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 NO3- occurs in two steps; in the first step, NO3- is reduced to NO2-; and in the second, unlike denitrification, NO2- is reduced to NH4+ without intermediates. There are two sets of NO3-/NO2- 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 (N2O-reductase) ratio favours DNRA. Recently, several crystal structures of NrfA have been presumed to producee N2O 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 NO2-/NO3- ratio, and concentrations of ferrous iron (Fe2+) and sulfide (S2-). Under low-redox conditions, a high C/NO3- ratio selects for DNRA while a low ratio selects for denitrification. When the proportion of both C and NO3- are equal, the NO2-/NO3- ratio modulates partitioning of NO3-, and a high NO2-/NO3- ratio favours DNRA. A high S2-/NO3- 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 NO3- to NH4+, it is essential for protecting NO3- from leaching and gaseous (N2O) losses and enriches soils with readily available NH4+-N to primary producers and heterotrophic microorganisms. Therefore, DNRA may be treated as a tool to reduce ground-water NO3- pollution, enhance soil health and improve environmental quality.