Agricultural Water Management 97 (2010) 1855–1860 Contents lists available at ScienceDirect Agricultural Water Management journal homepage: www.elsevier.com/locate/agwat Effects of changes in N-fertilizer management on water quality trends at the watershed scale Vinay Nangia a,∗ , Prasanna H. Gowda b , D.J. Mulla c a Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada, Ottawa, ON K1A 0C6, Canada b USDA-CPRL-ARS, PO Drawer 10, Bushland, TX 79012, United States c Department of Soil, Water, and Climate, 1991 Upper Buford Circle, 439 Borlaug Hall, University of Minnesota, Saint Paul, MN 55108, United States article info Article history: Received 10 March 2010 Accepted 30 June 2010 Available online 31 July 2010 Keywords: Tile drainage South-central Minnesota Climate change Nitrate-N loss abstract In this study, the ADAPT (Agricultural Drainage and Pesticide Transport) model was calibrated and val- idated for monthly flow and nitrate-N losses, for the 2000–2004 period, from two minor agricultural watersheds in Seven Mile Creek (SMC-1 and SMC-2) in south-central Minnesota. First, the model was calibrated and validated using the water quality data from the SMC-1 and again independently validated with the SMC-2 dataset. The predicted monthly flow and associated nitrate-N losses agreed reasonably with the measured trends for both calibration (r 2 = 0.81 and 0.70 for flow and nitrate-N losses, respec- tively) and validation (r 2 = 0.85 and 0.78 for flow and nitrate-N losses from SMC-1, and 0.89 and 0.78 for flow and nitrate-N losses from SMC-2, respectively) periods. The model performed less satisfacto- rily for the snowmelt periods than it did for the entire simulation period. Using the calibrated model, long-term simulations were performed using climatic data from 1955 to 2004 to evaluate the effects of climatic variability and N application rates and timing on nitrate-N losses. The predicted nitrate-N losses were sensitive to N application rates and timing. A decrease in the fall N application rate from 179.3 to 112 kg/ha decreased nitrate-N losses by 23%. By changing application timing from fall to spring at a rate of 112 N kg/ha, nitrate-N losses decreased by 12%. The predicted nitrate-N losses showed a linear response to precipitation with larger losses generally associated with wet years. A 25% increase in mean annual precipitation would offset reductions in nitrate-N loss achieved using better N fertilizer management strategies described above. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Amendments of nitrogen fertilizer improve crop yield up to a point, but excess application can be harmful to the environment. In addition to raising local water quality concerns, excess nitrate-N losses from Midwestern U.S. agricultural regions have been linked to hypoxia in the Gulf of Mexico (Burkart and James, 1999; Rabalais et al., 2001; Jaynes et al., 2001; Randall et al., 2003). High nitrate-N losses are associated with cropland with subsurface tile drainage systems that receive fall applications of fertilizer N at rates in excess of plant uptake requirements (Baker and Johnson, 1981; Baker and Melvin, 1994). Nitrate-N loads transported to surface water through subsurface tile drainage systems are a function of transport volume (amount of water) and nitrate-N concentration in the transported water. The amount of drainage water leaving the landscape is largely a function of climate, soil properties, and tile drainage depth, spac- ∗ Corresponding author. E-mail address: vinay.nangia@agr.gc.ca (V. Nangia). ing and intensity. Drainage is further influenced by the temporal distribution of precipitation within a particular year. Numerous field studies have shown that the corn (Zea mays L.)- soybean [Glycine max (L.) Merr.] rotation contributes significant losses of nitrate-N to subsurface tile drainage waters (Goldstein et al., 1998; Dinnes et al., 2002). In a four-year drainage study in Minnesota, flow weighted nitrate-N concentrations averaged 28 mg/L for continuous corn, 23 mg/L for a corn–soybean rotation, and <2 mg/L for alfalfa (Medicago sativa L.) and grass perennial crops (Randall et al., 1997). Precipitation and cropping system have the greatest impacts on nitrate-N losses from agricultural landscapes to surface waters (Randall and Mulla, 2001). A wide range of Best Management Practices (BMPs) have been studied for their impact on nitrate-N losses (Mulla, 2008). Randall et al. (2003) studied nitrate-N losses from a tile-drained Canisteo clay loam soil in southern Minnesota, and showed that nitrate-N losses from a corn–soybean rotation can be reduced by from 13 to 18% by either applying N in the spring or using nitrapyrin (NP) with late-fall applied ammonia. Davis et al. (2000) simulated long- term (1915–1996) nitrate-N losses in subsurface tile drainage for Minnesota climatic conditions at the plot scale using the ADAPT 0378-3774/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2010.06.023