Organic carbon, water repellency and soil stability to slaking under different crops and managements: a case study at aggregate and intra-aggregate scales

Abstract. This research studies the distribution of organic C and intensity of water repellency in soil aggregates with different size and in the interior of aggregates from Mediterranean soils under different crops (apricot, citrus and wheat) and management (conventional tilling and no tilling/mulching). For this, undisturbed aggregates were sampled and carefully divided in size fractions (0.25–0.5, 0.5–1, 1–2, 2–5, 5–10 and 10–15 mm) or peeled to obtain separated aggregate layers (exterior, transitional and interior). Organic C content in the fine earth fraction of soils under different crops did not show important variations, although it increased significantly from conventionally tilled to mulched soils. The distribution of organic C content in aggregates with different size varied among soils under different crops, generally increasing with decreasing size. At the intra-aggregate level, organic C concentrated preferably in the exterior layer of aggregates from conventionally tilled soils, probably because of recent organic inputs or leachates. In the case of mulched soils, higher concentrations were observed, but no significant differences among aggregate regions were found. The intensity of water repellency, determined by the ethanol method, did not show great variations among crops, but increased significantly from conventionally tilled to mulched soils. Coarser aggregates were generally wettable, while finer aggregates showed slight water repellency. Regardless of variations in the distribution of organic C in aggregate layers from conventionally tilled soils, great or significant differences in the distribution of water repellency at the intra-aggregate level were not found in any case. Finally, the intensity of water repellency was much more important than the concentration of organic C in the stability to slaking of aggregates.