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Using Vadose Zone Instruments and Post-Harvest Soil Nitrogen to Evaluate Nutrient Transport at an Agricultural Field Site
Citation:
Hutchins, S., J. Weitzman, Jacqueline Brooks, J. Compton, B. Faulkner, S. Pearlstein, R. Peachey, M. White, AND R. Coulombe. Using Vadose Zone Instruments and Post-Harvest Soil Nitrogen to Evaluate Nutrient Transport at an Agricultural Field Site. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-22/070, 2022.
Impact/Purpose:
This report describes fate and transport of nutrients at an agricultural site planted in corn. It can be used by researchers to design field studies with the goal of conserving nitrate in agricultural settings.
Description:
Water and nutrient transport were evaluated throughout the vadose zone underlying a sweet corn crop grown under different land management strategies from 2016-2020. One purpose of the study was to determine whether the use of an interseeded cover crop would facilitate nutrient uptake and thereby mitigate any potential impacts on groundwater. A second purpose was to ascertain whether post-harvest soil nitrate values could be correlated with vadose zone water nutrient concentrations and used to guide future fertilizer application rates to minimize groundwater impacts. A two-acre field was divided into two subfields and a monitoring system was placed in the center of each. The monitoring system for the south subfield consisted of a center well and three replicate lysimeters at three depths each. The monitoring system for the north subfield was identical, except that replicate soil moisture probes and tensiometers were also included at those three depths. Groundwater and lysimeter pore water samples were collected biweekly and analyzed for a suite of parameters, including standard water quality measures, nutrients, cations, and stable isotopes. Post-harvest soil cores were obtained each year in Sept 2016-2019 (WY16-WY19) from three to four depths at six locations around each of the two monitoring systems and analyzed for nutrients. Sweet corn was planted in either Jun or Jul each year from WY16-WY20. Following the first year of corn production with no cover crops in WY16, the corn in the north subfield was interseeded with either a Steptoe barley, triticale, or tall fescue cover crop on an annual basis, allowing comparison of the north field with cover crop and the south field without cover crops during the fall and winter. The field was irrigated throughout the summer growing season through fixed irrigation lines equipped with sprinklers. In Jun WY19 a bromide tracer test was conducted for one of these irrigation events through the established irrigation system to track the movement of irrigation water through the vadose zone. The field site setting was quite heterogeneous and exhibited some complex hydrogeologic characteristics that complicated data interpretation regarding the effects of cover crops and the transport of water and nutrients. Data from the bromide tracer study and water stable isotopes do not indicate uniform flow of water through the vadose zone, but instead indicated the potential for substantial channeling of water through preferential flow paths. Nitrate and phosphate concentrations were highly variable in the two center wells even before initiation of the cover crops in WY17. This variability and groundwater difference between the two wells precludes using the water quality data from these wells to assess the impact of the interseeded cover crop. Nitrate concentrations within the lysimeters tended to decrease with lysimeter depth, but nitrate variation between lysimeters at the same depth was very high. Evaluation of lysimeter nutrient data, whether from individual lysimeters or averaged values within each subfield, showed little difference in nitrate, phosphate, total phosphorus, or total Kjeldahl nitrogen concentrations between the north subfield interseeded with the cover crop or the south subfield without a cover crop. One of the objectives in this study was to determine whether post-harvest soil nitrate values could be correlated with lysimeter nitrate concentrations to better assess the impact for groundwater contamination. There was clearly no positive correlation between soil concentrations versus pore water concentrations. The uncertainty in the time required for infiltrating water to reach groundwater, coupled with the presence of preferential flow paths, precludes the direct use of post-harvest soil nitrate values in one year to predict groundwater nitrate concentrations in subsequent years for complex conditions such as those at this field site.