Constraints Placed by Community Diversity on the Enzymatic Response of Microbial Decomposer Communities to Climate Change in Southern California

Abstract:

The return of organic carbon to the atmosphere through terrestrial decomposition is mediated through the breakdown of complex organic polymers by extracellular enzymes produced by microbial decomposer communities. It is unclear how microbial diversity constrains enzymatic potential, making it difficult to predict future carbon cycling under climate change scenarios that could alter microbial community composition. To address this question, we deployed fine-pore nylon mesh “microbial cage” litterbags containing grassland litter with and without local inoculum across five sites in southern California, spanning a gradient of 4.0-24.5º C in mean annual temperature and 129-630 mm mean annual precipitation. Litterbags were deployed in October 2014 and collected in March and June 2015. Collected litterbags were assayed for mass loss and potential activity of nine extracellular enzyme classes. We hypothesized that extracellular enzyme potential would be greatest in litter transplanted to moister sites, given the importance of moisture as a driver of ecosystem function in southern California. We also hypothesized that litter inoculated with local microbiota would exhibit greater extracellular enzyme potential than litter containing only grassland microbes, with the assumption that local decomposer microbes would be more effective than grassland microbes at decomposing litter in their native environment. We found that potential extracellular enzyme activities varied significantly (p<0.01) by site for all nine enzyme classes. Six of the nine enzymes assayed (and six of the seven hydrolytic enzymes) failed to support our hypothesis, exhibiting significantly lower enzyme activity in the coldest and wettest site in comparison to the other four sites (p<0.01). Conversely, both oxidative enzymes assayed exhibited the greatest observed activity in the coldest, wettest site, supporting our hypothesis and indicating that hydrolytic and oxidative enzyme classes from the same microbial community respond independently to changes in temperature and moisture. The addition of local inoculum to grassland litter did not have an effect on mass loss or on observed potential enzyme activity for any of the nine enzymes assayed, suggesting that grassland microbial communities may adapt to future changes in climate.