Linking remote sensing with crop modeling for yield and nitrate leaching predictions in Minnesota

Upscaling crop yield and nitrate-N leaching loss from experimental sites to large areas under alternative crop rotations is crucial for assessing strategies and setting goals to protect groundwater quality at a regional scale. Nitrogen (N) rate field trials were used to calibrate the Environmental Policy Integrated Climate (EPIC) model for continuous-corn (Zea mays L.) (C-C), corn-soybean (Glycine max L.) (C-Sb), and alfalfa (Medicago sativa L.)-corn (A-C), with or without rye (Secale cereale L.) cover crop. Satellite estimates of crop evapotranspiration (ET_c) were used to upscale the EPIC model for crop yield and nitrate-N leaching, using the irrigation-water permitting data from 2010 to 2017 for 13,375 ha of sandy soils in Bonanza Valley, central Minnesota. Four alternative management scenarios were evaluated with EPIC: (1) reducing N fertilizer rate from the maximum return to N value (MRTN) (of 0.05 to a value of 0.1 (for the N price/crop value ratio), (2) adding rye cover crop at MRTN of 0.1, (3) irrigating with EPIC auto-trigger in scenario 2, and (4) converting 50% of C-C acreage in scenario 3 to A-C. Nash-Sutcliffe coefficients, normalized root-mean-square error, and R² values based on ET_c/crop yield for calibration and validation of the EPIC model ranged 0.95-0.54, 4.67-19.4, and 0.96-0.74; and 0.74-0.41, 7.99-23.4, and 0.88-0.55, respectively. Results indicate that corn yield at MRTN of 0.05 averaged 12.5, 13.2, and 13.4 t ha^-1 under C-C, C-Sb, and A-C rotations, while yields at MRTN of 0.1 were reduced by 4.1%, 3.5%, and 3.3%, respectively. The baseline scenario of C-C, C-Sb, and A-C rotations at MRTN of 0.05 had annual nitrate-N leaching losses of 51.8, 45.5, and 31.4 kg ha^-1, while MRTN of 0.1 reduced these losses by 9.1%, 5.0%, and 3.8%, respectively. Rye after corn and soybean reduced nitrate-N leaching losses in the MRTN of 0.1 scenario by 5.8% and 13.6%, respectively. EPIC auto-irrigation of corn, soybean, and alfalfa at MRTN of 0.1 reduced nitrate-N leaching losses with rye (relative to conventional irrigation) by 9.6%, 9.1%, and 8.5%, respectively. Further, replacing half of the C-C acreage with A-C rotation would provide a 6.1% reduction, resulting in a total reduction of 27.4% in nitrate-N leaching to groundwater when all alternative practices are combined. Overall, augmenting EPIC model with field-observed ancillary data and remote sensing successfully predicted the yield and NO₃-N leaching losses under different crop rotations, indicating opportunities to upscale field-scale agroecosystem simulations, particularly if used to calculate NO₃-N leaching on a long-term basis at the regional scales.