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 stable SOM. In part, this conflict is driven by uncertainty in how litter forms new stable SOM with living roots 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 selectively mobilized nitrogen from litter without additional carbon release, and transferred litter carbon into more stable, aggregate-associated SOM. Litter rapidly formed minerally-protected SOM, but native minerally-protected SOM was also rapidly lost. Overall, our fundings suggest that roots stimulate litter decomposition to selectively release nitrogen. We also highlight there may be a rapidly cycling component of minerally-protected SOM that is accessible to roots and microbes.