Nitrate and water under terraced dryland wheat production in Oregon

Dry land agriculture using summer fallow is a common crop productionpractice in the Columbia Plateau region of eastern Oregon. Farmed-over levelterraces are used to control surface water runoff and soil erosion. More than70 percent of the average annual precipitation around Pendleton, Oregon (350 -400 mm) falls as low intensity, long duration rainfall from September to March.Wetter soil zones typically occur above and below the terrace. These areashave a higher potential for crop production as well as for movement ofchemicals to ground water and to surface water where seepage occurs. Theextra nitrogen or water that could accumulate in these areas needs to beconsidered in managing these areas.The first objective of this study was to measure the distribution of nitratenitrogen (NO₃-N) and water in relation to farmed-over level terraces, and inferpotential solute flow patterns from changes in the measured distributions overtime. The second objective was to make recommendations regardingmanagement practices required for specific field locations to maximize cropproduction and minimize negative impacts on groundwater quality.Results indicate NO⁻₃ concentrations following harvest were < 4 mg kg⁻¹ of soil. Equivalent to soil solution concentrations between 27 and 20 mg L⁻¹ at 15 and 20 percent volumetric water content, respectively. Limited deep percolation of NO⁻₃ occurred below the root zone between harvest and planting. The NO⁻₃ concentrations below the root zone were < 1 to 15 mg kg⁻¹ following the summer fallow period. In August 1993, evidence exists that shows N applied fertilizer moved out of the surface 0.3 m and deeper into the profile. The redistribution of NO⁻₃ in the terrace channels of transects 1 and 2 strongly support this. Soil profiles that contain high residual concentrations of NO₃-N during the fallow period increase the potential for NO₃-N leaching below the root zone. Unusually heavy precipitation during normally dry periods or above normal winter precipitation increases the potential for NO₃-N leaching below the root zone.