Does microbial community affect soil organic carbon decomposition directly across ecosystems?

Background/Question/Methods

Soil organic carbon (SOC) decomposition is key in regulating SOC dynamics, which is pivotal in carbon biogeochemical cycle due to its large pool size and feedback to climate change. Unfortunately, the prediction of SOC dynamics based on current SOC decomposition models has large uncertainties. Most SOC decomposition models do not incorporate the direct influences of microbial decomposers and not represent SOC stabilization mechanisms. Recently, it is a hot topic to discuss how to incorporate microbes into SOC decomposition models across ecosystems but without much support from empirical studies. To address this question, we quantified the changes in microbial composition and functions in SOC/litter decomposition process from empirical studies, and explored the relations between microbial composition and functions and SOC/litter decomposition rate.

We searched publications for SOC/litter decomposition studies with time series data of mass loss or CO2 respiration, microbial community by using the phospholipid fatty acid (PLFA) method, and microbial functions indicated by enzyme activity. The PLFA biomass and enzyme activity measured at different times were normalized to the initial values available at the beginning of decomposition and plotted again decomposition time to find out the change patterns.

Results/Conclusions

Total PLFA biomass shows the decreasing trend in decomposition. This decreasing trend coincides with reduced labile organic carbon in decomposition, supporting that substrate quality is key to control SOC/litter mineralization. Differently, fungal and bacterial PLFA biomass have no apparent change trends in decomposition, which do not agree with the assumed microbial succession from bacteria dominance in early decomposition stage to fungi dominance in late decomposition stage. Enzyme activity generally shows no clear trend, except that phenol oxidase activity increased in decomposition. Mass loss or cumulative CO₂ respiration in decomposition was strongly correlated with cumulative enzyme activity in each of all decomposition studies but not in all data together. Moreover, the correlations between PLFA biomass and mass loss or cumulative CO₂ respiration are absent except few studies.

Taken together, this study provides empirical evidence that direct influences of microbes in SOC/litter decomposition is more likely to exist at local scales instead of global scales. To address how to incorporate microbes into SOC decomposition model, the priority is to study microbial cohorts rather than microbial composition (e.g., fungi and bacteria) that can different represent SOC mineralization pathways in SOC/litter decomposition.