Denitrification Is the Main Nitrous Oxide Source Process in Grassland Soils According to Quasi‐Continuous Isotopocule Analysis and Biogeochemical Modeling

Abstract

Isotopic composition of soil‐emitted nitrous oxide (N2O), especially the intramolecular distribution of 15N in N2O known as site preference (SP), can be used to track the two major N2O emitting soil‐processes nitrification and denitrification. Online analysis of SP in ambient air has been achieved recently, yet those approaches only allowed addressing large areas (footprints) on the basis of strong changes in surface atmospheric N2O concentrations. Here, we combined laser spectroscopy with automated static flux chambers to measure, for the first time, SP of low N2O fluxes with high sensitivity and temporal resolution and to explore its spatial variability. The measurements were then used to test the N2O isotope module SIMONE in combination with the biogeochemical model LandscapeDNDC to identify N2O source processes. End‐member mixing analysis of the data revealed denitrification as the predominant N2O source. This finding was independent of the soil water content close to the soil surface, suggesting that N2O production in the subsoil under high water‐filled pore space conditions outweighed the potential production of N2O by nitrification closer to the surface. Applying the SIMONE‐LandscapeDNDC model framework to our field site showed that the modeled SP was on average 4.2‰ lower than the observed values. This indicates that the model parameterization reflects the dominant N2O production pathways but overestimates the contribution of denitrification by 6%. Applying the stable isotope‐based model framework at other sites and comparing with other models will help identifying model shortcomings and improve our capability to support N2O mitigation from agricultural ecosystems. (© 2020 American Geophysical Union).