Trophic level mediates soil microbial community composition and function

Soil microbial communities drive ecosystem processes, and technological advances have led to an unprecedented understanding of these communities. Yet microbes are only one constituent of soil communities. Understanding how soil microbes will respond to changes in the trophic levels of soil food webs, particularly in combination with inputs of labile carbon resources, is vital for a complete picture of belowground dynamics. Here we manipulate the trophic levels of soil communities, creating a microbe treatment, a microbivore treatment, and two predator treatments that test between consumptive and non-consumptive effects. We then exposed these communities to glucose additions that simulate either the rhizosphere or bulk soil. We found that trophic levels, with and without glucose addition, lead to shifts in microbial community composition and function. Specifically, we observed that the presence of increasing trophic levels led to distinct bacterial communities compared to treatments containing only microbes, and the presence of the predator led to the most distinct shifts compared to the microbe treatment. Not surprisingly, soil respiration was greater in the rhizosphere compared to the bulk soil with the microbe treatment exhibiting greater and lesser respiration compared to the other treatments in the rhizosphere versus the bulk soil, respectively. However, the similarity in respiration between treatments was driven by different underlying processes where the presence of the predator leads to increased microbial biomass and microbial efficiency. In fact, trophic levels, compared to the availability of labile carbon, had a greater influence on microbial efficiency. This suggests that trophic levels of soil communities should be considered when attempting to understand the effect of soil microbial communities on ecosystem processes.