Abstract:
The internal transformations of nitrogen in terrestrial ecosystems exert strong
controls over nitrogen availability to net primary productivity, nitrate leaching into groundwater,
and emissions of nitrogen-based greenhouse gas. Here we report a reductive pathway
for nitrogen cycling in upland tropical forest soils that decreases the amount of nitrate
susceptible to leaching and denitrification, thus conserving nitrogen in the ecosystem. Using
15N tracers we measured rates of dissimilatory nitrate reduction to ammonium (DNRA) in
upland humid tropical forest soils averaging ;0.6 mg·g21·d21. Rates of DNRA were three
times greater than the combined N2O and N2 fluxes from nitrification and denitrification
and accounted for 75% of the turnover of the nitrate pool. To determine the relative importance
of ambient C, O2, and NO3 concentrations on rates of DNRA, we estimated rates
of DNRA in laboratory assays using soils from three tropical forests (cloud forest, palm
forest, and wet tropical forest) that differed in ambient C and O2 concentrations. Rates of
DNRA measured in laboratory assays ranged from 0.5 to 9 mg·g21·d21 in soils from the
three different forests and appeared to be primarily limited by the availability of NO3, as
opposed to C or O2. Tests of sterile soils indicated that the dominant reductive pathway
for both NO2 and NO3 was biotic and not abiotic. Because NH4 is the form of N generally
favored for assimilation by plants and microbes, and NO3 is easily lost from the ecosystem,
the rapid and direct transformation of NO3 to NH4 via DNRA has the potential to play an
important role in ecosystem N conservation.