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Sediment Enrichment Mechanisms of Organic Carbon and Phosphorus in a Well-Aggregated Oxisol

Department of Agronomy and Soil Science, Univ. of Hawaii at Manoa, 1910 East-West Road, Honolulu, HI 96822.

^* Corresponding author (ywan{at}hawaii.edu).

ABSTRACT

Mechanisms of organic carbon (OC) and P enrichment in sediment eroded from well-aggregated soils are not well understood. This study was conducted to examine why OC and extractable P (ext-P) were enriched in sediment with a well-aggregated Oxisol. Soil samples presented four field 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 aggregate size for all treatments, suggesting that OC is preferentially retained in small aggregates and, perhaps, protected from microbial attack. Extractable P concentration changed little with aggregate size for the NF treatment, but increased significantly with decreasing aggregate size for the STP and OW treatments and for liquid-P incubated NF soil. This difference was attributed to the blockage of interior sorption sites within large aggregates. Breakdown of dry STP and OW aggregates by 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 slaking dry aggregates, and further breakdown by stripping water-stable aggregates was more difficult and slow. Sediment enrichment observed in eroded sediment was caused by selective transport of P and OC-rich fine aggregates induced by the effects of aggregation blockage and aggregate breakdown by slaking and stripping. For the LTP treatment, however, ext-P concentration was independent of aggregate size, indicating that a uniform P distribution in aggregates can be reached given sufficiently long equilibration time.

Research supported by USDA Section 406, Grant Agreement No. 91-34135-6177.

Received for publication November 13, 1996.

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