SORBED CHEMICAL-TRANSPORT IN OVERLAND-FLOW .2. ENRICHMENT RATIO VARIATION WITH EROSION PROCESSES

Breakdown of soil aggregates by rainfall, loss of sediment, and the enrichment of fine particles, organic matter, and N in eroded sediment were measured in flume experiments with a constant rainfall rate of 100 mm h-1. Experiments covering two soil types with different soil surface configurations and slopes were designed to ensure both exclusion of runoff entrainment as a major erosion factor, and that throughout exposure to rainfall the soil surface was unprotected by any significant depth of water. Sediment concentration differed initially for different experimental configurations, but all approached a single value after equilibrium was attained. Slope length did not have any effect on sediment concentration when rainfall was the only erosive agent at work and no surface rilling occurred. A rainfall-induced mechanism of transient microrilling has been proposed for sediment transport when there is no significant depth of water or rills under rainfall. Aggregate size distribution of the eroded sediment was drastically different to that of the original soil. The percentage of fine particles in the eroded sediment was more than 50% of the total, while this size fraction constituted less than 5% of the original soil. The proportion of large aggregates in the eroded sediment was less than in the original soil, and the intermediate size fractions remained unchanged. This dramatic production of fine particles is due to direct raindrop impact on the exposed surfaces of soil aggregates. The resulting loss of fine sediment from large aggregates has been termed raindrop stripping, a process that can lead to significant enrichment of any sorbed chemical concentrated in the outer layers of soil aggregates. The peeling action of raindrops was shown to be responsible for the formation of the bulk of fine particles found in eroded sediment. This mechanism, together with the differential concentration of sorbed chemicals within the aggregates, were found to be the main cause of the enrichment of sorbed chemicals in the eroded sediment. The enrichment ratio of organic C and N both decreased with cumulative soil loss and duration of the erosion event.