Plant-microbe interactions in the rhizosphere shape carbon and nitrogen cycling in soil organic matter (SOM). However, there is conflicting evidence on whether these interactions lead to a net loss or increase of SOM. In part, this conflict is driven by uncertainty in how living roots and microbes alter SOM formation or loss in the field. To address these uncertainties, we traced the fate of isotopically labeled litter into SOM using root and fungal ingrowth cores incubated in a Miscanthus x giganteus field . Roots stimulated litter decomposition, but balanced this loss by transferring carbon into more persistent, aggregate associated SOM. Further, roots selectively mobilized nitrogen from litter without additional carbon release. Overall, our fundings suggest that roots can efficiently mine nitrogen and build persistent soil carbon.