Abstract Most models for plant litter decomposition link degradation rates to measures of climate or litter composition, rather than directly to microbial activity. We developed a model based on the premise that saprotrophic microbial communities maximize their productivity by optimizing their allocation of resources in the production of extracellular carbon, nitrogen and phosphorus-acquiring enzymes. In this model, enzyme activity indicators are used to estimate decomposition rates and to assess relative N and P availability. This approach may facilitate estimation of decomposition rates in the field and improve ecological forecasting.