FLOW-INDUCED TRANSPORT AND ENRICHMENT OF EROSIONAL SEDIMENT FROM A WELL-AGGREGATED AND UNIFORMLY-TEXTURED OXISOL

摘要

Sediment enrichment and transport from a highly weathered, well-aggregated, and uniformly-textured soil were determined using field rainfall simulation. Overland flow on an agriculturally managed ridge-and-furrow system was generated by applying four successive rainfall events: a dry run on bare soil with a rainfall intensity of 35 mm h(-1), a wet run on bare soil with the same rainfall intensity, a very wet run on bare soil with a rainfall intensity of 62 mm h-1, and a final run on partially plastic-mulched soil (to simulate pineapple culture) with a rainfall intensity of 50 mm h(-1). Stream power and shear stress of overland flow during the dry and wet runs did not exceed the critical level for rill flow detachment. and resulted in runoff sediment concentrations between 1.7-3.5 g L(-1), with 70-90% of sediment consisting of fine aggregates (< 0.063 mm). As flow rate and flow stream power and shear stress increased, sediment concentration increased to over 12 g L(-1) for the subsequent two runs. Elevated sediment concentration was associated with increased transport of sand-sized aggregates. Elimination of interrill erosion by plastic mulching reduced transport of fine aggregates in comparison with the bare soil. Due to the high clay content and uniform texture of the Oxisol, sediment from the four events displayed similar primary particle size distribution and no enrichment of clay particles. However, organic carbon (OC) and extractable phosphorus (Ext-P) were enriched in sediment. The magnitudes of enrichment ratio followed the order: dry run (1.2 for OC and 1.4 for Ext-P)> wet run (1.1 for OC and 1.2 for Ext-P)> very wet run(1.03 for OC and 1.1 for Ext-P), plastic mulch run (close to 1 for both OC and Ext-P). This enrichment was associated with selective transport of fine aggregates which were chemically richer than the coarser ones.