The uncertainty of wet spring weather, which has the potential to delay maize planting and other field operations, is an important reason that farmers perform land preparation activities such as N fertilizer application the previous fall. In Illinois, the majority of N fertilizer is applied either late in the fall (fall-applied N, FN) in the form of ammonia-usually with nitrification inhibitor-or early in the spring, often as ammonia, before or near the time of planting (spring-applied N, SN). Hence, designing N management practices which increase maize GY while minimizing negative environmental consequences is crucial for this region. At a broader scale, maize production systems in Illinois are located within the Mississippi River Basin and contribute approximately 20% of nitrate loading to the Gulf of Mexico. Excessive N losses contribute to climate warming due to the potential for increased soil nitrous oxide emissions, while also causing nutrient pollution of water resources. Complex interactions among weather patterns, soil properties, crop growth, and N loss pathways make it difficult to synchronize fertilizer management with crop N demand, which may result in under- or over-N application. In Illinois, maize occupies approximately 5.0 million ha (30% of total geographical area), and as a crop receives large amounts of N fertilizer. Maize grain yield (GY) has increased significantly in the past several decades, in part due to more efficient nitrogen (N) fertilizer use in high-yielding varieties. Positive impacts on both GY and NUE occurred in only 60% of simulations for this scenario, highlighting the challenge of simultaneously improving yield and NUE with a 15% N rate reduction in this region. The combined practice of reducing N fertilizer amounts from 224 kg N ha -1 to 190 kg N ha -1 and shifting from FN to SN resulted in a wide range of yield responses during 2011–2015, with the probability of increasing yields varying from 70% of simulation points within a watershed. ![]() Accordingly, differences in NUE for SN compared to FN were small (0.0–1.4 kg GY/kg N) when cumulative winter rainfall was 500 mm at both 168 kg N ha -1 and 224 kg N ha -1. When simulations were scaled up to the watershed level, results suggest that increases in average maize GY for SN compared to FN occurred in years with higher than average winter rainfall (2011, 2013), whereas yields were similar (+/- 4%) in 2012, 2014, and 2015. We simulated the effects of N fertilizer rate (0, 168, 190, 224 kg N ha -1) and application timing on maize grain yield (GY) across 3042 points in Illinois during 2011–2015 using the DSSAT-CERES-Maize model. Although these strategies generally suggest decreasing N rates and shifting the timing of N application from fall to spring, the spatiotemporal impacts of these practices on maize yield and fertilizer N use efficiency (NUE, kg grain yield increase per kg N applied) have not been assessed at the watershed scale using crop simulation models. ![]() Midwest to decrease the environmental footprint associated with nitrogen (N) fertilizer use. ![]() Nutrient loss reduction strategies have recently been developed in the U.S.
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