N₂O source partitioning in soils using ¹⁵N site preference values corrected for the N₂O reduction effect

Abstract

Rationale The aim of this study was to determine the impact of isotope fractionation associated with N2O reduction during soil denitrification on N2O site preference (SP) values and hence quantify the potential bias on SP‐based N2O source partitioning. Methods The N2O SP values (n = 431) were derived from six soil incubation studies in N2‐free atmosphere, and determined by isotope ratio mass spectrometry (IRMS). The N2 and N2O concentrations were measured directly by gas chromatography. Net isotope effects (NIE) during N2O reduction to N2 were compensated for using three different approaches: a closed‐system model, an open‐system model and a dynamic apparent NIE function. The resulting SP values were used for N2O source partitioning based on a two end‐member isotopic mass balance. Results The average SP0 value, i.e. the average SP values of N2O prior to N2O reduction, was recalculated with the closed‐system model, resulting in −2.6 ‰ (±9.5), while the open‐system model and the dynamic apparent NIE model gave average SP0 values of 2.9 ‰ (±6.3) and 1.7 ‰ (±6.3), respectively. The average source contribution of N2O from nitrification/fungal denitrification was 18.7% (±21.0) according to the closed‐system model, while the open‐system model and the dynamic apparent NIE function resulted in values of 31.0% (±14.0) and 28.3% (±14.0), respectively. Conclusions Using a closed‐system model with a fixed SP isotope effect may significantly overestimate the N2O reduction effect on SP values, especially when N2O reduction rates are high. This is probably due to soil inhomogeneity and can be compensated for by the application of a dynamic apparent NIE function, which takes the variable reduction rates in soil micropores into account. Copyright © 2016 John Wiley & Sons, Ltd.

The nitrogen cycle: A review of isotope effects and isotope modeling approaches

“…the differences between open and closed system calculations decrease with increasing residual substrate fraction f, so that adequate results can be achieved also with open system calculations if the time step is controlled appropriately (Wu et al, 2016).…”

Early season N<sub>2</sub>O emissions under variable water management in rice systems: source-partitioning emissions using isotope ratios along a depth profile

“…In reality, it can be assumed that neither perfect open or closed systems exist in nature and processes likely reflect a mixture of these dynamics. The use of one or the other case may bias results; therefore, we chose to take the mean of the two systems to estimate N 2 O reduction, nitrification/fungal denitrification and denitrification/nitrifier denitrification-derived N 2 O emissions (Decock and Six, 2013b;Wu et al, 2016). Due to a disproportionate number of missing values at 25 cm in the two WS treatments, we chose not to include data from this depth in our analysis and discussion.…”

Effect of soil saturation on denitrification in a grassland soil

“…It has been reported that the combination of the isotopic signatures of N 2 O potentially identifies the contribution of processes other than bacterial denitrification (Köster et al, 2015;Wu et al, 2016;Deppe et al, 2017). The question arises of to what extent the relationships between the δ 18 O and δ 15 N bulk and between δ 18 O and SP within the individual treatments denitrification dynamics are related.…”

“…Here we presented for the first time an extensive dataset with large range in product ratios and moisture to calculate the contribution of bacterial denitrification (%B DEN ) of emitted N 2 O from SP 0 . The uncertainty of this approach arises from three factors: (i) from the range of SP 0 endmember values for bacterial denitrification of −11 to 0 ‰ and 30 to 37 for hydroxylamine oxidation/fungal denitrification, (ii) from the range of net isotope effect values of N 2 O reduction (η N 2 O−N 2 ) for SP which vary from −2 to −8 ‰ , and (iii) system condition (open vs. closed) taken to estimate the net isotope effect (Wu et al, 2016).…”