Quantifying Changes to the Arctic N Budget following Permafrost Thaw

Permafrost soils in the Arctic region contain vast amounts of frozen dead biomass, composed of twice the amount of carbon (C) as in the atmosphere and a great quantity of nutrients. Due to climate change, these soils are thawing at greater depths each summer and will continue to do so in the future, rendering this frozen organic material accessible to decomposition. Most research thus far has focused on the amount of C that could potentially be released as CO₂ and CH₄ gases through the decomposition. However, the release of nitrogen (N) is estimated to have a significant impact on the region's ecosystems and global greenhouse gas budgets, as this excess N may boost vegetation growth, potentially enhancing its CO₂ uptake. However, it could also lead to increased N losses in the form of N₂O emissions and lateral export to water bodies. This research aims at synthesizing the historical and future changes of the terrestrial N budget in the Arctic following permafrost thaw.

Here, we provide estimates of past and future changes to the pan-Arctic N budget and how it is affected by permafrost thaw. We combine soil nitrogen data from Palmtag et al (2022) and CMIP6 model projections of active layer depth. The amount of N released through permafrost thaw is compared to estimates of changes in N deposition and biological fixation. Furthermore, we quantify the biologically available fraction of the N released from the permafrost and provide a first-order estimate on the consequences of this altered N cycling for Arctic vegetation biomass growth and CO₂ uptake based on published results from N fertilization field experiments.

Based on CMIP6 model output, we estimate that the mean active layer depth over the whole Arctic permafrost region will increase from an averaged depth of 1.3 m for the present day to 2.3 m depth following SSP 126, to 3.6 m depth following SSP 370, and to 3.9 m depth following SSP 585 scenarios for the time period 2080 - 2100. The additional N mobilized through this permafrost thawing translates to increases of 95 %, 167 % and 186 % of nitrogen in the active layer compared to present day. By 2100, with N inputs from permafrost thaw and assuming that 5 - 15 % of this becomes available as plant nutrition, vegetation biomass could increase by 16 – 50 g C m^-2 yr^-1, 31 – 96 g C m^-2 yr^-1, or a 35 – 106 g C m^-2 yr^-1 for the SSPs 126, 370 and 585, respectively, assuming a linear increase in vegetation biomass growth until 2100. These numbers would reflect a significant additional drawdown of CO₂ by the pan-Arctic vegetation, with relevance for global assessments.

 

Figure 1: Timeseries showing the amount of additional N in kg N / m² in the active layer, anomaly to 1880-1900. The dotted line indicates the change from observation-based models to future projection based on the SSP scenarios.