Effects of interrill erosion on the distribution of soil organic and inorganic carbon in different sized particles of Mediterranean Calcisols

In this study, the potential effect of the selective transport and deposition by interrill erosion on the spatial distribution of eroded soil and associated soil organic (SOC) and inorganic carbon (SIC) in carbonate-rich soils was investigated in Mediterranean cultivated Calcisols. Particular attention was paid to the role of calcium carbonate in the stabilization of SOC and its impact on soil structure and the settling behavior of soil particles in suspension. The settling velocity is a key variable in controlling the transport and fate of eroded sediment and sediment-associated carbon, which accounts for soil particle / aggregate size and density. The objectives were to analyze the settling behavior of cultivated soils at two contrasting slope positions, representative of erosion and deposition sites, in an interrill area and compare with those of uncultivated and undisturbed soils (not affected by deposition or erosion processes). Besides we examined the distribution of SOC and SIC associated with different particle size classes. Soils were fractionated into five size classes (>250, 125-250, 63-125, 32-63 and ≤32 μm) using a settling tube procedure and carbon contents (SOC and SIC, %) were measured at each settling size classes. The results indicated that the particle size selectivity of interrill erosion affects the distribution of soil particles and associated SOC and SIC spatial variability. As a result of erosion processes, cultivated soil at the upper part of the slope was enriched in finer fractions compared to the lower part where mainly coarse particles were deposited and the finer material was transported and washed out. Cultivation leads to a depletion of SOC and of the >250 μm fraction in comparison with uncultivated and undisturbed soil, where the SOC content was two times higher than in cultivated soil. Lighter and smaller settling size classes (≤63 μm) had the highest SOC content in uncultivated and cultivated downslope soils. This is contrary to what occurs with SIC that acts as a cementing agent and promotes aggregation formation affecting soil structure. Therefore, sand-sized and silt-sized carbonates were accumulated in coarser size fractions in these soils, mainly in downslope positions. This study provides a first insight into the role of selective processes by interrill erosion and their impact on soil organic/inorganic carbon distribution affected by different particle selective transport process, soil aggregate size distribution and associated carbon in Mediterranean agroecosystems.