COS 97-5
The resilience of microbial composition and its functioning in response to global change manipulations
Future global changes are predicted to alter ecosystem processes such as leaf litter decomposition. Such environmental changes may affect functioning directly, through changes in abiotic conditions, and indirectly, through changes in microbial and plant communities. The importance of microbial composition for functioning may also change over time, as microbial composition shifts in response to new environmental conditions. However, little is known about the resilience of microbial communities to global change pertubations, both in terms of their composition and functioning.
Here, we report the results of a three-year litterbag experiment, which manipulated microbial composition, plant litter substrate, and the abiotic environment in a California grassland. We assayed bacteria and fungal composition by sequencing of PCR-amplified ribosomal genes and estimating bacterial and fungal abundances. To assay functioning, we measured litter mass loss and chemistry over the course of the experiment. This design allowed us to disentangle whether changes in decomposition due to treatment manipulations (drought and nitrogen addition) were due to abiotic effects or changes in the microbial or plant communities. By tracking these effects over three years, we investigated the resilience of microbial composition and its effect on decomposition rate.
Results/Conclusions
Both drought and nitrogen addition altered bacterial and fungal composition. In the first year, microbial composition still reflected the microbial origin (i.e., whether the community inoculum originated from a drought or nitrogen plot; explaining 13-19% of total variance, P<0.001). These compositional differences indirectly affected leaf litter decomposition. Specifically, the effect of microbial composition on decomposition rate under drought conditions was as strong as the abiotic effect of drought. After the second year, however, bacterial composition was resilient to the initial global change treatment; composition reflected its current environment and/or the original of the leaf litter. In contrast, fungal composition was less resilient; after three years, there was still a significant effect of microbial origin on fungal composition (explaining 4-5% of total variance, P<0.01). At the same time, the functional effects of these compositional changes were only apparent during the first year of the experiment. In years two and three, litter mass loss depended only on the plot environment in the drought experiment. These results suggest that fungal composition on grassland leaf litter is less resilient than bacterial composition. At the same time, decomposition driven by bacteria and fungi appears to be resilient to drought and nitrogen perturbations after one year.