Woody encroachment increases carbon degradation rates and alters soil microbial community structure in restored and remnant mesic tallgrass prairie

Grasslands store approximately one-third of the global terrestrial carbon stock and microbial communities in grassland soil play an essential role in soil carbon sequestration. Despite being vital carbon reservoirs, grasslands face many challenges to carbon sequestration, perhaps most notably conversion for agricultural use and encroachment by woody plants. We used a temperate grassland system consisting of a reconstructed tallgrass prairie and an adjacent never cultivated remnant prairie both undergoing woody encroachment to understand how management history and woody encroachment influence degradation rates of multiple forms of organic carbon and microbial community structure. We measured rates of hydrolytic carbon-degrading extracellular enzymes and used 16S and ITS amplicon sequence data to categorize bacterial and fungal taxa into microbial groups of potential carbon use efficiency. We found that extracellular enzyme rates were higher in the remnant site and in encroached soils. Additionally, microbial groups with low potential carbon use efficiency—which are expected to contribute to soil C release—were generally more prevalent in the remnant site and in encroached soils. This could suggest that higher rates of organic carbon degradation occur with lower potential microbial carbon use efficiency. Our results contradict the widely held notion that never-cultivated remnant grasslands are reliable carbon sinks and suggest that woody encroachment can promote C release from grassland soils.