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

C B Pandey; Upendra Kumar; Megha Kaviraj; K J Minick; A K Mishra; J S Singh

doi:10.1016/j.scitotenv.2020.139710

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

Sci Total Environ. 2020 Oct 10:738:139710. doi: 10.1016/j.scitotenv.2020.139710. Epub 2020 Jun 2.

Authors

C B Pandey¹, Upendra Kumar², Megha Kaviraj³, K J Minick⁴, A K Mishra⁵, J S Singh⁶

Affiliations

¹ ICAR-Central Arid Zone Research Institute, Jodhpur 342003, Rajasthan, India. Electronic address: cbpandey5@rediffmail.com.
² ICAR-National Rice Research Institute, Cuttack 753006, Odisha, India. Electronic address: ukumarmb@gmail.com.
³ ICAR-National Rice Research Institute, Cuttack 753006, Odisha, India.
⁴ Department of Forestry and Environmental Resources, North Carolina State University, Raleigh, NC 27695, USA.
⁵ International Rice Research Institute, New Delhi 110012, India.
⁶ Ecosystem Analysis Lab, Centre of Advanced Study in Botany, Banaras Hindu University (BHU), Varanasi 221005, India.

PMID: 32544704
DOI: 10.1016/j.scitotenv.2020.139710

Abstract

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 (S^2-). 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 S^2-/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.

Keywords: Denitrification; Nitrate respiration; Nitrate-conservation; Redox potential of soils.

Publication types

Meta-Analysis
Review

MeSH terms

Ammonium Compounds*
Denitrification
Ecosystem*
Nitrates
Nitrogen*

Substances

Ammonium Compounds
Nitrates
Nitrogen