Nitrifier denitrification can be a source of N₂O from soil: a revised approach to the dual‐isotope labelling method

Abstract

Nitrifier denitrification (i.e. nitrite reduction by ammonia oxidizers) is one of the biochemical pathways of nitrous oxide (N 2 O) production. It is increasingly suggested that this pathway may contribute substantially to N 2 O production in soil, the major source of this greenhouse gas. However, although monoculture studies recognize its potential, methodological drawbacks prohibit conclusive proof that nitrifier denitrification occurs in actual soils. Here we suggest and apply a new isotopic approach to identify its presence in soil. In incubation experiments with 12 soils, N 2 O production was studied using oxygen (O) and nitrogen (N) isotope tracing, accounting for O exchange. Microbial biomass C and N and phospholipid fatty acid (PLFA) patterns were analysed to explain potential differences in N 2 O production pathways. We found that in at least five of the soils nitrifier denitrification must have contributed to N 2 O production. Moreover, it may even have been responsible for all NH 4 + -derived N 2 O in most soils. In contrast, N 2 O as a by-product of ammonia oxidation contributed very little to total production. Microbial biomass C and N and PLFA-distinguished microbial community composition were not indicative of differences in N 2 O production pathways. Overall, we show that combined O and N isotope tracing may still provide a powerful tool to understand N 2 O production pathways, provided that O exchange is accounted for. We conclude that nitrifier denitrification can indeed occur in soils, and may in fact be responsible for the greater proportion of total nitrifier-induced N 2 O production.

Nitrogen fertiliser-induced changes in N2O emissions are attributed more to ammonia-oxidising bacteria rather than archaea as revealed using 1-octyne and acetylene inhibitors in two arable soils

“…Two mainly N 2 O-yielding can explain the reason for the higher N 2 O production by AOB than AOA, NH 2 OH oxidation and nitrifier denitrification (Shaw et al 2006;Stein 2011;Schreiber et al 2012). It was reported that N 2 O emission from the oxidation of NH 2 OH contributed very little to total N 2 O production and nitrifier denitrification was believed to be the predominant process for N 2 O production in soils with 50 % < WFPS < 70 % (Kool et al 2010(Kool et al , 2011. Recently, Vajrala et al (2013) found that NH 2 OH was an intermediate of ammonia oxidation in AOA, but they would not be able to produce N 2 O like AOB probably due to lack of genes for a homologue of hydroxylamine oxidoreductase (HAO) known to be responsible for N 2 O formation in AOB (Hooper and Terry 1979).…”

Ammonia oxidation pathways and nitrifier denitrification are significant sources of N 2 O and NO under low oxygen availability

“…Isotope analysis of soil mineral N was performed on aliquots of the extracts using a diffusion technique (49 (36). Then, the contribution of the different NH 3 oxidation pathways (NN, ND, and NCD) to N 2 O production was calculated based on the combined 18 O and 15 N data according to the dual-isotope method of Kool et al (29,31). The method is based on the principle that N 2 O produced via NN, ND, and NCD obtains 0, one-half, and two-thirds of the O from H 2 O, respectively (24, 51) (Fig.…”

“…Wrage et al (24) conceived a dual isotope ( 15 N and 18 O) approach to distinguish the pathway of ND from other N 2 O-forming processes. Kool et al (29)(30)(31) further refined this method by including 18 O-labeled NO 3 − to quantify the exchange of oxygen atoms (O) between H 2 O and N oxides during denitrification and nitrification in soil. Although these methods have elucidated pathways and sources of N 2 O production, there is still a dearth of knowledge of the environmental factors (particularly O 2 availability) and substrates (fertilizer N source) controlling soil N 2 O production through ND.…”

Using stable isotopes to follow excreta N dynamics and N2O emissions in animal production systems

“…However, the assumption that there was negligible exchange of O between H 2 18 O and NO 3 2 was subsequently proven to be violated (Kool et al, 2007;Kool et al, 2009a and2009b). Thus, the dual isotope method was revised by introducing an additional 18 O-labelled NO 3 2 treatment so that O exchange during denitrification could be accounted for (Kool et al, 2010 and2011a), and it was subsequently shown that nitrifier denitrification made a significant contribution to the N 2 O fluxes in a number of soils examined. This method holds great promise for furthering our understanding of the role of biological processes in producing N 2 O and needs to be applied across a wider range of soils and agroecosystems, in particular.…”