Microbial Production and Consumption of Nitrate in an Annual Grassland

Gross nitrification rates (calculated by 15N pool dilution) ranged from 12 to 46% of gross mineralization rates during the growing season of annual grasses. Pools of NH4+ and NO3 (measured as N) remained below 7 and 4 ,ug/g soil, respectively, but turned over about once a day. Microbial assimilation of NO3- occurred at rates similar to previous estimates of plant uptake. Hence two common assumptions, that nitrifying bac-teria are poor competitors for NH4+ and that microbial immobilization of NO3- is insig-nificant, are not correct for this grassland system. Soil heterogeneity probably results in NH4+ availability to NH4+ oxidizers at some microsites, while NO3- assimilation by heterotrophic microorganisms occurs at other microsites where NH4+ is not available. Relatively high rates of NO3- production and consumption in an ecosystem with an annual mean hydrologic loss of NO3--N of only 3.3 kg/ha indicate the importance of NO3- in the internal N cycle of this ecosystem. Nitrification potential rates, which are an index of population size, declined during the dry season. However, a significant population remained viable when soil water potential was below -9 MPa, indicating that nitrifying bacteria can tolerate severe desiccation. A simple diffusion model demonstrates the dependence of NH4+ availability on soil moisture. Population decline during the dry season may result from both desiccation stress and a lack of substrate availability for maintenance energy of the population. Spatial compart-mentalization of sites of production and consumption of inorganic-N, along with diffusional constraints among such microsites, appear to be critical factors affecting N-cycling char-acteristics of the ecosystem