Grassland and forest soil nitrogen transformations and microbial function in the North American Midwest

The extent to which land use influences soil organic matter processing, nitrogen transformations, and the generation of CO₂ and N₂O in mesic grasslands is unclear. To quantify how land management influences these processes, we examined soil nitrogen availability, N₂O production, denitrification potential, and microbial biomass in grasslands and afforested grasslands in northeastern Kansas. Measurements were obtained every 3-4 weeks from June 2006 to the present, from plots representing five land management techniques. Nitrogen availability was greatest in all fertilized plots (14.3±0.8 μg_N g^-1soil); hayed plots had the lowest available N levels (4.4±0.3 μg_N g^-1soil). Differences between fertilized plots and all other land management practices for inorganic N were the greatest during the growing season; inorganic N was reduced and differences were minimized by December 2006. In fertilized plots, NO₃^- availability explained 87.0% of the variation in N₂O fluxes. Early succession, “old field” grasslands experienced the highest N₂O fluxes, with a maximum flux rate of 3.4 μg_N2O-N m^-2 d^-1. Denitrification potential data suggest that wooded plots possess similar potential for denitrification as fertilized grasslands.  Microbial biomass C:N ranged from 72.1 to 11.8, and fluctuated with season; values peaked in August for early succession and hayed grassland soils, and were relatively low (~30) for fertilized and wooded plots. These data suggest that mesic grasslands subjected to fertilization and afforestation experience changes in microbial structure that can directly influence soil N transformations.  Both fertilization and woody succession, processes occurring widely at the prairie/forest ecotone, can result in relatively high fluxes of N₂O.