Relationship between microbial composition and substrate use efficiency in a tropical soil

Soil organic matter (SOM) is crucial to soil health, supporting most of the soil properties that benefit plant growth and ecosystem services including carbon sequestration, nutrient recycling and water infiltration. Recent study is exposing the soil microbial community as not only decomposing SOM, but also providing the primary source of chemicals for its formation. All else equal, SOM formation is theoretically greatest when microbes maximise enzyme efficiency (ratio of enzyme activity to carbon loss from respiration) and biomass efficiency (biomass gain per unit substrate added). Our study examines the relationship between microbial composition and metrics of these two efficiencies. We hypothesised that both will increase with higher ratios of soil fungi to bacteria. We manipulated microbial composition through sustained use of selective microbial inhibitors in microcosms with tropical soil, alongside variation of litter quality and diversity and the presence or absence of a simulated root exudate. Both litter and inhibitor treatments significantly changed soil microbial composition and function, and enzyme efficiency and biomass efficiency were both higher in microbial communities with more fungi, supporting our hypothesis. Structural equation modelling suggested that the observed efficiency changes did indeed occur in part via changes in microbial composition after accounting for direct effects of treatments. Taken together the results provide some support for the hypothesis that soil fungi benefit SOM formation.