Resilience of extracellular enzymes to environmental change
Microbial enzymes play a fundamental role in ecosystem processes and nutrient mineralization. Therefore understanding enzyme responses to anthropogenic environmental change is important for predicting ecosystem function in the future. We used a reciprocal transplant design to examine the direct and indirect effects of drought and nitrogen (N) fertilization on litter decomposition in a southern California grassland. We asked whether changes in microbial enzymes promote resilience in litter decomposition, a critical ecosystem function. To address this question, we measured microbial biomass and the activities of nine extracellular enzymes in decaying litter over 3 years. We hypothesized that changes in fungal biomass and potential extracellular enzyme activity (EEA) would relate directly to litter decomposition responses. We also predicted that enzyme profiles from initially distinct microbial communities would converge when transplanted into the same environment.
We found that microbial enzyme function was adapted to the local environment and was not immediately resilient. Drought and N altered the efficiencies of EEA, defined as the mass of target substrate lost per unit potential EEA. Enzyme efficiencies declined with drought treatment, possibly because reduced water availability increased enzyme immobilization and reduced diffusion rates. In the N experiment, the efficiencies of b-glucosidase, b-xylosidase, and polyphenol oxidase were greater when microbes were transplanted into environments from which they originated, suggesting local adaptation. Enzyme profiles changed with N addition, and these differences persisted over time even when microbial communities were transplanted into the same treatment. Overall, our results indicate that ecological functions controlled by microbes may adapt to environmental change. Still, adaptation may require months or years, limiting the resilience of microbial function on short time scales.