneficial reforestationeffects which accrued during the century following reforestation,proved sufficient for the development of a clear soil profile with dis-tinct soil horizons. Detailed analysis showed that soil properties arenow gradually approaching the values determined in the surround-ing agricultural soils. A remarkable decrease in soil pH, carbonateleaching and soil organic matter accumulation were observed inthe newly developed soils of these gullies. Statistical evaluation hasrevealed that the A horizon of gully soils has properties significantlydifferent from the parent marl soil material, but very closely approx-imating the A horizon of surrounding agricultural soils. Irrespectiveof some controversial information in the literature concerning rela-tionships between the aggregate stability and individual soil proper-ties, the overall effect of the evolution of soil properties has resultedin an increase in aggregate stability in the A horizon of gully soils. Theaggregate stability is even so high that its difference from the sur-rounding agricultural soils is not statistically significant. The mainreason for the region's susceptibility to gully erosion can be derivedfrom the analysis of the aggregate stability in the studied soils. Thisshowed that landscape instability here can be attributed to thevery low aggregate stability of the parent marl material below a rel-atively stable soil cover. The obtained results also showed that refor-estation at the end of the 19th century with black locust was aneffective measure in changing unsuitable properties in the marl ma-terial exposed in these gullies. This reforestation was also beneficialin the development of soil profiles and for stabilization of the wallsof the permanent gullies in this region.

Acknowledgements

This research was supported by the project VEGA 1/0476/09, andpartially also by the Research and Development Operational Pro-gramme, the project Excellence Centre of Integral Territory FloodProtection, code ITMS 26240120004, funded by the ERDF.

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Fig. 6. Correlation diagram for relationship between aggregate stability and carbonatecontent in samples from the agricultural and gully soils.

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Fig. 7. Correlation diagram for relationship between aggregate stability and pH/H2O insamples from the agricultural and gully soils.

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