DNRA: A short-circuit in biological N-cycling to conserve nitrogen in terrestrial ecosystems.

This paper reviews dissimilatory nitrate reduction to ammonium (DNRA) in soils - a newly appreciated pathway of nitrogen (N) cycling in the terrestrial ecosystems. The reduction of NO occurs in two steps; in the first step, NO is reduced to NO; and in the second, unlike denitrification, NO is reduced to NH without intermediates. There are two sets of NO/NO reductase enzymes, i.e., Nap/Nrf and Nar/Nir; the former occurs on the periplasmic-membrane and energy conservation is respiratory via electron-transport-chain, whereas the latter is cytoplasmic and energy conservation is both respiratory and fermentative (Nir, substrate-phosphorylation). Since, Nir catalyzes both assimilatory- and dissimilatory-nitrate reduction, the nrfA gene, which transcribes the NrfA protein, is treated as a molecular-marker of DNRA; and a high nrfA/nosZ (NO-reductase) ratio favours DNRA. Recently, several crystal structures of NrfA have been presumed to producee NO as a byproduct of DNRA via the NO (nitric-oxide) pathway. Meta-analyses of about 200 publications have revealed that DNRA is regulated by oxidation state of soils and sediments, carbon (C)/N and NO/NO ratio, and concentrations of ferrous iron (Fe) and sulfide (S). Under low-redox conditions, a high C/NO ratio selects for DNRA while a low ratio selects for denitrification. When the proportion of both C and NO are equal, the NO/NO ratio modulates partitioning of NO, and a high NO/NO ratio favours DNRA. A high S/NO ratio also promotes DNRA in coastal-ecosystems and saline sediments. Soil pH, temperature, and fine soil particles are other factors known to influence DNRA. Since, DNRA reduces NO to NH, it is essential for protecting NO from leaching and gaseous (NO) losses and enriches soils with readily available NH-N to primary producers and heterotrophic microorganisms. Therefore, DNRA may be treated as a tool to reduce ground-water NO pollution, enhance soil health and improve environmental quality.