In situ decomposition of above and belowground plant biomass of the native grass species Andropogon glomeratus (Walt.) B.S.P. and exotic Imperata cylindrica (L.) Beauv. (cogongrass) was investigated using litter bags over the course of a 12 month period. The above and belowground biomass of the invasive I. cylindrica always decomposed faster than that of the native A. glomeratus. Also, belowground biomass of both species decomposed at a consistently faster rate when placed within an invaded area consisting of a monotypic stand of I. cylindrica as opposed to within a native plant assemblage. However, there was no similar such trend observed in the aboveground plant material. The microbial communities associated with the invaded sites often differed from those found in the native vegetation and provide a possible causal mechanism by which to explain the observed differences in decomposition rates. The microbial communities differed not only compositionally, as indicated by ordination analyses, but also functionally with respect to enzymatic activity essential to the decomposition process. This study supports the growing consensus that invasive plant species alter normal ecological processes and highlights a possible mechanism (alteration of microbial assemblages) by which I. cylindrica may alter an ecosystem process (decomposition).