Junguang Wang

Tensile Strength and Friability of Ultisols in Sub-Tropical China and Effects on Aggregate Breakdown Under Simulated Rainfall

Abstract Many studies have shown the effects of mechanical properties on soil erosion. However, most have focused on the strength of bulk soil, with little attention paid to the strength of the aggregates. The objective of this study was to assess the effects of aggregate tensile strength and friability on aggregate breakdown under simulated rainfall. Different size aggregates of Ultisols, derived from Shale (S) and Quaternary red clay (Q), were exposed to simulated rainfall of 60 mm h−1. Fragment size distribution and splash loss were compared. The results showed that tensile strength decreased when aggregate size increased. For 1- to 2-mm aggregates, tensile strength of Q was greater than S, but the opposite was true for 10- to 20-mm aggregates. Estimates of friability from the coefficient of variation method were different with aggregate sizes tested and always gave larger numerical values than the volume dependence method. Aggregate breakdown was closely related to tensile strength because both depend on aggregate microstructure. Good correlations were observed between tensile strength and normalized mean weight diameters under different amounts of cumulative rainfall. As aggregate size increased, mean weight diameter of fragments increased, but normalized mean weight diameter and splash loss decreased. The tendency that aggregate splash decreased with increasing aggregate size was more obvious for the soil with low friability. For all size classes, the aggregates of soil with low friability produced more splash materials. It might be possible, with further experimentation, to develop empirically based criteria for stability of soil to erosion using tensile strength and friability.

Effects of soil type and rainfall intensity on sheet erosion processes and sediment characteristics along the climatic gradient in central-south China

Soil erosion poses a major threat to the sustainability of natural ecosystems. The main objective of this study was to investigate the effects of soil type and rainfall intensity on sheet erosion processes (hydrological, erosional processes and sediment characteristics) from temperate to tropical climate. Field plot experiments were conducted under pre-wetted bare fallow condition for five soil types (two Luvisols, an Alisol, an Acrisol and a Ferralsol) with heavy textures (silty clay loam, silty clay and clay) derived separately from loess deposits, quaternary red clays and basalt in central-south China. Rainfall simulations were performed at two rainfall intensities (45 and 90mmh-1) and lasted one hour after runoff generation. Runoff coefficient, sediment concentration, sediment yield rate and sediment effective size distribution were determined at 3-min intervals. Runoff temporal variations were similar at the high rainfall intensity, but exhibited a remarkable difference at the low rainfall intensity among soil types except for tropical Ferralsol. Illite was positively correlated with runoff coefficient (p<0.05). Rainfall intensity significantly contributed to the erosional process (p<0.001). Sediment concentration and yield rate were the smallest for the tropical Ferralsol and sediment concentration was the largest for the temperate Luvisol. The regimes (transport and detachment) limiting erosion varied under the interaction of rainfall characteristics (intensity and duration) and soil types, with amorphous iron oxides and bulk density jointly enhancing soil resistance to erosive forces (Adj-R2>88%, p<0.001). Sediment size was dominated by <0.1mm size fraction for the Luvisols and bimodally distributed with the peaks at <0.1mm and 1-0.5mm size for the other soil types. Exchangeable sodium decreased sediment size while rainfall intensity and clay content increased it (Adj-R2=96%, p<0.01). These results allow to better understand the climate effect on erosion processes at the spatial-temporal scale from the perspective of soil properties.