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Feather mosses in boreal forests form a dense ground-cover that is an important driver of both nutrient and carbon cycling. While moss growth is highly sensitive to moisture availability, little is known about how moss eff ects on nutrient and carbon cycling are aff ected by the dynamics of moisture input to the ecosystem. We experimentally investigated how rainfall regimes aff ected ecosystem processes driven by the dominant boreal feather moss Pleurozium schreberi by manipulating total moisture amount, frequency of moisture addition and moss presence/absence. Moisture treatments represented the range of rainfall conditions that occur in Swedish boreal forests as well as shifts in rainfall expected through climate change. We found that nitrogen (N) fi xation by cyanobacteria in feather mosses (the main biological N input to boreal forests) was strongly infl uenced by both moisture amount and frequency, and their interaction; increased frequency had greater eff ects when amounts were higher. Within a given moisture amount, N fi xation varied up to seven-fold depending on how that amount was distributed temporally. We also found that mosses promoted vascular litter decomposition rates, concentrations of litter nutrients, and active soil microbial biomass, and reduced N release into soil solution. Th ese eff ects were usually strongest under low moisture amount and/or frequency, and revealed a buff ering eff ect of mosses on the decomposer subsystem under moisture limitation. Th ese results highlight that both the amount and temporal distribution of rainfall, determine the eff ect of feather mosses on ecosystem N input and the decomposer subsystem. Th ey also emphasize the role of feather mosses in mediating moisture eff ects on decomposer processes. Finally, our results suggest that projected shifts in precipitation in the Swedish boreal forest through climate change will result in increased moss growth and N 2 fi xation but a reduced dependency of the decomposer subsystem on feather moss cover for moisture retention.

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