Petagrams of Nitrogen released from the Permafrost affect Arctic Ecosystem Fluxes under any Climate Scenario

Arctic permafrost-affected soils serve as one of Earth’s most significant terrestrial reservoirs of nitrogen (N), storing an estimated 55 Pg of total N within the first three meters of the soil. Ongoing anthropogenic climate change is causing permafrost soils to thaw at greater depths each summer, rendering part of vast pools of N stored in the permafrost accessible to plant uptake and microbial processing. However, little is known of the magnitudes and consequences of the potential increase of N availability due to permafrost thaw, as the mobilisation and mineralisation of this N has the potential to alter ecosystem productivity, as well as increase emissions of reactive nitrogen gases, such as nitrous oxide (N₂O). This research aims to quantify nitrogen dynamics across the Arctic following permafrost thaw from the historical period to the end of the twenty-first century.

In our study, we first estimate the total amount of soil N that will be mobilized through thawing. To do this, we combine a vertically-resolved high spatial resolution soil C and N dataset with future projections of pan-Arctic active layer depth changes derived from five CMIP6 models across four climate change scenarios. Vertical mean soil N profiles were thereby determined for different land cover types of the tundra, taiga, wetlands and barren biomes. Secondly, we estimated the amount of permafrost organic soil N that is rapidly mineralized to bioavailable forms, was determined from a temperature-dependent mineralisation flux estimation. Finally, we perform a first-order estimation of the impact of the additional bioavailable N for Arctic vegetation NPP and N₂O emissions. 

Across the CMIP6 models, the pan-Arctic mean maximum active layer depth is projected to increase by an additional 1 - 2.65 m by 2100 (SSP 1-2.6 to 5-8.5), relative to present-day conditions. This increase corresponds to a potential cumulative release of 20 - 44 Pg total N by the end of the century, of which 4.4 - 5.5 % may be mineralised rapidly under projected soil warming. Putting these estimates into context with our novel budget of present-day pan-Arctic N fluxes, we show that the addition of reactive nitrogen from the permafrost will consist of an important part of N sources to the Arctic in the future (40-50 %).  

Based on our synthesis of N fertilisation effects on Arctic vegetation net primary productivity (boreal ANPP: 14.1 (+/- 3.55) g C / g N, BNPP: 2.82 g C / g N; tundra ANPP: 2.66 (+/- 2.22) C / N, BNPP: 2.29 (+/- 4.41) g C / g N), we furthermore quantify that the release of N from the permafrost could increase  NPP by 150 – 400 Tg C yr-1 until year 2100. In a similar approach based on soil manipulation experiments, we also estimate a potential additional 0.12 – 0.8 Tg N yr-1 in N₂O emissions by the year 2100.

Our results show that permafrost thawing will significantly alter the Arctic N budget, having very likely substantial impacts for both terrestrial NPP and as N₂O emissions.