We aimed to clarify the individual and interactive effects of temperature increase during snow-free seasons and snow depth change (increase/decrease) on litter decomposition and microbial community in cool-temperate semi-natural grassland. We conducted a 2-year in situ composite warming experiment comprising temperature increase (ca. 2°C) using infrared heaters during snow-free seasons and manual snow depth manipulation (±50% in snow depth) in Japanese grassland. Changes in litter mass remaining and litter carbon-to-nitrogen ratio (C/N ratio) were assessed by litter bag methods. Microbial biomass and community structure were determined by phospholipid fatty acid analysis. Litter decomposition constant (k) was low in the plots with temperature increase during snow-free seasons (0.56) and with less snow cover (0.57), but combining these two treatments resulted in acceleration of decomposition (k = 0.70); probably, decreased decomposition in the cold climate of early spring resulting from advanced snow melting was compensated for by higher temperature. Differences in mass loss among the treatments were well explained by litter C/N, microbial biomass and microbial community structure. The plots with a high mass loss showed lower litter C/N ratio, larger microbial biomass and different microbial community structure comparing to the plots with low mass loss. Our results showed the complex responses of litter decomposition to summer and winter climate change and combination of less snow cover and summer warming seemed to accelerate the decomposition in cool-temperate semi-natural grassland.
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