Microbial production and consumption of nitrate in an annual grassland

Gross nitrification rates (calculated by [sup.15.N] pool dilution) ranged from 12 to 46% of gross mineralization rates during the growing season of annual grasses. Pools of [NH.sub.4.sup.+] and [NO.sub.3.sup-] (measured as N) remained below 7 and 4 [mu]g/g soil, respectivelyd, but turned over about once a day. Microbial assimilation of [NO.sub.3.sup.-] occurred at rates similar to previous estimates of plant uptake. Hence two common assumptions, that nitrifying bacteria are poor competitors for [NH.sub.4.sup.+] and that microbial immobilization of [NO.sub.3.sup.-] is insignificant, are not correct for this grassland system. Soil heterogeneity probably results in [NH.sub.4.sup.+] availability to [NH.sub.4.sup.+] oxidizers at some microsites, while [NO.sub.3.sup.-] assimilation by heterotrophic microorganisms occurs at other microsites where [NH.sub.4.sup.+] is not available. Relatively high rates of [NO.sub.3.sup.-] production and consumption in an ecosystem with an annual mean hydrologic loss of [NO.sub.3.sup.-] -N of only 3.3 kg/ha indicate the importance of [NO.sub.3.sup.-] 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.sub.4.sup.+] 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 amont such microsites, appear to be critical factors affecting N-cycling characteristics of the ecosystem.