Divergence between plant and microbial allocation strategies explains continental patterns in microbial enzyme allocation and alters biogeochemical fluxes

Background/Question/Methods

Allocation tradeoffs can shape ecological and biogeochemical phenomena at local to global scale. Plant allocation strategies drive major changes in ecosystem carbon cycling. Microbial allocation to enzymes that decompose carbon vs. organic nutrients may similarly affect ecosystem carbon cycling. Current solutions to this allocation problem prioritize stoichiometric tradeoffs implemented in plant ecology. These solutions split investment between carbon and nitrogen degrading enzymes in an attempt to match resource uptake stoichiometry to microbial biomass stoichiometry. These solutions may not maximize microbial growth and fitness under all conditions, because organic nutrient decomposition is a viable strategy to acquire carbon. I created multiple allocation strategy frameworks and simulated microbial growth using a microbial physiology driven biogeochemical model. I then tested predictions from multiple allocation models against a continental data set (n=651) of sediment enzyme data and substrate stoichiometry.

Results/Conclusions

I demonstrate that prioritizing stoichiometric tradeoffs does not optimize microbial resource allocation, while exploiting organic nutrients as carbon resources does. Analysis of a continental-scale enzyme data set supports the allocation patterns predicted by this framework. Modeling suggests large deviations in soil C loss and N-mineralization based on which allocation strategy is implemented. Furthermore, this allocation framework challenges current methods of inferring microbial carbon use efficiency based on enzyme activity ratios. If microbes primarily decompose organic nutrients to acquire carbon in some environments, then the assumption that resource supply ratios match microbial threshold element ratios is violated, and carbon use efficiency calculations will not be possible. Given the importance of allocation tradeoffs in other organisms and in representing ecosystem carbon cycling, understanding microbial enzyme allocation strategies will likely improve our understanding of ecosystem element cycling, and by extension global climate.