THE importance of nitrate (NO3-) in the internal nitrogen cycle of undisturbed coniferous ecosystems has not been widely recognized(1,2). Nitrate concentrations in soils from these forests tend to be low, and assays measuring net nitrification usually show exceedingly slow rates(3-4). It may be, however, that microbial assimilation of NO3- is substantial in these soils, and that net nitrification rates greatly underestimate gross rates(5). Here we use a N-15 isotope-dilution technique in intact soil cores to measure gross rates of nitrification and microbial assimilation of NO3- in eleven undisturbed forest ecosystems of New Mexico and Oregon. We found that gross nitrification rates were surprisingly high in all of the forests examined. Net nitrification rates poorly predicted gross rates because the soil microbial communities had the capacity to assimilate almost all of the NO3- produced. To our knowledge, this is the first report of gross nitrification and NO3- assimilation rates in intact soil samples from a large number of contrasting forest ecosystems. Our results contradict previous assumptions that nitrification rates are low in mature coniferous forests and suggest that current models greatly underestimate the role of the microbial community in preventing NO3- loss.