Cover crops in organic cotton influence greenhouse gas emissions and soil microclimate

Abstract

Cover crops in organic cotton systems can offset the carbon loss typically observed in conventional systems. However, their effects on greenhouse gas (GHG) emissions and soil microclimate are poorly understood. Our objective was to investigate the effects of cover crops on soil carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄) emissions and soil moisture and temperature dynamics in organic cotton systems. To achieve this, we used static chamber techniques with soil sensors in a field study near College Station, TX, from 2020 to 2022. Cover crops tested were oat (Avena sativa L.), Austrian winter pea (Pisum sativum L.) (AWP), turnip (Brassica rapa subsp. rapa), a mixture of all three, and a fallow control. In the first year of organic transition (2020), mixed species treatment enhanced CO₂ emission by 39.6%, 34.4%, and 40% than AWP, turnip, and control, respectively. Compared to the control, N₂O emissions were lower in AWP, turnip, and oat treatments by 77%, 57.2%, and 53% in 2020. Weed pressure and drought in 2021 and 2022 neutralized cover crops’ effect on soil GHG emissions. Soils generally acted as net CH₄ sinks, but the uptake did not differ among the treatments. Cover crops depleted soil moisture during their growing period, but surface residues helped retain more moisture during the cotton season. Compared to fallow, mixed species and AWP were observed to reduce soil temperature fluctuations. Therefore, in transitioning, organic systems effects of cover crops on soil GHG emissions can vary depending on weather, weed management, and the cover crop types.