Grandy, A. Stuart (Michigan State University, W.K. Kellogg Biological Station, 3700 E, Gull Lake Drive, Hickkory Corners, MI, 49060; Phone: 269-671-2336; Email: grandya1@kbs.msu.edu)

Physical and Microbial Controls over Trace Gas Fluxes Following Initial Cultivation

A.S. Grandy *, T.D. Loecke, G.P. Robertson

Tillage reduction can contribute to soil carbon sequestration and greenhouse gas abatement in agriculture by slowing the turnover of organic C and N. An unanswered question, however, regards the stability of stored C and N following subsequent tillage of soils at maximum soil organic matter equilibrium. We investigated the responses of CO₂ and N₂O fluxes, nitrifier and denitrifier enzyme dynamics, and soil C and N pools for two years immediately following initial cultivation of a previously uncultivated midsuccessional community in Southwest Michigan. Carbon dioxide fluxes increased immediately after cultivation and remained at levels greater than 150% of those in reference sites. Increased CO₂ fluxes were associated with the release of particulate organic matter from protected anaerobic microsites within aggregates, which declined between 30 and 50%. Nitrous oxide fluxes increased from an average of about 0.8 to 3 g N₂O-N ha^-1 d^-1 following cultivation and were related to substantial soil nitrate increases. The distribution and activity of nitrifier and denitrifier enzymes were additionally altered by cultivation and may indicate a microbial-level control over trace gas flux changes. These results suggest that soil carbon and nitrogen change following the onset of cultivation are faster than commonly assumed and suggest further that recently sequestered soil carbon may be at significant risk for loss upon the resumption of tillage or other soil disturbance.