Fractionation factors for stable isotopes of N and O during N₂O reduction in soil depend on reaction rate constant

Abstract

Nitrous oxide (N(2)O) is a major greenhouse gas that is mainly produced but also reduced by microorganisms in soils. We determined factors for N and O isotope fractionation during the reduction of N(2)O to N(2) in soil in a flow-through incubation experiment. The absolute value of the fractionation factors decreased with increasing reaction rate constant. Reaction rates constants ranged from 1.7 10(-4) s(-1) to 4.5 10(-3) s(-1). The minimum, maximum and median of the observed fractionation factors were for N -36.0 per thousand, -1.0 per thousand and -9.3 per thousand and for O -74.0 per thousand, -6.9 per thousand and -26.3 per thousand, respectively. The ratio of O isotope fractionation to N isotope fractionation was 2.4 +/- 0.3 and it was independent from the reaction rate constants. This leads us to conclude that fractionation factors are variables while their ratio in this particular reaction might be a constant.

Evidence for the predominance of denitrification as a source of N2O in temperate agricultural soils based on isotopologue measurements

“…Using the flux-weighted average d 15 N for soil NO 3 À at KBS of 2.5% [Ostrom et al, 1998] to represent the nitrogen isotope value of the substrate, the NIE for our data, 7 to À19%, falls within or above the range of values indicative of N 2 O production from denitrification (À13 to À28%). However, like Perez et al [2001] [Menyailo and Hungate, 2006;Ostrom et al, 2007;Sutka et al, 2008;Vieten et al, 2007;JinuntuyaNortman et al, 2008]. Variation in a fractionation factor can result from changes in the relative importance of the reaction rate, constants for diffusion and enzymatic reduction that occur during an experiment [Jinuntuya-Nortman et al, 2008;Sutka et al, 2008].…”

The fate of N<sub>2</sub>O consumed in soils

“…Concentrations of N 2 O were measured by GC-ECD (SRI8610C Gas Chromotograph; SRI Instruments Inc., Las Vegas, N.V., USA). By varying the concentration of N 2 O in the gas mixture, we were able to determine rates of gross production and gross consumption of N 2 O from a linear regression fitted through the measured net N 2 O fluxes against N 2 O concentrations (Vieten et al, 2007). The δ 15 N of soil organic matter (SOM) was measured (on three or more sub-samples) before and after an 11 to 29 day incubation period ( …”

Can N2O stable isotopes and isotopomers be useful tools to characterize sources and microbial pathways of N2O production and consumption in tropical soils?

“…References: a, [Sutka et al, 2006;Ueda et al, 1999;Yoshida et al, 1984]; b, [Pérez et al, 2006]; c, [Barford et al, 1999;Snider et al, 2008;Sutka et al, 2006;Thompson et al, 2004;Toyoda et al, 2005;Yoshida et al, 1984]; d, [Mariotti et al, 1981[Mariotti et al, , 1982Menyailo and Hungate, 2006;Ostrom et al, 2007;Pérez et al, 2006;Snider et al, 2008;Wada et al, 1991;Well et al, 2006]; e, [Barford et al, 1999;Ostrom et al, 2007;Thompson et al, 2004;Wahlen and Yoshinari, 1985;Webster and Hopkins, 1996;Yoshida et al, 1984]; f, [Mandernack et al, 2000;Menyailo and Hungate, 2006;Ostrom et al, 2007;Vieten et al, 2007]; g, [Sutka et al, 2003[Sutka et al, , 2004.…”

“…Thus, " a /" 18 = +1.6 ± 0.3. We must next make the assumption that an average value for " 18 of ∼−15‰ for the reduction of N 2 O [Menyailo and Hungate, 2006;Ostrom et al, 2007;Vieten et al, 2007] is relevant for the soils of this study (at least for the sake of our back of the envelope calculation here). Thus, we derive a value for " a of −24‰.…”

“…Finally, the enrichment factors are either not constant (e.g., for denitrification) or not different enough to distinguish among processes (e.g., for nitrification). For example, it has been shown that the enrichment factors for 15 N and 18 O during N 2 O reduction to N 2 decrease with increasing the reaction rate, thus producing smaller depletions in 15 N and 18 O in the products [Vieten et al, 2007]. This variability in the magnitude of the enrichment factors prevents the partitioning of the relative contribution of each process to be determined by a simple isotope mass balance approach.…”