The potential impact of soil carbon content on ground water nitrate contamination

A potential buildup of nitrate in the ground water resources of the eastern Sandhills of Nebraska has been projected to occur due to the intensive use of nitrogen fertilizer on irrigated cropland. A root-zone nitrate leaching study in this area revealed that soils with a high carbon concentration had minimal leaching compared to soils with lower concentrations. Soils high in carbon have an active population of denitrifying bacteria possibly causing denitrification and in turn reduction of nitrate leaching. Denitrifying bacteria are principally heterotrophic using soil organic carbon for both an energy and carbon source. The objective of this research was to interpret how root-zone denitrification affected nitrate leaching and ground water contamination by nitrate. A modified version of a solute transport model developed for the Eastern Sandhills was used to assess the risk of nitrate contamination for combinations of fertilizer and irrigation rates and for various soil carbon levels. The first attempt was to make risk assessment with eight farm management practices for cells with increasingly greater carbon levels until only those cells with the greatest carbon level were kept in production. Results of this assessment showed that even with excessive fertilizer and irrigation rates, risk of nitrate leaching was reduced as the minimum carbon level was increased. However, since less cropland was leaching nitrate with each successive risk calculation, the impact that root-zone denitrification had in nitrate leaching reduction could not be definitively determined. This prompted a model modification of the risk calculation procedure which kept all cropland in production and computed nitrate leachate risk for increasingly higher artificial carbon levels during successive risk calculations. Changing carbon levels was still more detrimental on nitrate leaching rates than changing farm management practices. Copyright (C) 1996 IAWQ.