Soil Nitrogen Dynamics Under Conservation Management in Continuous Cotton in West Tennessee.

Conservation management practices can reduce soil nitrogen (N) exports from agroecosystems. To better understand how conservation practices impact the fate of N compared to conventional management, this study assessed how tillage (disk, no-till), N fertilizer rate (0, 67 kg N ha^-1), and winter cover crops (none, hairy vetch, winter wheat) affected soil N pools and transformation rates in a long-term continuous cotton system. All treatments were in a randomized complete block design replicated four times, and management systems have been in place since 1981. During 2016, total extractable N was similar across management treatments, though soil inorganic N (NO₃ and NH₄) levels were higher in vetch plots. Higher rates of N mineralization and nitrification no till and vetch plots suggested a combined management effect on N transformation rates. Microbial biomass N (MB_N) pools in tilled treatments increased rapidly between the cover crop kill and two weeks after cotton planting, whereas MB_N pools in no till treatments gradually increased and did not peak until two weeks before cotton harvest. This suggests a tillage effect on net N microbial immobilization rates, except in the no-till, 67 kg-N wheat cover treatment. These data indicate that no till combined with hairy vetch winter cover can increase labile soil N. Although higher concentrations of inorganic N were measured under 67 kg-N ha^-1 application, the implications of N loss via leaching or N₂O emissions have not yet been assessed. Further, our results suggest that disk tillage may contribute higher rates of MB_N uptake at the time of cotton planting, which may significantly influence the fate of immobilized N during the crop season. This hypothesis will be explored with future analysis of plant N uptake and microbial enzyme activity.