Dissimilatory nitrate reduction to ammonium in upland tropical forest soils

The internal transformations of nitrogen in terrestrial ecosystems exert st rong controls over nitrogen availability to net primary productivity, nitra te leaching into groundwater, and emissions of nitrogen-based greenhouse ga s. Here we report a reductive pathway for nitrogen cycling in upland tropic al forest soils that decreases the amount of nitrate susceptible to leachin g and denitrification, thus conserving nitrogen in the ecosystem. Using N-1 5 tracers we measured rates of dissimilatory nitrate reduction to ammonium (DNRA) in upland humid tropical forest soils averaging similar to0.6 mug.g( -1).d(-1). Rates of DNRA were three times greater than the combined N2O and N-2 fluxes from nitrification and denitrification and accounted for 75% of the turnover of the nitrate pool. To determine the relative importance of ambient C, O-2, and NO3 concentrations on rates of DNRA, we estimated rates of DNRA in laboratory assays using soils from three tropical forests (clou d forest, palm forest, and wet tropical forest) that differed in ambient C and O-2 concentrations. Rates of DNRA measured in laboratory assays ranged from 0.5 to 9 mug.g(-1).d(-1) in soils from the three different forests and appeared to be primarily limited by the availability of NO3, as opposed to C or O-2. Tests of sterile soils indicated that the dominant reductive pat hway for both NO2 and NO3 was biotic and not abiotic. Because NH4 is the fo rm of N generally favored for assimilation by plants and microbes, and NO3 is easily lost from the ecosystem, the rapid and direct transformation of N O3 to NH4 via DNRA has the potential to play an important role in ecosystem N conservation.