Process-based analysis of aggregate stability effects on sealing, infiltration, and interrill erosion

Aggregate stability indices are correlated with soil erosion rates. Although aggregate stability influences soil erodibility and aggregate breakdown product size distributions, these indices are usually unsatisfactory because of interacting physicochemical considerations affecting infiltration and erosion. We examined the spatial variability in structural and depositional seal composition, and the influence of breakdown fragment size distribution on seal hydraulic resistance and interrill erosion. Two Alfisols, Villamblain clay loam and Blosseville silt loam, were sieved to retain <2.0-cm aggregates and packed in 0.25-m(2) soil trays. Soil trays were rained upon for 4 h at 23 mm h(-1). Infiltration, runoff, and subseal pressure head were measured throughout the experiment, and surface roughness and depositional seal area were measured at selected times during the event. Aggregate stability, hydraulic resistance, and fragment size distribution of the seal matrix were also determined. Mean weight diameter (MWD) of aggregates after mechanical breakdown by stirring was greater for Villamblain (1.70 mm) than for Blosseville (0.92 mm) (r(2) = 0.94, P < 0.001). Blosseville seals had finer fragments than those of Villamblain (r(2) = 0.85 P < 0.001): fewer than 28% of fragments >1 mm and more than 40% <0.1 mm, compared with 60 and 15% for Villamblain. Depositional seals had finer fragments than structural seals for both soils. For the same runoff rate (15 mm h(-1)), soil loss was greater for Blosseville (2 g m(2) min(-1)) than for Villamblain (0.4 g m(2) min(-1)) since breakdown products were finer and more transportable. Seal hydraulic resistance was correlated (r(2) = 0.53, P < 0.01) with MWD of seal fragments.