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Our understanding of the principal controls on litter decomposition is critical for our capacity to predict how global changes will impact terrestrial ecosystems. Although climate, litter quality and soil organisms clearly modulate carbon (C) and nutrient turnover, land-use change affecting plant species composition and structure can alter the relative importance of such controls. We took advantage of prior land-use changes of intentional planting of exotic forest species along a broad precipitation gradient [250-2200mm mean annual precipitation (MAP)] in Patagonia, South America, where we established five paired sites in natural vegetation and adjacent 35-year-old pine plantations. We explored direct and interactive effects of precipitation and plant community structure on litter decomposition with insitu decomposition, common litters and reciprocal transplants, in addition to an evaluation of microenvironmental changes. Surface litter decomposition in natural vegetation (NV) was similar in all sites along the gradient, independent of litter quality, MAP or soil characteristics, while mass loss demonstrated a significant positive linear relationship with MAP in pine plantations (PP). Decomposition of common litters in PP was markedly reduced with respect to NV, which was > 50% faster at the arid extreme of the gradient. C:N ratios predicted decomposition only in PP, and differences in decomposition were highly correlated with impacts of vegetative cover on incident solar radiation.Synthesis. Concurrent changes in plant cover in NV with increasing MAP resulted in reduced incident solar radiation at the soil surface and decreased the relative importance of photodegradation as a control on surface mass loss. These changes eclipsed direct effects of water availability, litter quality and soil nutrients. In contrast, increased shade and recalcitrant litter with afforestation in PP sites combined such that photodegradation was entirely eliminated as a control and biotic decomposition was much reduced. While afforestation projects are promoted as a strategy to mitigate increased atmospheric carbon dioxide due to human activity, our results highlight that primary controls of litter decomposition were substantially altered with unexpected consequences for the C balance of these ecosystems.