Global soil nitrous oxide emissions in a dynamic carbon-nitrogen model

Abstract. Nitrous oxide (N₂O) is an important greenhouse gas that also contributes to the depletion of stratospheric ozone. Due to its high temporal and spatial heterogeneity, a quantitative understanding of terrestrial N₂O emission and its variabilities and responses to climate change are challenging. We added a soil N₂O emission module to the dynamic global land model LM3V-N, and tested its sensitivity to mechanisms that affect the level of mineral nitrogen (N) in soil such as plant N uptake, biological N fixation, amount of volatilized N redeposited after fire, and nitrification-denitrification. We further tested the relationship between N₂O emission and soil moisture, and assessed responses to elevated CO₂ and temperature. Results extracted from the corresponding gridcell (without site-specific forcing data) were comparable with the average of cross-site observed annual mean emissions, although differences remained across individual sites if stand-level measurements were representative of gridcell emissions. Processes, such as plant N uptake and N loss through fire volatilization that regulate N availability for nitrification-denitrification have strong controls on N₂O fluxes in addition to the parameterization of N₂O loss through nitrification and denitrification. Modelled N₂O fluxes were highly sensitive to water-filled pore space (WFPS), with a global sensitivity of approximately 0.25 TgN per year per 0.01 change in WFPS. We found that the global response of N₂O emission to CO₂ fertilization was largely determined by the response of tropical emissions with reduced N₂O fluxes in the first few decades and increases afterwards. The initial reduction was linked to N limitation under higher CO₂ level, and was alleviated through feedbacks such as biological N fixation. The extratropical response was weaker and generally positive, highlighting the need to expand field studies in tropical ecosystems. We did not find synergistic effects between warming and CO₂ increase as reported in analyses with different models. Warming generally enhanced N₂O efflux and the enhancement was greatly dampened when combined with elevated CO₂, although CO₂ alone had a small effect. The differential response in the tropics compared to extratropics with respect to magnitude and sign suggests caution when extrapolating from current field CO₂ enrichment and warming studies to the globe.