Experimental design
We incubated isotopically enriched litter in soil ingrowth cores and traced the fate of litter C and N into SOM over one growing season. Our experimental design included 3 levels of root/hyphal ingrowth: root and fungal ingrowth (root), root exclusion and fungal ingrowth (fungal), and root and fungal exclusion (none) and 3 fertilization treatments: no fertilization (control), high-level inorganic fertilization (high N), or organic fertilization (organic). We randomly selected 5 plots from each fertilization treatment from those which had successful rhizome establishment during initial plot development. Within each plot, we replicated each ingrowth core treatment twice, where we installed ingrowth cores by 2 of the plot’s 25 plants (SI Fig. 3b ). This resulted in a total of 90 experimental ingrowth cores (3 cores x 3 fertilization treatments x 5 plots x 2 locations/plot).