PHOSPHORUS TRANSPORT IN RUNOFF FROM A SMALL AGRICULTURAL WATERSHED

Impacts of phosphorus (P) discharges on receiving waters depend on the magnitude of loadings and their P bioavailability which varies for va rious phosphorus forms. To advance the understanding of P transport in agricultural runoff, the main nonpoint source of P, seasonal and shor t-term variabilities in P forms were studied. Total phosphorus (TP), t otal particulate phosphorus (TPP) and total suspended sediment (SS) co ncentrations were measured in runoff from the Nissouri Creek agricultu ral watershed. About 54% of TP in runoff was associated with sediment particles (>0.45 mum). 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), a nd SP and the organic matter concentration in suspended sediments (OS) . None of the phosphorus forms studied correlated well with runoff vol umes. TP, SP and TSS concentrations varied substantially during runoff events and during the year. Higher proportion of particulate P were t ypically observed at the time when runoff volumes were high, often dur ing the early phase of runoff. Highest TP and SP concentrations were m easured in spring and fall runoffs exceeding those in summer runoffs b y an order of magnitude. The suspended sediments concentrations in spr ing and fall runoffs were, however, two orders of magnitude higher tha n those in the summer runoffs.