Impacts of long-term warming and enhanced nitrogen and sulfur deposition on carbon sink potential in a boreal peatland

In order to assess peatland carbon sink potential under multiple global change perturbations, we examined the individual and combined effects of long-term warming and enhanced nitrogen (N) and sulfur (S) deposition on ecosystem CO₂ exchange at one of the longest-running experiments on peatlands, Degerö Stormyr poor fen, Sweden. The site has been treated with NH₄NO₃ (15 times ambient annual wet deposition), Na₂SO₄ (6 times ambient annual wet deposition) and elevated temperature (air +3.6 C) for 23 years. Gross photosynthesis, ecosystem respiration and net CO₂ exchange were measured weekly during June-August using chambers. After 23 years, two of the experimental perturbations: N addition and warming individually reduced net CO₂ uptake potential down to 0.3-0.4 fold compared to the control mainly due to lower gross photosynthesis. Under S only treatment ecosystem CO₂ fluxes were largely unaltered. In contrast, the combination of S and N deposition and warming led to a more pronounced effect and close to zero net CO₂ uptake potential or net C source. Our study emphasizes the value of the long-term multifactor experiments in examining the ecosystem responses: simultaneous perturbations can have nonadditive interactions that cannot be predicted based on individual responses and thus, must be studied in combination when evaluating feedback mechanisms to ecosystem C sink potential under global change.