QUANTIFYING NITRATE TRANSPORT RATES IN THE UNSATURATED ZONE BELOW AGRICULTURAL FIELDS

Each year in Illinois, billions of pounds of nitrogen fertilizers are applied to agricultural fields. Although fertilizer application is necessary to sustain and maximize agricultural production, much of this nitrogen is released into the environment as nitrate. The purpose of this study was to quantify vertical nitrate transport rates below agricultural fields, specifically looking at movement: 1) associated with normal opposed to wet soil conditions; 2) below soybean and corn fields; 3) based on soil grain size; and 4) based on organic matter content of the soil. During the summer and fall of 2004, samples were collected at the Illinois State University Research Farm from three fields, two corn and one soybean, representing three different soil types. Measured nitrate concentrations identified “bulges” that could be traced vertically through the unsaturated zone over the course of the study. Based on the data, nitrate transport rates of 0.62 cm/day to 2.63 cm/day were measured. Most nitrate movement occurred during normal conditions, rather than after heavy precipitation events. The soils where corn was growing had lower nitrate concentrations than the soils growing soybeans. Within the coarser grained soils, the nitrate transport rate was faster. The results also demonstrated that soils high in organic matter content have lower nitrate concentrations than soils with lower organic matter content. Based on the results of the study, a large amount of nitrate is utilized by plants at the beginning of the growing season. Once plants reach a certain growth stage, nitrate is no longer utilized. As the plants expire they begin to decompose releasing nitrogen into the soil. The conversion of the nitrogen to nitrate correlates to elevated nitrate concentrations measured at the end of the growing season. Data collected at each of the sampling location also indicates nitrate being detected throughout the entire soil profile. Deeper nitrate is attributed to residual nitrate from previous growing seasons or possibly from the application of fertilizer at the end of the previous growing season. With no active crop growth, nitrate applied at the end of the previous growing season would be less impeded and would migrate through the soil system.