Evaluation of N mass balance and soil nitrate as indicators of N leaching to groundwater in a Pacific Northwest dairy grass field
Washington State Department of Ecology
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Dairy farms in the U.S. are expected to use nitrogen (N) mass balance techniques to determine appropriate agronomic manure application rates for forage crops. In addition, soil nitrate sampling is increasingly used as an indicator of the amount of N available at the end of the growing season for leaching to groundwater. We conducted a 4-1/2- year study that quantified the major N inputs, outputs, and residuals (soil and groundwater) at a commercial dairy field overlying a shallow water table aquifer in the Pacific Northwest. A purpose of the study was to evaluate N mass balance and post-harvest soil nitrate for effectiveness in representing effects on groundwater nitrate. A simple spreadsheet model that takes into account hydrologic characteristics of the site was useful for quantifying the mass of nitrate reaching the water table whether originating from the current year’s N application or from internal loading due to past applications. However without groundwater nitrate data for comparison, even 4-5 year means for N mass balance and post-harvest soil nitrate did not correspond with early winter groundwater nitrate, despite intensive sampling of N balance components.