Sally D. Logsdon, USDA-ARS National Laboratory for Agriculture and the Environment, Ames, IA
Landscape and management often result in uneven nutrient loads within a field. The hypotheses of this study are that 1) phosphorus accumulates at low areas in the landscape adjacent to waterways, and 2) nitrate at lower landscape positions will be decreased in the subsoil due to denitrification and leaching losses. The purpose of this study was to determine if high nutrient loads accumulate in zones close to outlets (waterways, ditches, tiles, etc.) with potential to contribute to offsite loss of nutrients. Soil was sampled within transects in three fields, and three wells were installed in each field. For two fields, the transects were close to grassed waterways, and runoff was monitored in one of the fields. The data show the hypotheses was not consistently correct. Surface nutrient concentrations related more to recent management than landscape in a field that had recent swine manure application before sampling, and showed a zone of high nutrient load. Another field did show a relation between landscape factors and nutrient levels in the surface soil. However, in this field groundwater nitrate levels were highest in the low elevation site closest to the waterway, contrary to the hypothesis. Groundwater nutrient levels were a result of several factors reflecting both local soil and management, and surface and subsurface lateral transport across the landscape.