Analyses of Agricultural Nutrient Sources of Impairment and Effects on Downstream Waters (SSWR 4.02C.1-b)

Project Leads:  Stephen R. Hutchins, Barton R. Faulkner

Project Duration:  10/01/15 – 09/30/19

Background:  Some states are using post-harvest deep soil nitrate values to help determine whether ground waters underlying agricultural fields are at risk from contamination by nitrate and to promote more effective use of chemical and organic fertilizers.  Thresholds of 45 ppm NO3-N have typically been used, but the number can vary from state to state, as does the depth from which the samples are acquired.  Further, there has been little rigorous testing under common agricultural practices to determine whether these threshold levels are truly protective of ground water.  The research objective is to determine whether a simple metric like post-harvest soil nitrate values can be used to establish threshold values so as to preclude ground water contamination for the protection of ground and surface water quality.

Research Approach:  This will be done by an evaluation of data collected from the operation of two separate field studies, one in Oregon and one in Oklahoma.  The field studies will be designed to not only include ground water monitoring wells and post-harvest soil core collections, but also vadose zone instrumentation, including lysimeters, soil moisture probes, and tensiometers to monitor transport of water and nutrients throughout the year.  This monitoring network will be installed in each of two subfields at each site, so that differences in land management strategies can also be evaluated at each site.  At the Oregon site, a comparison will be made between one subfield planted in sweet corn and the other subfield planted in sweet corn with an interseeded cover crop.  At the Oklahoma site, both subfields will be planted in a rotation of sorghum and winter wheat, and a comparison will be made between conventional application of fertilizer and precision application of fertilizer using enriched N strips with sensor monitoring for optimizing fertilizer application.  A secondary research objective, then, is to determine whether changes in land management practices at each site will have effects on transport of nutrients into ground water.  Water samples will be collected biweekly or monthly from wells and lysimeters, and will be analyzed not only for nitrogen and phosphorus, but for stable isotopes and other water quality parameters.  This information will be correlated with sensor readings from tensiometers and soil moisture probes to characterize the transport of water and nutrients through the vadose zone within each subfield.  Field monitoring will continue for three years to evaluate seasonal and annual effects.

Research Collaboration:  GWERD researchers will be collaborating with NHEERL’s Western Ecology Division and Oregon State University researchers at the Oregon site, and with Oklahoma State University researchers at the Oklahoma site.

Research Products:  A comprehensive EPA report will be published in September 2019 that will cover all aspects of the research at the two field sites.  It is expected that additional journal articles will be published focusing on the vadose zone transport studies.