Two paired plant-soil cores were located at each sampling point and 12 sampling points were located within the field. The available NH4+ of one plant-soil pair was enriched to an abundance of 5% atom excess with (15NH4)2SO4. Available NO3 -was low (0.1 ug/g) when the experiment was initiated and the pool was increased to 1 ug/g with K15NO3. Six replicate cores from each treatment were harvested one week after isotope addition and the remainder was harvested at the end of the growing season. Cores were analyzed for plant total N, soil total N, microbial biomass N, and available NH4+-N and NO3--N and their 15N abundance. Additional samples were collected to measure rates of denitrification using the acetylene inhibition assay. Rates of plant uptake, microbial immobilization and nitrification were calculated using isotope dilution techniques.