Transport and fate of nitrate within soil units of glacial origin

Abstract

Questions concerning the influence of soil type and crop cover on the fate and transport of nitrate (NO 3 − ) were examined. During a growing season, soils derived from glacial material underlying either corn or soybeans were sampled for levels of NO 3 − within the pore water. Measured levels of NO 3 − ranged from below detection limit to 14.9 g NO 3 − per kilogram of soil (g/kg). In fields with the same crop cover, the silty-clayey soil exhibited a greater decrease in NO 3 − levels with depth than the sandier soil. Crop uptake of NO 3 − occurs within the root zone; however, the type of crop cover did not have a direct impact on the fate or transport during the growing season. The soils underlying soybeans had an increase in NO 3 − levels following harvest, suggesting that the decomposition of the soybean roots contributed to the net gain of NO 3 − in the shallow soil. For all of the soil types, conditions below 100 cm are conducive for microbial denitrification, with both a high water saturation level (>60%) and moderate organic carbon content (1–2%). At depths below 100 cm, temporal differences in NO 3 − levels of over a magnitude, up to a 95% reduction, were recorded in the soil units as the growing season progressed. Physical properties that control the transport of NO 3 − or denitrification have a larger influence on NO 3 − levels than crop type.