Is the isotopic composition of nitrous oxide an indicator for its origin from nitrification or denitrification? A theoretical approach from referred data and microbiological and enzyme kinetic aspects

Abstract

Literature data on the isotopic composition of nitrous oxide indicate a general predominance of the alpha-15N-isotopomer and a parallel 18O-enrichment in N2O from nitrification and denitrification, respectively. As the kinetic isotope effects on any individual reactions of the two processes lead to depletions of the heavy isotopes of nitrogen and oxygen in the products, the observed enrichments could mainly be caused by enzymatic reduction of NO, provided it occurs via a symmetric intermediate like hyponitrite; infrared data are in favour of large differences between the binding constants of the isotopomers and isotopologues of this compound. As a matter of fact one of the mechanisms discussed for the nitric oxide reductase from certain microorganisms implies the parallel binding of two NO molecules and the formation of a symmetrical intermediate, while that of the enzyme from other microorganisms reduces NO in a sequential mechanism. In addition, isotope effects on the reduction of N2O to N2 must contribute to the observed isotope characteristics of N2O, especially in context with denitrification. Therefore, the known enzymatic reaction pathways suggest that the alpha-15N-isotopomer preference and the 18O-signature of the produced N2O is not essentially characteristic for its origin from nitrification or denitrification, respectively, but rather from the involved population of microorganisms and the type of their nitric oxide reductases. This has to be confirmed experimentally.

Constraints on N<sub>2</sub>O budget changes since pre-industrial time from new firn air and ice core isotope measurements

“…So far, it has not been possible yet to determine the relative contribution of these different oxygen sources in today's N 2 O budget, because no consistent picture has yet emerged (Gros et al, 2004). Recent studies have also emphasized the importance of the population of microorganisms involved in the 18 O-enrichment (Schmidt et al, 2004;Sutka et al, 2003).…”

Nitrous oxide distribution and its origin in the central and eastern South Pacific Subtropical Gyre

“…The primary source of O in the N 2 O produced by ammonium oxidation is dissolved O 2 (Ostrom et al, 2000), since it is expected that δ 18 O (N 2 O) will be greater than 40‰. Furthermore, the preferential breakage of (Schmidt et al, 2004). The source of the oxygen atom in N 2 O produced by nitrite reduction is nitrite (Aerssens et al, 1986;Averill, 1996).…”

“…This is the main advantage of the N 2 O isotopomer ratio over the conventional δ 15 N ratio. Nevertheless, the lack of knowledge of the specific biochemical mechanisms involved in N 2 O production makes it difficult to "predict" which isotopomer signature corresponds to which particular processes (Schmidt et al, 2004). The few works that present isotopomer results in marine environments have found positive SP values associated with nitrification (Popp et al, 2002;Toyoda et al, 2002) or denitrification (Westley et al, 2006;Yamagishi et al, 2005).…”

Site selective real-time measurements of atmospheric N<sub>2</sub>O isotopomers by laser spectroscopy

“…For example, the 15 N depletion in N 2 O produced by autotrophic nitrification was found to be considerably higher as compared to heterotrophic denitrification (Koba et al, 2009;Sutka et al, 2006;Toyoda et al, 2005;Yoshida, 1988). On the other hand, processspecific effects on δ 15 N bulk might be masked by shifts in the precursor signature (Well et al, 2008), and theoretical considerations indicate a major impact of the involved bacterial species (Schmidt et al, 2004). In contrast to δ 15 N bulk , the site preference is considered to be independent of the isotopic composition of the precursor and, thus, supplies clear process information even if the isotopic signature of the substrate for N 2 O production is lacking (Frame and Casciotti, 2010;Ostrom et al, 2007;Sutka et al, 2006;Toyoda et al, 2002;Yamagishi et al, 2007;Well and Flessa, 2009).…”