Effects of land use conversion on soil aggregate stability and organic carbon in different soils

Efectos del cambio de uso del suelo en la estabilidad de agregados y carbono orgánico en diferentes suelos

Vladimir Ćirić ^*, Maja Manojlović, Milivoj Belić, Ljiljana Nešić, Srdan Šeremešić

University od Novi Sad, Faculty of Agriculture, Trg D. Obradovića 8, 21000 Novi Sad, Serbia. (vciric@polj.uns.ac.rs). * Author for correspondence.

Received: October, 2012.
Approved: August, 2013.

Abstract

Aggregate stability is an important factor of the soil functioning. Greater aggregate stability leads to greater soil organic carbon (SOC) preservation, while SOC acts as a key cementing agent in aggregation processes. The objective of this study was to investigate the effects of native vegetation conversion in soil aggregate stability and SOC concentration. The investigation was conducted in the Vojvodina Province, Serbia, in July 2009. Undisturbed soil samples were taken from Haplic Chernozem, Haplic Fluvisol and Gleyic Vertisol, at a depth ranging from 0 to 20 cm. A completely randomized experimental design was used with three replicates. Each soil type was considered under treatments 1) cropland >100 years, 2) native meadow and 3) native deciduous forest. The means were compared by the Tukey test (p≤0.05). The sampling distance between different land use areas was less than 200 m. Wet sieving was performed in order to obtain four size classes of stable aggregates (8000-2000, 2000-250, 250-53 and <53 ,µm). The soil organic carbon concentration in aggregate classes was determined by the dichromate wet oxidation method. The conversion of native vegetation to cropland caused the MWD reduction of 78 % in Haplic Chernozem, 55 % in Haplic Fluvisol and 50 % in Gleyic Vertisol, and the largest decrease was recorded in the content of aggregates 2000-8000 µm. The reduction of the SOC concentration in sand-free aggregates occurred mainly in the aggregates 532000 µm amounting to 48 % in Gleyic Vertisol and 52 % in Haplic Chernozem, whereas in Haplic Fluvisol was 52 % in the aggregates 8000-2000. The silt and clay fraction (<53 µm) showed the highest level of SOC preservation. Due to the high concentration of SOC and clay, Gleyic Vertisol showed lower susceptibility to aggregate stability deterioration and greater ability for SOC preservation than Haplic Chernozem and Haplic Fluvisol. This study also indicated the necessity for sand-free correction in coarse-textured soils.

Key words: aggregate stability, soil organic carbon, land use change, soil type, soil structure.

Resumen

La estabilidad de los agregados es un factor importante en el funcionamiento del suelo. La mayor estabilidad de los agregados conduce a una mayor conservación del carbono orgánico del suelo (COS), mientras que COS actúa como un agente de cementación clave en los procesos de agregación. El objetivo de este estudio fue investigar los efectos de la conversión de la vegetación nativa en la estabilidad de los agregados del suelo y la concentración de COS. La investigación se realizó en la provincia de Vojvodina, Serbia, en julio del 2009. Muestras inalteradas de suelo de los tipos Chernozem Háplico, Fluvisol Háplico y Vertisol Gléyico se tomaron a una profundidad de 0 a 20 cm. El diseño experimental fue completamente al azar, con tres repeticiones. Cada tipo de suelo fue considerado con los tratamientos: 1) tierras de cultivo > 100 años, 2) pradera nativa y 3) bosque caducifolio nativo. Las medias se compararon con la prueba de Tukey (p≤0.05). La distancia de muestreo entre las diferentes áreas de uso del suelo fue menor a 200 m. Se realizó un tamizado en húmedo para obtener cuatro clases de tamaño de agregados estables (8000-2000, 2000-250, 250-53 y <53 µm). La concentración de carbono orgánico del suelo en clases de agregados se determinó por el método de oxidación en húmedo de dicromato. La conversión de vegetación nativa a tierras de cultivo causó la reducción de 78 % de MWD en Chernozem Háplico, 55 % en Fluvisol Háplico, 50 % en Vertisol Gléyico, y el mayor descenso se registró en el contenido de agregados de 2000 a 8000 µm. La reducción de la concentración de COS en agregados sin arena fue principalmente en los agregados de 53 a 2000 µm, que fue 48 % en Vertisol Gléyico y 52 % en Chernozem Háplico, mientras que en Fluvisol Háplico fue 52 % en los agregados 8000-2000 µm. La fracción de limo y arcilla (<53 µm) mostró el mayor nivel de conservación de COS. Debido a la concentración alta de COS y arcilla, el Vertisol Gléyico mostró menor susceptibilidad al deterioro de estabilidad de agregados y mayor capacidad para la conservación de COS que Chernozem Háplico y Fluvisol Háplico. Este estudio también indicó la necesidad de la corrección para agregados libres de arena en suelos de textura gruesa.

INTRODUCTION

Soil structure is a major physical property of soil which significantly affects soil fertility, plant nutrition and the environment in general. It is typically expressed via aggregate stability and related indices such as the mean weight diameter (MWD) and the geometric mean diameter (GMD). Soils with unstable aggregates have high susceptibility to water, wind erosion and nutrient leaching. Generally, land use change such as the conversion of natural ecosystems to croplands leads to a rapid deterioration of aggregate stability. Such changes have occurred in European soils long time ago (Janssens et al., 2003).

Primary aggregation mechanisms differ between soil types (Bronick and Lal, 2005) and, therefore, the effects of land use change on the degree of aggregation increase/decrease should be analyzed separately for different soil types. According to the conceptual model for soil aggregation, microaggregates join together and form macroaggregates (Tisdall and Oades, 1982). The cementing agents which take part in aggregation processes are soil organic carbon (SOC), biota, ionic bridging, clay and carbonates (Bronick and Lal, 2005). The aggregate hierarchy is subjected to change when SOC is not the main cementing and stabilizing agent in the soil (Fernández-Ugalde et al., 2010). When SOC content is low, the stability of macroaggregates is controlled by CaCO₃, while the stability of microaggregates is correlated with clay (Boix-Fayos et al., 2001). Levy and Mamedov (2002) also indicate the correlation between aggregate stability and clay content, but not with SOC.

There are four main mechanisms involved in the aggregate disruption: slaking caused by the compression of entrapped air during wetting; by differential swelling; by raindrop impact; and physico-chemical dispersion due to osmotic stress (Le Bissonnais, 1996). Undisturbed natural ecosystems (forests, meadows, pastures) have favorable soil structure and high SOC concentration, in contrast to croplands which are mixed and turned during soil cultivation.

The SOC level is closely associated with soil structure. Aggregates physically protect SOC while SOC is a binding agent in the aggregation process. The SOC is an extremely valuable natural resource (Lal, 2004) and its content in the soils worldwide is 1500 Pg (1 Pg=10¹⁵ g) in the 0-100 cm layer. The soil type shows a significant relationship with SOC, reflecting the effects of parent material (Wang et al. , 2008), soil genesis and soil-forming processes (melanization, vertization or fluvial sedimentation). The SOC tends to be lost when grasslands, forests or other native ecosystems are converted to cropland (Smith, 2008). Additionally, Manojlovic et al. (2011) report a lower SOC concentration in grassland than in forest and cropland soils due to the SOC concentration in the thin surface layer of grassland soils formed on rocky substrate. Most agricultural soils have lost 30 to 75 % of their SOC pool, or 30 to 40 t C ha^-1 (Lal et al., 2007).

Aggregate stability and SOC concentration and preservation depend, to a large extent, on soil texture. The effect of SOC on structural stability is more pronounced in soils containing low clay levels (Wuddivira and Camps-Roach, 2007). Such soils also have a low SOC concentration in the microaggregate fraction since most of the sand occurs in this fraction. Therefore, it is best to calculate SOC concentration on the basis of sand-free aggregates.

The objective of this study was to understand the behaviour of different soil types during the native vegetation conversion. The hypothesis was that aggregate stability and SOC changes due to cultivation are different in various soil types and occur mostly in macroaggregates.

The study was carried out in the Vojvodina Province, Serbia, in the southernmost part of the Pannonian Basin (46° 11' - 46° 37' N, 18° 51' - 21° 33' E). The Vojvodina Province is the warmest and driest part of the Pannonian Basin with 11.0 °C means annual temperature, average of 88 frosty days (24 % per year), 602 µm mean annual precipitation and 76 % mean annual relative humidity. Most coµmon relief units are river plains (~70 m altitude), loess terraces (70-90 m altitude) and loess and sand plateaus (90-120-200 m altitude). Vojvodina has 2 150 600 ha which is characterized by intensive agriculture fostering the conventional tillage system with corn, wheat and soybean as the most coµmon crop rotation. The investigated cropland has been under the conventional tillage for more than 100 years. Meadows consist mainly of a combination of mesophytes (Dactylis glomerata, Bromus mollis, Festuca pratensis, Cirsium arvense) and grasses (Poa sp., Stipa sp., Festuca sp., Cynodon sp., Panicum sp.). Fagus sp., Quercus sp., Populus sp. and Salix sp. predominate in the areas under deciduous forests. Cropland (1 650 000 ha or 77 %) constitutes the largest part of the agricultural land (1 790 000 ha or 83 %) in the Vojvodina Province. Meadows and grasslands account for a much smaller area. Forests cover 140 717 ha or 6 % of the agricultural land.

The objects of study were the following soil types which differ in texture: Haplic Chernozem (medium-textured), Haplic Fluvisol (coarse-textured) and Gleyic Vertisol (fine-textured). A completely randomized experimental design was used and with three treatments for each soil type: 1) cropland >100 years, 2) native meadow and 3) native deciduous forest. The sampling distance between the different treatments was not larger than 200 m in order to ensure soil comparability in each location. The treatment means were compared using Tukey test (p≤0.05). The calculations and statistical analyses were carried out with Statistica 10.0, StatSoft, Inc.

The undisturbed soil samples were taken from the surface horizon (0-20 cm), with three replicates approximately 10 m apart. The air-dried samples were used for the analysis of aggregate stability. The method of Elliot (1986) was adapted for aggregate separation. Briefly, 100 g of air-dried soil was capillary wetted on a 2000 µm sieve and suspended for 2 min in deionised water at room temperature. The 8000-2000 µm fraction was obtained by moving the sieve 3 cm up and down with 30 repetitions during 2 min, breaking the surface of the water with each stroke. The aggregates were collected and backwashed in an aluminium pan. The soil and the water which passed through the sieve were poured into a smaller-sized sieve. Sieving was repeated with reduced vertical movements, 20 times for 250 µm and 10 times for the 53 µm sieve. The obtained stable aggregates were dried at 50 °C and weighed. Therefore, four size classes of stable aggregates were obtained (8000-2000, 2000-250, 250-53 and <53 µm).

With these weights, the MWD diameter was calculated with the following equation (Hillel, 2004):

where w_i is the weight percentage of each aggregate size class with respect to the total sample and x_i is the mean diameter of each aggregate size class (µm).

GMD (µm) was calculated according to Hillel (2004):

where w_i is the weight percentage of each aggregate size class with respect to the total sample and x_i is the mean diameter of each aggregate size class (µm).

where SOC is the soil organic carbon concentration of aggregate size class (g kg^-1), and sand is the sand concentration of aggregate size class (g kg^-1).

RESULTS AND DISCUSSION

The presence of stable macroaggregates is a prerequisite for favorable structure. The results showed a different distribution of stable aggregates among the tested soil types (Figure 1). Haplic Fluvisol had a more uniform distribution of stable aggregates than Haplic Chernozem and Gleyic Vertisol. Microaggregates (53-250 µm) tended to have an increased average concentration in Haplic Fluvisol as a result of high total sand concentration determined in this soil. Conversely, the content of macroaggregates (250-8000-µm) reached 84 and 88 % in Haplic Chernozem under meadow and forest, and it was even higher in Gleyic Vertisol under meadow (89 %) and forest (96 %). The soil structure in Haplic Chernozem was favourable due to the enriched high-quality humus, high base saturation and bioturbation (Altermann et al., 2005). These results do not agree with those presented by Tobiasová (2011) who found a more favorable soil structure in Haplic Fluvisol in comparison with Haplic Chernozem. Gleyic Vertisol was the most favorable soil structure due to vertization processes and a very high concentration of binding agents such as SOC and clay. The SOC decreases the wettability and increases the cohesion of aggregates, thus increasing aggregate stability (Chenu et al., 2000).

The distribution of stable aggregates was strongly affected by the conversion of native vegetation to cropland followed by long-term cultivation. As much as 93 % of large macroaggregates were lost in Haplic Chernozem, 72 % in Haplic Fluvisol and 66 % in Gleyic Vertisol. Long-term application of conventional tillage practices caused the breakdown of large macroaggragetes and exposure of organic matter to oxidation processes leading to both aggregate stability decline and SOC loss (Tisdall and Oades, 1982; Elliot, 1986; Kay, 1990). The results are similar to those reported by Balashov and Buchkina (2011), who found that long-term agricultural management decreased the content of water-stable aggregates in Haplic Chernozem. The conversion of natural forest on Haplic Fluvisol to meadow and cropland decreased the >250 µm aggregate fraction by 9-16 % and 29-47 % (Gajic et al, 2010). In Haplic Chernozem, the reduction of large macroaggregates due to cropping resulted in a significant increase of microaggregates, and silt and clay fractions (<53 µm). The content of small macroaggregates (2000-250 µm) was increased in Gleyic Vertisol converted to cropland. Silt and clay fractions were increased in Haplic Fluvisol converted to cropland. Based on the increase of different aggregate size classes in different soils (following the native vegetation conversion), the susceptibility of soils to land use change can be assessed. Gleyic Vertisol showed the highest resistance to long-term tillage because it retained a high content of macroaggregates for a long time after land use change. This could be associated with high concentrations of SOC compounds and clay in the studied soil. Haplic Chernozem and in particular Haplic Fluvisol showed a lower resistance to the destructive action of the applied management practices, which resulted in the formation of a large amount of <250 µm aggregates in croplands. These results are similar to those resported by DeGryze et al. (2004) of a higher proportion of 53-250 µm aggregates in the cultivated soil compared with the native vegetation. An increase in aggregates <0.84 µm is considered as a negative process which makes the soil susceptible to wind erosion (Chepil, 1953).

The values of MWD and GMD (Figure 2) decreased in all the analyzed soil types in the following order: forest > meadow > cropland. The recorded GMD values had a similar distribution pattern and were highly correlated with MWD (r=0.99). All of the three soil types had significantly higher MWD and GMD values under forest and meadow than under cropland. Abrishamkesh et al. (2011) observed significantly greater MWD and GMD values in forest soils than in a long-term cultivated tea garden. High values of MWD in natural soils are associated with the absence of tillage and the presence of hydrophobic substances which coat the aggregates, slowing the entry of water into soil micropores and preventing the deterioration of soil aggregates (Blair et al., 2006). In this study, the reduction of MWD induced by long-term cultivation was 78 % in Haplic Chernozem, 55 % in Haplic Fluvisol and 50 % in Gleyic Vertisol, while GMD was decreased by 61 % in Haplic Chernozem, 41 % in Haplic Fluvisol and 37 % in Gleyic Vertisol. The most significant decrease in structure indices was noted in Haplic Chernozem located at the experimental station. Therefore, this soil type was exposed to more intensive annual cultural practices in comparison with Haplic Fluvisol and Gleyic Vertisol.

The average SOC concentrations in whole aggregates and in sand-free aggregates were significantly higher under native vegetation than under cropland in the three soil types, and was 2-45 % lower under cropland than under meadow or forest in Haplic Fluvisol. The corresponding values for Haplic Chernozem and Gleyic Vertisol were lower by 3743 % and 52-53 %. In prairie loess soil, Martens et al. (2003) found higher SOC concentrations under forest (46 %) and pasture (25 %), than under cropped soil. In this study, differences in SOC concentration were higher when calculated on sand-free basis. In Haplic Fluvisol there were slight differences in SOC concentration between cropland and meadow. When SOC concentration was calculated on a sand-free basis, Haplic Fluvisol under cropland showed significantly lower values than the meadow soil. This support the sand correction proposed by Six et al. (1998). When sand content in the soil is low (<100 g kg^-1), sand particles are incorporated in soil aggregates (John et al., 2005) and, consequently, sand-free correction is not necessary. Since the increased SOC concentration in Haplic Fluvisol was twice as high in the 53-250 µm sand-free aggregate fraction than in whole aggregates, the sand-free correction is recoµmended for the coarse-textured soils containing >400 g kg^{- 1} sand at least.

In the studied land use systems, SOC concentrations were significantly higher in whole and sand-free aggregate size fractions >53 µm than in the <53 µm size fraction. The only exception were the whole aggregates in Haplic Fluvisol due to the high sand content of the soil. The low SOC concentration in the <53 µm fraction might be due to the soil's high silt content, low adsorption capacity and ability to bind SOC. Highest average SOC concentrations were obtained in sand-free aggregates of the 2000250 µm and 250-53 µm size fractions. Moreover, in Haplic Chernozem and Gleyic Vertisol, the above size fractions of sand-free aggregates had highest average SOC losses after native vegetation conversion (Table 3). Lower SOC losses were recorded in the 2000-8000 µm fraction of sand-free aggregates and the lowest in the silt and clay fraction. Haplic Fluvisol showed a higher average SOC loss in the 8000-2000 µm fraction of sand-free aggregates than in the 250-2000 µm and 53-250 µm size fractions. Increased cultivation intensity induces the loss of SOC-rich macroaggregates and the gain of SOC-depleted microaggregates, resulting in an overall loss of SOC (Six et al., 2000). The SOC is more stable in microaggregates than in macroaggregates (Puget et al. , 2000).

The reduction of the total SOC concentration in sand-free aggregates was 48 % in Gleyic Vertisol, 52 % in Haplic Chernozem and 52 % in Haplic Fluvisol. The slightest reduction of the SOC concentration in Gleyic Vertisol is probably a consequence of the high clay concentration in the soil. Moreover, the silt and clay fraction showed a high potential for SOC preservation due to the fact that this fraction showed the lowest SOC losses in all the three soil types. This might be a result of the stabilizing effect of clay on SOC. Organo-mineral complexes such as clay and silt limit the microbial access to intra-aggregate carbon (Bossuyt et al., 2002).

CONCLUSIONS

The conversion of meadow and forest soils to cropland leads to a noticeable deterioration in the aggregate stability of Haplic Chernozem, Haplic Fluvisol and Gleyic Vertisol, which mainly occurs in large macroaggregates. The reduction of SOC concentration in sand-free aggregates mostly occurred in the aggregates 53-2000 µm in Gleyic Vertisol and Haplic Chernozem, and in the aggregates 20008000 µm in Haplic Fluvisol. There was a higher level of aggregate stability and SOC preservation in Gleyic Vertisol in comparison with Haplic Chernozem and Haplic Fluvisol. The slit and clay fractions (53<µm) were the best protectors of SOC.

ACKNOWLEDGEMENTS

This paper was funded by the Ministry of Education and Science of R. Serbia (the project TR 31027). We would like to express our gratitude to Prof. Borivoj Pejić, Ph. D., for the support and valuable suggestions.

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