Effects of long-term warming and enhanced nitrogen and sulfur deposition on microbial communities in a boreal peatland

With ongoing environmental change, it is important to understand ecosystem responses to multiple perturbations over long time scales at in situ conditions. Here, we investigated the individual and combined effects of 18 years of warming and enhanced nitrogen and sulfate deposition on peat microbial communities in a nutrient-poor boreal mire. The three perturbations individually affected prokaryotic community composition, where nitrogen addition had the most pronounced effect, and its combination with the other perturbations led to additive effects. The functional potential of the community, characterized by shotgun metagenomics, was strongly affected by the interactive effects in the combined treatments. The responses in composition were also partly reflected in the functional gene repertoire and in altered carbon turnover, i.e. an increase of methane production rates as a result of nitrogen addition and a decrease with warming. Long-term nitrogen addition and warming-induced changes caused a shift from Sphagnum-dominated plant communities to vascular plant dominance, which likely transact with many of the observed microbial responses. We conclude that simultaneous perturbations do not always lead to synergistic effects, but can also counteract and even neutralize one another, and thus must be studied in combination when attempting to predict future characteristics and services of peatland ecosystems.