Strong long-term interactive effects of warming and enhanced nitrogen and sulphur deposition on the abundance of active methanogens in a boreal oligotrophic mire

Peatlands play a key role in the carbon cycle by being a considerable source of atmospheric methane. Thus, an understanding of the microbial production of methane is important in relation to environmental changes of peatlands. We applied real-time PCR on the mcrA gene and transcript to investigate the peat methanogen community response to the combined effect of 18 years of simulated warming and deposition of nitrogen (N) and sulphur (S) at a boreal oligotrophic mire in Sweden. The long-term effects of the experimental treatments on the methanogens was highly dependent on interactions between the treatment factors Enhanced N deposition amplified the effect of warming, resulting in a further increase of the abundance of active methanogens. The effect of the perturbations was modulated by the depth horizon, with the strongest effect at the water level, where the interaction between enhanced N and S deposition, and warming, resulted in an increase of active methanogens. These results indicate that increasing average temperatures and simultaneously higher N deposition rates will substantially increase the methanogenic activity in northern ombrotrophic peatlands. These findings strongly highlight the importance of accounting for any possible interactive perturbation effects when investigating the response of peat methanogens to environmental change.