Long‐term cover cropping and nitrogen fertilization impacts on net global warming potential of continuous no‐till cotton cropping system

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.