Nitrous oxide (N2O) is a strong greenhouse gas and an important ozone-depleting substance, released primarily from microbial production in soils via pathways such as nitrification and denitrification. The extent of these pathways – controlled by many competing factors, in particular soil moisture – is a key uncertainty in the nitrogen cycle, and critical to understand the fate of reactive nitrogen. Future climate scenarios predict increased summer drought for European grasslands, as well as heavier winter precipitation. The effects of changes in precipitation and climate on specific N2O production and consumption pathways are unknown which complicates efforts to mitigate emissions.
This study presents the first online isotopic measurements of N2O emitted from grassland soils subjected to strong drought and rewetting, to directly investigate N2O production and consumption pathways. Automated LICOR chambers were directly interfaced with Picarro spectrometers to monitor CO2 and CH4 fluxes as well as N2O fluxes and isotopic composition. The abundance of keystone microbes and soil NO3/NH4 concentration and δ15N are brought together with the N2O isotope data – as well as NanoSIMS measurements of soil N distribution at the microscale – to gain a detailed view of the factors controlling microbial N2O pathways in drought-affected soils.
Results/Conclusions:
We found that N2O emissions following fertilization of grasslands are denitrification-dominated, and the main drivers of variability in baseline emissions and pathways were microbial functional gene abundances and soil moisture. During drought, denitrification dominated grassland N2O production and dynamics, contributing >70% of emissions. This finding is in direct contrast to expectations of high nitrification in dry soils, and shows that significant reactive nitrogen is lost to gas production throughout drought periods. N2O emissions in drought-affected soils were due to a clear reversible enrichment in nitrogen-bearing organic matter compared to oxygen on microaggregates, thus suggesting a strong role for chemo- or codenitrification. Following rewetting, hysteresis was evident for both total N2O flux and denitrification contribution, which were significantly higher during rewetting than for control and drought plots at the same soil moisture range.
This study shows that precipitation changes induce strong, non-linear changes in N2O emissions and metabolic pathways in grassland soils. We illustrate that feedbacks between climate change-induced precipitation changes and N2O emission pathways are sufficient to account for the accelerating atmospheric N2O growth rate observed over the past decade. We expect these feedbacks will be amplified in the coming decades as climatic extremes increase in severity.