Benchmarking impact of nitrogen inputs on grain yield and environmental performance of producer fields in the western US Corn Belt

Benchmarking crop yields against nitrogen (N) input levels can help provide opportunities to improve N fertilizer efficiency and reduce N losses on maize in the US Corn Belt by identifying fields most likely to benefit from improved N management practices. Here, we evaluated a large producer database that includes field-level data on yield and applied N inputs from 9280 irrigated and rainfed fields over a 7-year period (2009–2015) in Nebraska (USA). A spatial framework, based on technology extrapolation domains, was used to cluster each field into spatial units with similar climate and soil type that represent 1.3 million ha of US farm land sown annually with maize. Three metrics were employed to evaluate agronomic and environmental performance: partial factor productivity for N inputs (PFPN, ratio between yield and N inputs), N balance (difference between N inputs and grain N removal), and yield-scaled N balance (ratio between N balance and yield). Nitrogen inputs included N from fertilizer and N contained in applied irrigation water. Average yield and N inputs were 40 and 44% higher in irrigated versus rainfed fields. The N balance was ca. 2-fold greater in irrigated versus rainfed fields (81 versus 41 kg N ha⁻¹). Of the total number of field-years, 58% (irrigated) and 15% (rainfed) had N balance ≥ 75 kg N ha⁻¹, which was considered a threshold to identify fields with potentially large N losses. Very large (> 150 kg N ha⁻¹) and negative N balance estimates were not apparent when analysis was based on field averages using a minimum of three years' data instead of individual field-years. Nitrogen balance was smaller for maize crops following soybean compared to continuous maize. Despite the larger N balance (on an area basis), irrigated fields exhibited smaller yield-scaled N balance relative to rainfed fields. The approach proposed here can readily be adopted to benchmark current use of N fertilizer for other cereal-based crop systems, inform policy, and identify opportunities for improvement in N management.