Phosphorus Transport from a Field Receiving Long-Term Liquid Dairy Manure Application in a Karst Landscape

Agricultural runoff is a leading non-point source contributor to water quality impairment in the United States and is associated with eutrophication of surface waters. Phosphorus (P) is often the most limiting nutrient for eutrophication in freshwaters. The objectives of this study were to characterize the P forms in surface runoff from an agricultural field that has received long-term applications of liquid dairy manure and to determine the forms of soil P that occur within a sinkhole feature located within the application field. Three 21-m x 6 m bermed plots were established to collect storm water runoff from a portion of the study site which drains into the sinkhole. The runoff collected was analyzed for total P (TP), dissolved reactive phosphorus (DRP), total dissolved P (TDP), dissolved organic phosphorus (DOP), and particulate P (PP). Soils were sampled from various elevations within the sinkhole feature and were analyzed for TP, total organic P (TOP), Mehlich-3 extractable P, and the maximum P sorption capacity. The results showed a precipitous increase in TP of surface runoff after manure application, from 2.2 g ha-1 to 21.9 g ha-1. The majority of P leaving the field as runoff prior to manure application was associated with the PP fraction (63% of TP). Surface runoff from two rainfall events occurring after diary manure was applied consisted predominately of DRP (67% of TP). Dissolved reactive P in runoff ranged from 0.02 to 0.2 mg L-1 before manure application and 7.1 to 17.1 mg L-1 after application. It has been reported that DRP concentrations in the low ìg/L range can negatively stimulate aquatic vegetation growth in P-limited waters, which indicates the runoff leaving the study site has the potential to impair water quality. A statistically significant difference existed for soil TP concentrations at different elevations within the sinkhole feature, ranging from 3116 mg kg-1 in the base to 914 mg kg-1 in the higher elevations of the sinkhole. A slight increase in TOP concentrations from the base (407 mg kg-1) to the higher elevations of the sinkhole (513 mg kg-1) occurred. The measured maximum P sorption capacity of soils within the sinkhole feature showed that these soils can potentially sorb between 284-379 mg kg-1 of P indicating that the soils are not P saturated and are capable of fixing additional P from runoff