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

Abstract

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.