Exploring temporal and spatial variation of nitrous oxide flux using several years of peatland forest automatic chamber data

Abstract. The urgent need to mitigate climate change has evoked a broad interest in better understanding and estimating nitrous oxide (N₂O) emissions from different ecosystems. Part of the uncertainty in N₂O emission estimates still comes from an inadequate understanding of the temporal and small-scale spatial variability of N₂O fluxes. Using 4.5 years of N₂O flux data collected in a drained peatland forest with six automated chambers, we explored temporal and small-scale spatial variability of N₂O fluxes. A Random forest with conditional inference trees was used to find immediate and time-lagged relationships between N₂O flux and environmental conditions across seasons and years with different environmental conditions.

The temporal variation of N₂O flux was large, and the daily mean flux varied between –11 and 1760 µg N₂O m⁻² h⁻¹. Three of the six measurement chambers had a maximum N₂O flux of less than 400 µg N₂O m⁻² h⁻¹, while the fluxes in the other three chambers exceeded 1000 µg N₂O m⁻² h⁻¹. Spatial differences in the flux persisted over time, and despite the high small-scale spatial variability, the temporal patterns of the fluxes were relatively similar across the chambers. Soil moisture as well as air and soil surface temperature were the most important variables in the random forest, with lagged soil moisture also considered important. N₂O flux responded to soil wetting with a time lag of 1–7 days, but the length of the time lag varied spatially and between seasons indicating interactions with other spatially and temporally variable environmental conditions.

The high temporal variation in N₂O flux was related to a) seasonally variable environmental conditions, with the highest N₂O fluxes measured after summer dry-wet cycles and winter soil freezing, and b) to annually variable seasonal weather conditions, which lead to high year-to-year variability in N₂O budget. Changes especially in the frequency of summer precipitation events and in winter temperature and snow conditions may increase the variability of annual N₂O emissions if the variability in summer and winter weather conditions increases due to climate change.