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Anthropogenic changes in climatic conditions, such as the timing and amount of rainfall, can have profound biotic and abiotic consequences on grassland ecosystems. Grassland's plant and animal phenology are adapted to the ecosystem's wet and cold winters and hot and dry summers and changes to this pattern will have profound consequences in aboveground community structure. Changes in climatic conditions and aboveground communities will also affect the soil biogeochemistry and microbial communities. Soil microbes are an essential component in ecosystem functioning, as they are the key players in nutrient cycling. This thesis investigated the direct and indirect effects of climate change on the structure, composition and abundance of grassland soil microbial communities. The research used the high-throughput technique of 16S rRNA microarrays (Phylochip) to detect changes in the abundances and activities of soil bacterial and archaeal taxa in response to changes in precipitation patterns, aboveground plant communities, and soil environmental conditions. The research took advantage of alongterm climate change experiment that simulated both an increase and an extension of the current winter season in northern California. Five years into the experiment, soil samples and aboveground plant diversity were collected before and after each treatment for two consecutive years. The variability in soil microbial communities after natural wet-dry rainfall events was also investigated. Results showed that, at the community level, soil microbial communities are very robust and resilient to intensified or extended rainfalls during the winter but under extreme and unusual weather events their community structure can be altered. On the other hand, an increased in moss biomass in the plots that received additional water during the spring and fluctuations in soil moisture content (precipitation models and wet-dry patterns) caused changes in soil environmental conditions which in turn affected the activity and abundance of some microbial taxa/guilds. Soil organic carbon and inorganic nitrogen were among the environmental variables that correlated the most with these changes in microbial groups. Considering the great importance soil microbes have in ecosystem functioning, the approach developed here will find application for monitor responses of keystone microbial species/guilds to future changes in climatic conditions. These responses should be taken in consideration for future soil management and conservation practices, and the impacts included in future climate change models.

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