To assess soil microbial response to global warming in cool temperate semi-natural grassland, we conducted an in situ warming experiment in grassland located in the mountains of central Japan. Five pairs of plots (control and warmed) of Zoysia japonica were established. For each pair of plots, one was warmed by ca. 2 °C using infrared heaters during the growing seasons of 2009–2011. Above-ground biomass of Z.japonica was estimated using the modified point-frame method. Soil organic matter contents, soil total carbon and nitrogen contents, as well as inorganic nitrogen (ammonium and nitrate) contents were determined from soil samples. Total phospholipid fatty acid (PLFA) contents and PLFA compositions were determined and used as indices for total microbial biomass and community structure, respectively. Analyses showed that the warming increased the above-ground biomass of Z. japonica significantly. Soil organic matter and soil total nitrogen contents were significantly decreased, while soil ammonium content was significantly increased in the warmed plots. Soil microbial biomass (especially fungal biomass) was lower in the warmed plots, probably reflecting higher temperature, lower soil water content, and/or depletion in available nutrients. The significant decrease in fungal biomass, in combination with our PLFA composition data, suggests that the soil microbial community structure shifted from a fungal-dominated to a bacteria-dominated one, causing changes in community-level physiological activity.
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