Tuesday, November 14, 2006
198-2
Estimating N Losses from Agricultural Fields (NLEAP).
Marvin Shaffer, Shaffer Consulting, 8225 Firethorn Dr, Loveland, CO 80538, Ronald Follett, USDA-ARS, Soil Plant Nutrient Research, 2150 Centre Ave, Bldg D, Fort Collins, CO 80526-8119, and James Wallace, Colonial Soil & Water Conservation District, P.O. Box 190, 2502 New Kent Highway, Quinton, VA 23141.
Nitrogen (N) losses from farm fields are difficult to measure both temporally and spatially. Modeling provides a practical method to make estimates of N losses for the large number of soil-crop management-climate conditions that exist across farms in an area or region. The Nitrogen Loss and Environmental Assessment Package (NLEAP) model was applied to farms in eastern Virginia with corn-soybean (fs) and corn-small grain-soybean (ss) rotations (mostly notill) on several coarse-textured soils for five years. Extensive prior information (past 5 years) on crop rotations, nutrients applied, tillage, and crop yields were collected for each field. Corresponding daily precipitation and air temperature records were obtained for the same 5-year period from the Richmond climate station. Model simulations were run for each field over the period of record and simulated soil residual NO3-N values were compared with observed values collected near the end of this period (R2 =0.6). N losses for NO3-N leached, N2O soil gas emissions, NH3 volatilized, and surface N washoff were simulated monthly for the 5 years. N loss events happened intermittently, but distinct pulses with significant NO3-N leaching occurred in some autumn and spring months after significant precipitation and during summers after occasional very large storm events. N2O emissions showed small pulses, but these were mostly associated with spring and summer applications of ammonia-containing fertilizer. Likewise small spikes of NH3 volatilization occurred after surface applications of ammonia fertilizers. NO3-N leaching was associated with specific storms and moved elevated residual soil NO3-N from fertilizer applications or carry-over from previous crops plus mineralization of soil organic matter. For these coarse textured soils, management to minimize soil residual NO3-N appears to be important at all times given the potential for major storm events. The management scenarios showing the lowest long-term NO3-N leaching were those that generally minimized soil residual NO3-N.