Phosphorus Transport in Runoff from a Small Agricultural Watershed

Abstract

Impacts of phosphorus (P) discharges on receiving waters depend on the magnitude of loadings and their P bioavai lability which varies for various phosphorus forms. To advance the understanding of P transport in agricultural runoff, the main nonpoint source of P, seasonal and short-term variabilities in P forms were studied. Total phosphorus (TP), total paniculate phosphorus (TPP) and total suspended sediment (SS) concentrations were measured in runoff from the Nissouri Creek agricultural watershed. About 54% of TP in runoff was associated with sediment particles (>0.45 µm). The remaining 46% of TP was transported in the dissolved form, as soluble P (SP). Significant correlations were found between TPP and the inorganic fraction of suspended sediments (IS), and SP and the organic matter concentration in suspended sediments (OS). None of the phosphorus forms studied correlated well with runoff volumes. TP, SP and TSS concentrations varied substantially during runoff events and during the year. Higher proportion of paniculate P were typically observed at the time when runoff volumes were high, often during the early phase of runoff. Highest TP and SP concentrations were measured in spring and fall runoffs exceeding those in summer runoffs by an order of magnitude. The suspended sediments concentrations in spring and fall runoffs were, however, two orders of magnitude higher than those in the summer runoffs.