Soil Aggregate‐ and Particle‐Associated Organic Carbon under Different Land Uses in Nepal

Soil aggregation is an important process of C sequestration and hence a useful strategy to mitigate the increase in concentration of atmospheric CO 2 We studied water stability of soil aggregates (WSA) and soil organic carbon (SOC) associated with aggregates and primary particles in surface (0–10 cm) and subsurface (10–20 cm) layers of cultivated ( khet , irrigated lowland, and bari , rainfed upland) and forest lands (dense Shorea forest, degraded forest and shrub land, pine –Shorea forest, Shorea–pine–Schima forest, and Schima–Castanopsis forest) in a mountain watershed of Nepal. Macroaggregates (>2 mm) were abundant in forest soils (41–70%) while microaggregates (<0.5 mm) were abundant (56–63%) in cultivated lands. Pine mixed forest contained more macroaggregates in both layers. Mean WSA in the surface soil was highest in Shorea–pine–Schima forest (96%) and lowest in khet (74%). Macroaggregates in the surface layers contained 14.9 to 24.8 and 5.5 to 20.7 g kg −1 SOC in cultivated and forest soils, respectively, while microaggregates contained 12.5 to 30.8 and 11.9 to 25.4 g kg −1 SOC, respectively. The forest soils contained more sand (639–834 g kg −1 ) and fewer clay particles (49–95 g kg −1 ) than the cultivated soils. Soils under natural forest, however, were characterized by higher SOC associated with all primary particles. Cultivated soils contained higher amounts of clay but less clay‐associated SOC than forest soils. The relation between clay content and clay‐associated SOC was explained by a quadratic function ( R 2 = 0.45, P = 0.002).