Simultaneous Production and Consumption of Soil N₂O Creates Complex Effects on Its Stable Isotope Composition

Abstract

The stable N and O isotope composition of soil and soil‐respired N2O is increasingly measured, yet a solid theoretical framework for interpreting the data remains to be developed. Here, the physical processes that affect soil N2O and its isotopes are embedded in a diffusion/reaction model. Numerical experiments are compared to data to demonstrate how various soil processes influence depth profiles and surface fluxes of soil N2O, δ15NN2O, and δ18ON2O. Model predictions and data suggest that the isotope composition of the net N2O soil flux, in soils that have N2O consumption, is a function of the net flux rate, and the isotope differences between the atmosphere and the biological source. Asymptotically large negative or positive δ15Nflux and δ18Oflux values occur as the net soil N2O flux approaches zero from positive or negative flux rates, respectively. This implies that the isotopic imprint of soil fluxes on the global atmospheric N2O pool is more variable than previously suggested. Additionally, the observed isotope values in static flux chambers are possibly complicated by the fact that consumption fluxes increase as the concentration in the chambers increases. This work reveals that even simple chamber flux measurements may possess isotope effects imparted by consumption during the chamber measurement and suggests ways to experimentally test this possibility. Additionally, simple methods to estimate depth‐dependent net production/consumption and its isotope effects are suggested. However, understanding the gross rates of the production and consumption of soil N2O remains an elusive goal.