Spatial variation in soil microbial processes as a result of woody encroachment depends on shrub size in tallgrass prairie

As woody plants encroach into grassland ecosystems, we expect altered plant-soil interactions to change the microbial processes that affect soil carbon storage and nutrient cycling. Specifically, this research aimed to address how (1) soil chemistry, (2) microbial nutrient demand, and (3) the rate and source of potential soil C mineralization vary spatially under clonal woody shrubs of varying size within a mesic grassland. We collected soil samples from the center, the midpoint between the center and edge, the edge, and the shrub-grass ecotone of multiple Cornus drummondii shrubs across a shrub-size gradient in infrequently burned tallgrass prairie. Total soil carbon and nitrogen increased with shrub size at every sampling location but the edge. Microbial demand for nitrogen also increased as shrubs increased in size. Across all shrub sizes and sampling locations, potential soil carbon mineralization rates were higher when microbes broke down proportionally more shrub-derived (C3) organic matter than grass-derived (C4) organic matter. Our results suggest that the spatio-temporal context of woody encroachment is critical for understanding its impact on belowground microbial processes. In this ecosystem, a longer period of occupancy by woody plants increases potentially mineralizable soil carbon.