The goal of the proposed session is to review current thinking and explore future research directions regarding the effects of plant genotypic and microbial diversity on ecosystem processes, and how these relationships might determine ecosystem response to global change. We are particularly interested in exploring the potential for interactions between these two trophic levels – e.g., how plant genotypic diversity indirectly influences ecosystem processes via its impact on microbial community composition, and vice-versa. Ecologists have a longstanding interest in the relationship between biodiversity and ecosystem processes. For example, the influence of species richness and/or functional group diversity on productivity in plant communities has been explored extensively, and has been the source of considerable debate among ecologists. More recently, it has become clear that intraspecific richness can act as a functional surrogate for interspecific richness in species-depauperate systems where a single foundation species forms the dominant structural component of the community. Thus, the genotypic diversity of producers (e.g., cottonwoods) has been shown to increase ecosystem productivity. Although the fundamental importance of microorganisms to ecosystem processes such as decomposition has long been recognized, efforts to link function to specific members of the microbial community have historically been very difficult. Recently, advances in molecular techniques have revolutionized microbial ecology by enabling investigators to characterize variation in community composition more or less in situ, as well as to explore its link to biogeochemical functions. As a result, numerous recent studies have demonstrated a correlation between microbial community composition and ecosystem process rates like decomposition, and there is growing evidence in support of a link between environmental change and microbial structure-function relationships. Given the intimate relationship between producers and decomposers, the effects of biodiversity on ecosystem processes is likely regulated to a large degree by complex interactions between these trophic levels. For example, microbially-mediated decomposition is dependent upon organic matter inputs from plants, and thus it no surprise that plant community composition can influence both microbial structure and function. Recently, investigators have demonstrated that within-species variation can have a similar effect. It is also likely that microbial community composition influences plant productivity - based upon both model predictions and empirical evidence that microbes have important direct (via mutualistic or pathogenic symbioses) and indirect (via competitive or facilitative interactions) affects on plants, but there have been few experimental tests of this relationship.