Nitrogen balances and losses in conservation cropping systems across a tile-drained landscape in Ohio, United States

Productions of corn ( Zea mays L.) and soybean ( Glycine max L.) in the midwestern United States are the primary source of nitrogen (N) degrading local and downstream surface waters. Conservation crop rotation involves growing a series of crop phases in a field, reducing fallow periods and enhancing N demand. The objective of this study was to contrast conventional rotations of corn–soybean (CS) with conservation rotations of corn–soybean–winter wheat ( Triticum aestivum L.; CSW) as N management tools using a mass balance approach. We calculated N balances (∑Inputs – ∑Outputs) and loads, as both nitrate-N (NO3−-N) and total N (TN), for fields with CS ( n = 18) and CSW ( n = 12) rotations to examine crop- and rotation-specific patterns of N surplus, deficit, and loss. Using data from all individual years ( n = 169), we found median N balance indicated surplus N in corn phases (CSW-corn: 112 kg N ha−1; CS-corn: 51 kg N ha−1) compared to N deficits in wheat (−1.3 kg N ha−1) and soybean (CS-soybean: −110 kg N ha−1; CSW-soybean: −92 kg N ha−1) phases. Median N loss was least in wheat (8 kg NO3−-N ha−1; 11 kg TN ha−1) and soybean phases (CS-soybean: 18 kg NO3−-N ha−1, 21 kg TN ha−1; CSW-soybean: 17 kg NO3−-N ha−1, 23 kg TN ha−1) and greatest in corn phases (CS-corn: 31 kg NO3−-N ha−1, 35 kg TN ha−1; CSW-corn: 27 kg NO3−-N ha−1, 34 kg TN ha−1). The median of average annual N balance was greater in CSW (14 kg N ha−1) than CS fields (−29 kg N ha−1), yet the medians of average annual N loss were similar (e.g., CSW: 19 kg NO3−-N ha−1; CS: 22 kg NO3−-N ha−1). These results suggest that including winter wheat into the CS rotation may have the potential to address N surplus pools and reduce N loss to downstream waters.