Denitrifying pathways dominate nitrous oxide emissions from managed grassland during drought and rewetting

Nitrous oxide (N2O) is a strong greenhouse gas released primarily from microbial nitrification (oxic) and denitrification (anoxic) in soils. The extent of these pathways controlled by many factors, in particular soil moisture is a key uncertainty in the global N cycle. Future climate scenarios predict increased summer drought for European grasslands. The effect of precipitation changes on N2O production pathways are unknown, complicating efforts to mitigate emissions. This study presents the first online isotopic measurements of N2O emitted from grassland soils subjected to drought and rewetting. Automated chambers were interfaced with laser spectrometers to monitor N2O fluxes and isotopic composition. The abundance of nitrifiers and denitrifiers, soil physicochemical properties, and NanoSIMS measurements of microscale soil N distribution were brought together with N2O isotope data to gain a detailed view of N2O production in drought-affected soils [1]. First results from in situ measurements which also include measurements of N2O isotopic composition in the soil profile through a summer drought and a winter freeze-thaw period will also be presented. Unexpectedly, isotopic measurements showed that denitrifying pathways dominated N2O emissions under both drought and control conditions (Fig. 1). Denitrification during drought was linked to a reversible, drought-induced enrichment in N-bearing organic matter on microaggregates, and suggested a strong role for the chemo-/co-denitrification pathways. Throughout rewetting, fluxes were highly variable and denitrification dominated emissions. Both total N2O flux and denitrification contribution were significantly higher during rewetting than for control plots at the same soil moisture range (Fig. 1). The observed feedbacks between precipitation changes and N2O emissions are sufficient to account for the accelerating atmospheric N2O growth rate observed globally over the past decade. Figure 1 caption: Effect of water-filled pore space on N2O fluxes and denitrification fraction during drought and rewetting. Large, dark points show the mean value for all monoliths in each treatment for each 2% WFPS bin, small points show individual 12-hour mean values. Figure from [1]. [1] Harris et al. (2021) Science Advances, doi: 10.1126/sciadv.abb7118