Enriched nutrient availability strengthens the net C uptake of the northernmost ecosystem station in Greenland

Although the Arctic tundra is an essential contributor to the global carbon (C) cycle, there is a lack of reference sites from where full C exchange dynamics can be characterized under harsh conditions and remoteness. The Greenland Ecosystem Monitoring (GEM) programme efforts have envisioned integrated and long-term activities to contribute to the basic scientific understanding of the Arctic and their responses to climate changes. Here we present 20+ years across the 2008-2018 period of C flux and ancillary data from two twin ecosystem stations in Greenland: Zackenberg (74°N) and Kobbefjord (64°N). In this project we show that Zackenberg fen has a significant higher C sink strength in a higher latitude during regularly shorter growing seasons compared to Kobbefjord fen. This ecosystem acted as a sink of CO₂ uptaking on average -50 g C m^-2 (range of +21 to -90 g C m^-2), more than twice compared to Kobbefjord (-18 g C m^-2 as average and range of +41 to -41 g C m^-2). We found that Zackenberg is a nutrient richer fen - the increased C uptake strength is associated with 3 times higher levels in soils of dissolved organic carbon and 5 times more plant nutrients, including dissolved organic nitrogen, nitrates. Additional evidences from in-situ sampling point to higher leaf area index (140%), foliar nitrogen (71%), and leaf mass per area (5%) in the northernmost site supporting the nutrient richer hypothesis. To test this overarching hypothesis, we further used the Soil-Plant-Atmosphere (SPA) model. We can explain ~68%, ~80% and ~67% of the variability of daily net ecosystem exchange of CO₂, photosynthesis and respiration respectively applying the model parameterization previously used in Kobbefjord but with increases in initial C stocks, leaf mass per area, N content and Q₁₀ of foliar and root respiration rates. Therefore, we conclude that the limitations of plant phenology timing in Zackenberg regarding net C uptake have not only been counterbalanced but also intensified due to richer compositions of nutrients and minerals. More high-temporal monitoring activities in Arctic ecosystems are needed not only to allow straightforward comparisons of key biogeochemical processes but also to help us understand the underlying differences in sensitive and rapidly changing ecosystems.