SEDIMENT ENRICHMENT MECHANISMS OF ORGANIC-CARBON AND PHOSPHORUS IN A WELL-AGGREGATED OXISOL

Mechanisms of organic carbon (OC) and P enrichment in sediment eroded from well-aggregated soils are not well understood. This study was con ducted to examine why OC and extractable P (ext-P) were enriched in se diment with a well-aggregated Oxisol. Soil samples presented four fiel d treatments: no fertilization (NF), short-term P fertilization (STP), long-term P fertilization (LTP), and organic waste fertilization (OW) , Organic C in dry-sieved aggregates increased with decreasing aggrega te size for all treatments, suggesting that OC is preferentially retai ned in small aggregates and, perhaps, protected from microbial attack, Extractable P concentration changed little with aggregate size for th e NF treatment, but increased significantly with decreasing aggregate size for the STP and OW treatments and for liquid-P incubated NF soil, This difference,vas attributed to the blockage of interior sorption s ites within large aggregates. Breakdown of dry STP and OW aggregates b y simulated storms produced fine aggregates that were richer in OC and ext-P than the original larger ones. Most of the fine aggregates were created in the early phase of rainfall by sinking dry aggregates, and further breakdown by stripping water-stable aggregates was more diffi cult and slow, Sediment enrichment observed in eroded sediment was cau sed by selective transport of P and OC-rich fine aggregates induced by the effects of aggregation blockage and aggregate breakdown by slakin g and stripping, For the LTP treatment, however, ext-P concentration w as independent of aggregate size, indicating that a uniform P distribu tion in aggregates ran be reached given sufficiently long equilibratio n time.