Reexamining the Rhizosphere As a Microbial Hotspot Across Tree Species Identity and Diversity, Using a Novel Spatial Extracellular Enzyme Assay.

It has been well established that the plant rhizosphere is a hub of microbial growth and activity, and that plant species traits such as root architecture and rhizodeposition alter microbial activity in the rhizosphere. While there is evidence that some tree species traits are plastic and may be altered by tree species diversity or neighborhood composition, there is little information regarding how plastic or sensitive root-associated microbial activity is to altered neighboring forest composition or diversity. It is also poorly understood how the hotspot of microbial activity in the rhizosphere scales up to influence the activity of surrounding soils.

In this study, we used a novel spatial in situ extra-cellular enzyme activity approach to measure the phosphatase activity associated with roots of four tree species either in monoculture or in mixtures of 2, 4, or 8 tree species. The platform of our research is the Biodiversity-Ecosystem Function (BEF) China project, which is a large forest BEF project situated in subtropical China. At each tree individual, we gently exposed roots in a soil surface continuum and then overlaid an agarose gel embedded with a flurescent phosphatase substrate. After a period of exposure, we then took the gels directly to a field laboratory and measured them using a plate reading flourometer set for high resolution scanning. Our results show that different tree species have different associated levels of enzyme activity relative to surrounding root-free soil, but that activity near roots always decreases sharply when moving away from the root surface. Thus, rhizosphere activity may be strongly localized, and knowing the total soil root growth, density, and traits may be important for predicting how root-associated microbial activity plays a role in decomposition across larger spatial scales.