Soil Greenhouse Gas Emissions Under Long-Term Conservation Management in a Continuous Cotton Cropland in West Tennessee.

Conservation practices such as no-tillage and cover crops are thought to help mitigate atmospheric greenhouse gas (GHG) concentrations through building soil organic matter. However, some studies have shown that both no-till and cover crops can increase GHG emissions. It is possible that these results are confounded by perturbations caused when management practices are newly implemented. There is a clear lack of data from long-term sites where experimental plots are sell equilibrated to the management systems. Starting in 2016, we measured fluxes of nitrous oxide (N₂O), methane (CH₄) and carbon dioxide (CO₂) in twelve combinations of tillage (disk, no-till), N fertilizer rate (0, 67 kg N ha^-1), and winter cover crops (none, hairy vetch, winter wheat) under continuous cotton production for 35 years. During the cotton growing season, the largest daily fluxes of N₂O (36.9±11.9 g N ha^-1 d^-1) occurred in tilled plots regardless of cover crop or fertilization rate. However, over the entire year, the largest fluxes were observed during winter cover crop growth (63.0±21.4 g N ha^-1 d^-1). Overall, N₂O fluxes were lower in no-till compared to tilled soils, save those under hairy vetch cover crop. These results, combined with our observation of higher rates of net mineralization and net nitrification in no-till and vetch plots, suggest vetch cover crops may stimulate both GHG and inorganic N production. We observed seasonal patterns in CH₄ flux with net CH₄ production during Spring and early Summer (from 0.2±0.8 to 4.8±3.2 g C ha^-1 d^-1), switching to net CH₄ consumption by late summer (from -6.3±3.4 to 0.8±0.5 g C ha^-1 d^-1). Our results highlight the need for improved understanding of the soil microbial and physical processes driving N and C transformations, and the interactive effect of conservation management practices.