Simulated long-term effect of wheat cover crop on soil nitrogen losses from no-till corn-soybean rotation under different rainfall patterns

Soil nitrogen dynamics is affected by the interaction of cover crops and rainfall amount and distribution in rainfed agricultural production systems. There is a lack of understanding on soil nitrogen losses from cover crop-based based cropping systems receiving different rainfall amounts. A simulation model, RZWQM2 (Root Zone Water Quality Model), was calibrated and validated with field measurements, and used to determine the long-term effect of a winter wheat (Triticum aestivum L.) cover crop on soil nitrogen balance in an 80-yr (1938–2017) no-tillage and rainfed corn (Zea mays L.)–soybean (Glycine max L.) rotations in northeastern Mississippi, United States. Based on annual soil nitrogen dynamics, the model also estimated nitrate losses to deep percolation during wheat, corn, and soybean growth periods under different seasonal rainfall patterns. A record of seasonal rainfall for 80 consecutive years (1938–2017) was classified as ‘wet’, ‘normal’, ‘dry’ years using frequency analysis approach, and the classifications were matched chronologically to each of these three crop growth periods as model input parameters. Results showed that, mean annual net nitrogen mineralization and annual denitrification were respectively 15% and 9% higher under wheat cover crop (WCC) than under winter fallow (WF) across 80 simulation years. Average annual nitrate leaching was 20% less in the WCC than in the WF system (68.2 vs. 84.8 kg N ha−1). The difference in annual nitrate leaching between the two treatments is equivalent to 17% of the total fertilizer nitrogen-applied annually for corn-soybean system. Compared to WF, the WCC system during autumn to spring (early October to early April) reduced simulated annual nitrate leaching losses by 19.6, 18.6, and 8.2 kg N ha−1 in wet, normal, and dry years, respectively. Long-term integration of wheat cover crop into a no-till corn-soybean rotation is a promising agronomic practice for reducing nitrate leaching and also fertilizer-nitrogen input in the humid northeastern Mississippi, especially in wetter fallow seasons from early October to early April.