Microbial Production and Consumpution of Nitrate in an Annual Grassland

Abstract

Gross nitrification rates (calculated by 1 5 N pool dilution) ranged from 12 to 46% of gross mineralization rates during the growing season of annual grasses. Pools of NH 4 / and NO 3 — (measured as N) remained below 7 and 4 mg/g soil, respectively, but turned over about once a day. Microbial assimilation of NO 3 — occurred at rates similar to previous estimates of plant uptake. hence two common assumptions, that nitrifying bacteria are poor competitors for NH 4 / and the microbial immobilization of NO 3 — is insignificant, are not correct for this grassland system. Soil heterogeneity probably results in NH 4 / availability to NH 4 / oxidizers at some microsites, while NO 3 — assimilation by heterotrophic microorganism occurs at other microsites where NH 4 + is not available. Relatively high rates of NO 3 — production and consumption in an ecosystem with an annual mean hydrologic loss of NO 3 — —N of only 3.3 kg/ha indicate the importance of NO 3 — 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 NH 4 / 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 compartmentalization of sites of production and consumption of inorganic—N, along with diffusional constraints among such microsites, appear to be critical factors affecting N—cycling characteristics of the ecosystem.