Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/17/2007
Publication Date: 3/1/2008
Citation: Nangia, V., Gowda, P., Mulla, D., Sands, G. 2008. Water quality modeling of fertilizer management impacts on nitrate losses in tile drains at the field scale. Journal of Environmental Quality. 37:296-307.
Interpretive Summary: Hypoxia in the Gulf of Mexico is a major environmental concern. A reduction in nitrate-N loading by 30% has been recommended to reduce hypoxia in the Gulf of Mexico. Nitrate-N loadings from the Upper Mississippi River Basin accounts for roughly 35% of the nitrate-N entering the Gulf of Mexico, yet this area covers less than 20% of the Mississippi River Basin. This study evaluated alternative best management practices that include varying depth and spacing of subsurface tile drains, and rate and timing of N-fertilizer application to reduce nitrate-N loading at the field scale. Results indicate that it is possible to achieve the recommended reduction in nitrate-N loading by decreasing depth and increasing spacing of tile drains.
Technical Abstract: Nitrate-N loadings entering the Mississippi River from tile drained row crop farms in the Upper Midwest U.S. contributes to hypoxia in the Gulf of Mexico. Strategies are needed to reduce nitrate-N losses to the Mississippi River. This paper evaluates the effect of fertilizer rate and timing and tile drain depth and spacing on nitrate-N losses in two commercial row crop fields (11 ha and 9.3 ha) located in southern Minnesota. The Agricultural Drainage and Pesticide Transport (ADAPT) model was calibrated and validated using monthly flow and nitrate-N losses measured in tile drains from these fields during 1999-2003. For the calibration period, there was good agreement between observed and predicted flow and nitrate-N discharges, with r**2 values of 0.87 and 0.85, respectively. During validation, there was excellent agreement between observed and predicted flow and nitrate-N discharges, with r2 values of 0.92 and 0.91 for an 11-ha field, and 0.94 and 0.90 for a 9.3-ha field, respectively. The calibrated model was used to evaluate the effects of tile drain spacing and depth, and rate and timing of N fertilizer application on nitrate-N losses with a 50 year record (1954-2003) of daily precipitation. Reductions in nitrate-N losses are possible by decreasing depth or increasing spacing of tile drains, and by changing N fertilizer application timing from fall to spring. For instance, for a spacing of 40 m, reducing tile depth from 1.5 m to 0.9 m reduced nitrate-N losses by 31%. For a tile depth of 1.5 m, increasing spacing from 27 to 40 m reduced nitrate-N losses by 50%. For an N application rate of 180 kg/ha (tile spacing 27 m, tile depth 1.5 m), there was an 8% reduction in nitrate-N losses when switching from a fall to spring application. Further reductions in nitrate-N losses require reductions in N application rates. A twelve percent reduction in nitrate losses was found when the application rate was reduced from 180 kg/ha to 135 kg/ha (tile spacing 27 m, tile depth 1.5 m).