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Soil aggregation is an important process of C sequestration and hence a useful strategy to mitigate the increase in concentration of atmospheric CO2. 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 ban, 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 g-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 (R² = 0.45, P = 0.002). [ABSTRACT FROM AUTHOR]

Soil surface crusting, a common phenomenon on cultivated soils, has major implications for agriculture and the environment because of itseffects on soil hydrological properties, erosion and crop establishment. The objectives of this study were to evaluate land use controls on crust formation and the hydraulic response of soils to crust development for a Patancheru series soil (clayey skeletal, mixed, isohyperthermic Udic Rhodustalf) in south-central India. Soil aggregates, obtained from cultivated (PL) and naturally vegetated fallow (NV) land, were packed into sample trays and subjected to laboratory rain simulation to form crusts. Thin sections and visual observation indicated that crust development reached a more advanced stage in the PL case compared with NV following 90 min of rain at 40-80 mm h-1 intensity. This was reflected in a thicker crust layer with fewer voidsin the former and a less smooth surface with partially disintegratedaggregates in the latter. The hydraulic response of the soil surfacewith the progression of crust development indicated a more permeableand less dense crust formed on NV than on PL soil. The results suggested that NV soil aggregates were more stable and that crust formation is more gradual for stable aggregates compared with the less stablePL aggregates. A structural crust-type formed on the Patancheru soilby means of parallel subprocesses involving translocation and illuviation of aggregate disruption by products, and raindrop compaction and particle rearrangement. Copyright (c) 1999 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Sealing and crusting of soil surfaces have dramatic effects on waterinfiltration into and runoff from soils, thereby greatly influencingerosion processes. This study focused on the effect of the initial stage of crusting on inter-rill erosion processes for a crust-prone Alfisol sampled from south-central India. Soil aggregates ranging from 274 to 8 mm collected from ploughed (PL) and naturally vegetated (NV)treatments were subjected to rainfall simulation under laboratory conditions. Runoff from PL soil aggregates was 2-275 times higher, while percolation was 20-100% lower, than for NV aggregates. Soil wash and splash losses were 075-3 times greater for PL than for NV soil. Runoff and inter-rill erosion were significantly higher during the wet simulation run compared with the dry run. The results indicated that NV soil aggregates were more resistant to breakdown from raindrop impact and slaking, and subject to less rapid sealing, than PL soil. Total soil loss was influenced most by initial aggregate stability and the extent of seal development. Splash and wash losses of soil both increased as a result of surface sealing regardless of soil condition for short (30-60 min) rainfall durations. High drying rates resulted inthe highest crust bulk densities. Increased crust strength for PL soil compared with NV soil reflected the greater susceptibility of cultivated soil to surface sealing and crusting. [ABSTRACT FROM AUTHOR]