Climate change rivals fertilizer use in driving soil nitrous oxide emissions in the northern high latitudes: Insights from terrestrial biosphere models

Nitrous oxide (N₂O) is the most important stratospheric ozone-depleting agent based on current emissions and the third largest contributor to increased net radiative forcing. Increases in atmospheric N₂O have been attributed primarily to enhanced soil N₂O emissions. Critically, contributions from soils in the Northern High Latitudes (NHL, >50°N) remain poorly quantified despite their exposure to rapid rates of regional warming and changing hydrology due to climate change. In this study, we used an ensemble of six process-based terrestrial biosphere models (TBMs) from the Global Nitrogen/Nitrous Oxide Model Intercomparison Project (NMIP) to quantify soil N₂O emissions across the NHL during 1861–2016. Factorial simulations were conducted to disentangle the contributions of key driving factors, including climate change, nitrogen inputs, land use change, and rising atmospheric CO₂ concentration​, to the trends in emissions. The NMIP models suggests NHL soil N₂O emissions doubled from 1861 to 2016, increasing on average by 2.0 ± 1.0 Gg N/yr (p < 0.01). Over the entire study period, while N fertilizer application (42 ± 20 %) contributed the largest share to the increase in NHL soil emissions, climate change effect was comparable (37 ± 25 %), underscoring its significant role. In the recent decade (2007–2016), anthropogenic sources contributed 47 ± 17 % (279 ± 156 Gg N/yr) of the total N₂O emissions from the NHL, while unmanaged soils contributed a comparable amount (290 ± 142 Gg N/yr). The trend of increasing emissions from nitrogen fertilizer reversed after the 1980 s because of reduced applications in non-permafrost regions. In addition, increased plant growth due to CO₂ fertilization suppressed simulated emissions. However, permafrost soil N₂O emissions continued increasing attributable to climate warming; the interaction of climate warming and increasing CO₂ concentrations on nitrogen and carbon cycling will determine future trends in NHL soil N₂O emissions. The rigorous interplay between process modeling and field experimentation will be essential for improving model representations of the mechanisms controlling N₂O fluxes in the Northern High Latitudes and for reducing associated uncertainties.