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

Peatlands in the northern hemisphere have accumulated more atmospheric C during the Holocene than any other terrestrial ecosystem, making peatlands longterm C sinks of global importance. Projected increases in N deposition and temperature make future accumulation rates uncertain. 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 increased annual precipitation intensified the negative effects of N. The temperature effect was comparable to an experimental application of almost 4 g N m-2 year-1 for each 1 oC increase. 91 · Our results indicate that current rates of N deposition in a warmer environment 92 will strongly inhibit C sequestration by Sphagnum-dominated vegetation