Reconciling high-temporal-resolution field measurements of N2O isotopic composition with a biogeochemical model revealed denitrification as the main N2O source
Abstract
Increased emissions from agricultural soils are the most important anthropogenic contribution to the elevated atmospheric abundance of N2O, which is a potent greenhouse gas and the most important contributor to stratospheric ozone destruction. In soils, microorganisms predominately produce N2O by nitrification and denitrification. Biogeochemical models simulate these processes and are increasingly used to assess N2O mitigation strategies. However, the lack of field methods allowing the direct partitioning of N2O to source processes entails a large uncertainty with regard to their relative contribution. The intramolecular distribution of 15N in the N2O molecule, also known as site preference, provides unique information to disentangle the relative contribution of nitrification and denitrification. On-line analysis of the changes in intramolecular N2O isotopic composition in ambient air has been achieved recently, but this approach alone i) does not allow an explicit spatial allocation of the isotopic signature and ii) is limited to events with a strong increase in N2O concentrations, which may obscure short term variations of the dominant N2O producing processes, e.g. due to day-time temperature or radiation changes. To address these shortcomings, we combined laser spectroscopy based on-line analysis of the intramolecular N2O isotopic composition [1] with the biogeochemical model LandcapeDNDC and SIMONE, a stable isotope model for nutrient cycles [2]. N2O emissions were spatially allocated by coupling the laser spectrometer to automated static flux chambers, which also enabled higher sensitivities for periods with low N2O fluxes and higher temporal resolution than previous studies [3, 4]. This approach allowed us assessing the relative contributions of nitrification and denitrification during a field campaign between August and December in 2017 on a grassland site close to Beromünster, Central Switzerland. Both N2O isotope measurements and model results indicated continuous predominance of denitrification and hardly any temporal variability in N2O SP. This finding was independent of the water content close to the soil surface, suggesting that N2O production in the subsoil under high water filled pore space conditions outweighed the production of N2O by nitrification closer to the surface. The modelled isotopic composition was offset by 1.9 ‰ with respect to the observed values. This indicates that the model parametrization reflects the dominant N2O production pathway but slightly overestimates the contribution of denitrification. To our knowledge, this is the first observation-based application and validation of a biochemical soil model using N2O isotopes. The application of this stable isotope based model validation approach at other sites and the comparison with other models will help to identify potential shortcomings and improve our capability to support N2O mitigation strategies using real-time measurements of stable N2O isotopes and corresponding soil chemistry and emission models. References 1. Ibraim, E., et al., Isotopes in Environmental and Health Studies, 2018. 54(1): p. 1-15. 2. Denk, T.R.A., et al., Soil Biology & Biochemistry, 2017. 105: p. 121-137. 3. Ibraim, E., et al., Biogeosciences Discuss., 2018. 2018: p. 1-27. 4. Wolf, B., et al., Biogeosciences, 2015. 12(8): p. 2517-2531.
- Publication:
-
EGU General Assembly Conference Abstracts
- Pub Date:
- April 2019
- Bibcode:
- 2019EGUGA..21.1924I