This study examines how human-induced vegetation change alters microbiological community composition and biogeochemical function in soils and will provide new insights into the interactions between vegetation, microbes, and ecosystem function. Active conversion of grasslands to forests, or afforestation, has accelerated in the past several decades and represents a major global change factor. However, there is considerable uncertainty about the sustainability of short rotation plantations and soil carbon storage. One of the biggest uncertainties in sustainability and soil carbon storage is the interaction of vegetation change and microbial dynamics. Combining novel molecular techniques, such as tRFLPs, qPCR, and microarrays, with ecological and biogeochemical analyses provides a new opportunity to link microbial and soil function to vegetation change. We hypothesize that the shift from grass to forest will cause a shift in the microbial community increasing the fungal biomass versus bacterial. Molecular techniques will be used to evaluate if the functional capacity of microbial community also changes. Afforestation of temperate South American grasslands provides an excellent opportunity to study vegetation-microbe interaction. These regions are historically treeless, but 30-45 thousand ha per year was afforested through the 1990s. Preliminary results show that afforestation of grassland soils leads to a significant shift in microbial community composition, a significant increase in ammonium and nitrate, and a net loss of soil carbon and nitrogen.