Long-term cover cropping and nitrogen fertilization impacts on net global warming potential of continuous no-till...

The introduction of cover crops, owing to their positive effects on soil organic carbon (SOC) sequestration, is a potential management practice that can mitigate agricultural greenhouse gas emissions. In this study, we leveraged a 42-year-old continuous cotton (Gossypium hirsutum L.) experiment under no tillage, to evaluate the effect of hairy vetch (Vicia villosa; HV) and no cover crop (NC) under N rates of 0 (no fertilizer [NF]) and 67 kg N ha^-1 (fertilized [F]), on net global warming potential (GWP) and greenhouse gas intensity (GHGI). The annual SOC sequestration rate was not significantly different in the F (115.1 kg ha^-1 year^-1) and HV (107.4 kg ha^-1 year^-1) treatments compared to the NF (103.2 kg ha^-1 year^-1) and NC (110.8 kg ha^-1 year^-1) treatments. Soil under HV and F treatments behaved as a net source of GHGs in 2022, with a GWP of 243 and 294 kg CO₂-eq ha^-1 year^-1, respectively. By contrast, in 2023, these treatments were net sinks of GHGs. Despite the increase in cotton lint yield under legume cover cropping and N fertilization, the GHGI followed the same trend as the net GWP, being net source of GHG in 2022 and a net sink in 2023. Nearly all estimated C gains were offset by N₂O emissions under these treatments in 2022-2023. Our results indicate that GHG mitigation through the adoption of legume cover cropping within cotton systems in humid subtropical climates is constrained by low soil C sequestration potential and elevated N₂O emissions.