Temporal Disconnect of Seasonal Plant Nutrient Demand and Thaw Depth implies an Increasing Source of N2O in High-Latitude Permafrost Ecosystems

Thawing and degradation of permafrost in high latitudes could have an important effect on the Earth’s greenhouse budget. Implications of increased nutrient availability resulting from thawing of nutrient-rich permafrost, however, remain poorly assessed, despite nutrients having being identified as a strong present-day constraint for plant growth and microbial activity in the high latitudes. In our pan-arctic scale study, we extend the terrestrial ecosystem model QUINCY, which already couples C-N-P cycles in soil and vegetation, for a better representation of high-latitude processes. With this model version, we perform historical simulations at the site-level over 1960-2019. Averaged over high-latitude grassland sites, our simulations show an average increase in the soil active layer depth of 0.1m and an increased gradient of biologically-available P and N at the permafrost front. In spite of this, only 11 % of the simulated increase over the GPP (+34%) is a result of increased nutrient supply from permafrost organic matter degradation. This owes to spatial and temporal decoupling of the simulated vegetation growth peak (mid-to-late-July), the time period where plant nutrient demand is the highest, and the maximum of the seasonal thaw depth (mid-to-late August), the time period in which nutrients in the deep active layer would potentially be available for uptake. As a result, increased nitrogen at the permafrost front and alternating aerobic-anaerobic conditions contribute to enhancing nitrification and denitrification in the model, causing a weak source of N₂O to the atmosphere of 0.7 kg N ha^-1 yr^-1, which undergoes a considerable upward trend of up to 0.1 kg N ha^-1 decade^-1, locally,over the simulation time frame. Considering the vastness of the permafrost domain, and that N₂O emissions from these regions have been largely neglected in the past, these results imply that high latitudes could be a considerable and growing contributor to the global atmospheric N₂O budget.