SEDIMENT AND PHOSPHORUS DELIVERY FROM ALFALFA SWARDS

Alfalfa fields comprise a significant proportion of land managed by animal feeding operations in the upper Midwest, yet studies of sediment and phosphorus (P) delivery from these systems are sparse. Sediment and P delivery from two alfalfa swards (<0.04 ha) were monitored during 11 natural runoff events (2003-2004) in south-central Wisconsin. Total solids (TS) and P loads were standardized by rainfall erosivity (EI30) after verifying that linear relationships existed between these parameters for the data collected in this study. Standardized TS and particulate P (PP) loads generally exhibited increases during secondary storms (successive storms following an initial storm in a series). These increases were observed regardless of the rainfall erosivity of secondary storms. Such increases were attributed to progressive degradation of the soil conditions, which were detected by the variable representing “erosivity-standardized” TS load (K. ). Results from one series of four events beginning 1 d following manure application were examined using exploratory data analysis and yielded three primary observations. Firstly, increase in the fine-sediment fraction (<2 .m) as this series progressed suggested that broadcast manure application did not significantly protect the soil surface. Lack of protection from manuring was attributed to minimal contact time, or incubation period, between manure and soil, exacerbated by the early onset of runoff immediately following manure placement. Secondly, runoff initially transported primarily dissolved P forms, but the dissolved reactive P (DRP) fraction of total P (TP) declined throughout the series. Finally, a decreasing ratio of volatile to total solids, accompanied by an increasing fraction of fine particles, suggested that manure-derived particulates were exhausted from the surface by the first few events following manure application. Other findings included: (1) significantly greater DRP concentrations during late season runoff (November); and (2) declines in DRP and total dissolved P (TDP) concentration with increased TS delivery. By conducting research using natural runoff, this study strengthened and confirmed results of sediment and P delivery patterns observed in studies undertaken at smaller scales.