||Per unit area, soils harbor the greatest diversity on earth largely in the fungal and bacterial pool. These communities, which also harbor a diversity of functions, are responsible for degrading and mineralizing the carbon and nutrients that enter the soil. Soil microbial communities are directly and indirectly responsible for the mineralization of terrestrial soil carbon and nutrients, thus understanding the links between soil communities and root function and soil processes is a major theme in ecosystem and global change ecology, but one that remains poorly understood. Soil properties and biological communities vary significantly by ecosystem and soil type, but current soil carbon cycling models fail to incorporate these interacting factors. For example, mycorrhizal fungi, which associate with plants and are important for nutrient uptake, are an integral link to carbon cycles in ecosystems because they exchange soil organic matter-derived nutrients for plant-assimilated carbon, yet their activity, and how their regulation of carbon dynamic might change by soil type, is often not considered in models. The failure to consider the role of the integrated biological community –microbes, mycorrhizal fungi, and plant roots – on soil carbon turnover may render models incapable of predicting local-scale responses to environmental change. Our project will fill this knowledge gap by modeling and experimentally manipulating the biological community in situ across temperate, tropical and boreal ecosystems, thus increasing our ability to predict and model the extent to which soils in terrestrial ecosystems will be an atmospheric net C source or sink.