Nitrogen Retention and Loss in Terrestrial Ecosystems: The Role of Dissimilatory Nitrate Reduction to Ammonium.
Whendee L. Silver, University of California at Berkeley, Department of Environmental Science, Policy and Management, 137 Mulford Hall 3114, Berkeley, CA 94720
Dissimilatory nitrate reduction to ammonium (DNRA) is an anaerobic microbial process that has the potential to contribute to nitrogen retention in ecosystems by decreasing the size of the nitrate pool and converting nitrogen to a form that is readily used by plants and microorganisms. We report on rates of DNRA in a wide range of terrestrial ecosystems and compare rates of DNRA with denitrification to nitrous oxide (N2O), a potent greenhouse gas. Rates of DNRA were greatest in soils with fluctuating redox conditions. These soils experience episodic nitrifying events during aerobic periods that provide substrate for DNRA when soils are anaerobic. Maximum potential rates of DNRA were 6 ug g-1 d-1 in humid subtropical and temperate forests and were equivalent to gross nitrification rates. Under ambient soil conditions DNRA exceeded N2O emissions at all sites except lowland tropical forests, where DNRA and N2O fluxes were similar. When we induced an anaerobic environment, N2O production exceeded DNRA. Warming of temperate and boreal forest soils increased both nitrogen retention via DNRA and gas loss, but rates of N2O production were proportionally greater in warmer soils than DNRA. Fertilization with nitrogen did not significantly increase rates of either flux. In the boreal forest DNRA was negatively correlated with the ratio of labile C to NO3-; N2O fluxes were not well correlated with any of our field or laboratory measures of C and N cycling or redox. Our results suggest that nitrogen retention via DNRA can be substantial and may help decrease N2O fluxes. However, greater relative sensitivity of N2O production with low redox and increased temperature suggests a potential decrease in nitrogen retention via DNRA in a warmer, wetter world.