Soil nutrient variability and groundwater nitrate-N in agricultural fields

Landscape may result in uneven nutrient loads within a field. The objective of this study was to determine the influence of landscape on soil carbon and nutrient levels, and on levels of nitrate-N in groundwater. Soil samples were collected in three fields, two transects each, 30 sites in each field. The soil morphology was characterized for the profile, and soil organic carbon and nutrient levels were determined for 0-0.15 and 0.15-0.3 m depths. Each field had wells installed at three of the sites. One field showed a wide range of landscape variability, and significant effects of curvature on soil carbon and nutrient levels. Another field showed no significant effect of slope or curvature on soil carbon and nutrient levels because the nutrient levels were quite variable, including high spikes. The third field had less variable landscape trends but still showed a few significant effects on soil carbon and nutrient levels. Nitrate-N levels remained high in two of the nine wells (20 to 50 mg L^-1), suggesting that additions of nitrate-N at the concave or converging sites replaced any losses. Median nitrate-N levels at the other seven well sites were lower, ranging from 8 to 17 mg L^-1. Influence of landscape on soil carbon and nutrients was more detectable when the landscape factors were highly variable without excessive variability in soil nutrient properties.