The multiple element limitation (MBL MEL) model is a process based ecosystem model describing the plot scale dynamics of plants and soils. It is quantitative synthesis of the Mooney-Bloom-Chapin Resource-Optimization hypothesis of plant nutrition (Mooney 1972, Bloom et al. 1985, Chapin et al 1987; also called the "functional equilibrium hypothesis," Farrar and Jones 2000) within a whole-ecosystem context (Rastetter et al. 1997a). The resource-optimization hypothesis predicts how plants should allocate their internal assets (biomass, proteins, carbohydrate...) to acquire resources from the environment (CO2, NH4, NO3, water, light...). In an environment where resource concentrations do not change, the optimum allocation of internal assets is one where all resources in the environment equally limit production (Chapin et al. 1987); otherwise too many assets would be expended toward acquiring a non-limiting resource and a reallocation of those assets toward limiting resources would therefore increase production. Changes in resource availability and in metabolic requirements through time complicate this picture, but the overall concept still applies: plants should constantly adjust the distribution of their internal assets to approach a more balanced rate of resource uptake from the environment (e.g., shifts in root-shoot ratios, allocation of N). Selective pressure should strongly favor species that maintain a near optimum allocation pattern. We use an analogous approach to simulate soil microbial processes. However, because changes in microbial community composition and acclimation of microorganisms to changes in the environment are much faster than acclimation in vegetation, we treat the reallocation of effort for soil processes as instantaneous.