Climatic modifiers of the response to nitrogen deposition in peat‐forming Sphagnum mosses: a meta‐analysis

Summary• Peatlands in the northern hemisphere have accumulated more atmospheric carbon (C) during the Holocene than any other terrestrial ecosystem, making peatlands long‐term C sinks of global importance. Projected increases in nitrogen (N) deposition and temperature make future accumulation rates uncertain. • Here, we assessed the impact of N deposition on peatland C sequestration potential by investigating the effects of experimental N addition on Sphagnum moss. We employed meta‐regressions to the results of 107 field experiments, accounting for sampling dependence in the data. • We found that high N loading (comprising N application rate, experiment duration, background N deposition) depressed Sphagnum production relative to untreated controls. The interactive effects of presence of competitive vascular plants and high tissue N concentrations indicated intensified biotic interactions and altered nutrient stochiometry as mechanisms underlying the detrimental N effects. Importantly, a higher summer temperature (mean for July) and increasedannual precipitation intensified the negative effects of N. The temperature effect was comparable to an experimental application of almost 4 g N m −2  yr −1 for each 1°C increase. • Our results indicate that current rates of N deposition in a warmer environment will strongly inhibit C sequestration by Sphagnum ‐dominated vegetation.