Ingrowth core construction and installation
Ingrowth core treatments included root and fungal ingrowth (root), root exclusion and fungal ingrowth (fungal), and root and fungal exclusion (none) (SI Fig. 2 ). Each ingrowth core was constructed with 10 cm long, 4.5 cm diameter rigid plastic 5 mm mesh tubing. The top 2.5 cm of each core was inserted into 5cm long PVC collars and attached with elastic sealant. Mesh bases were sewn onto each core with 12 lb. nylon fishing line and each core was wrapped with mesh that was glued on with 100% silicon adhesive. Root and fungal ingrowth (root) cores were constructed with 1.5 mm polyacrylic mesh that allowed fine root ingrowth. Root exclusion (fungal and none) cores were constructed with 50 um nylon mesh that was too fine for root ingrowth but allowed hyphal ingrowth (Phillips et al., 2012). Root and fungal exclusion (none) cores were constructed with the same root exclusion mesh and were also twisted once or twice a week to break off hyphae and prevent significant fungal ingrowth and establishment (SI Fig. 2 ).
Ingrowth cores were prepared in the lab using isotopically enriched litter amendments and soil harvested from the corresponding plot. In April 2021, soils from the top 10 cm were collected from each future ingrowth core location and were brought back to the lab where they were sieved to 2 mm and stored at 5°C when not being processed. Soils were homogenized within each nutrient treatment (control, high N, or organic) and were mixed with sand that had been acid washed and separated from particles less than 53 um diameter in a 9:1 soil:sand ratio to prevent soil compaction. 250 mg of isotopically enriched corn leaf litter, generated as described in Ridgeway et al., 2022, was used as the substrate in each ingrowth core. This addition rate was selected to be lower than litter production at the site to limit experimental artefacts from introducing a new decomposition substrate and high enough to ensure that the 13C inputs were traceable into SOM pools. This litter had a %C of 41.7% (±0.17%), C:N of 18.8 (±0.64), δ13C of 7020 (±49), and δ 15N of 34,800 (±310) and was dried and coarsely ground. Each core was filled with corresponding soil, and the labeled litter was gently mixed in to the top 2 cm.
Within 5 days of initial soil collection, the assembled cores were transported to the field location where they were installed into the top 10 cm of soil in each corresponding treatment plot (SI Fig.3a ). This occurred in April 2021 when miscanthus shoots were beginning to emerge. Within each plot, ingrowth cores were installed 8” north of visibly emerged miscanthus shoots (SI Fig. 3b ). After 20 weeks, the ingrowth cores were carefully cut from the soil in September 2021 and were brought back to the lab for processing. Although each treatment combination began with a planned replicate of n=10, two cores were removed from analysis due to animal interference. Additionally, five cores intended for the root exclusion fungal ingrowth treatment (fungal) were invaded by roots. After determining that these cores did not significantly vary from the rest of the root ingrowth (root) cores, these were also analyzed as root ingrowth (root) cores. Given these adjustments, the total replication ranged from 5-15 for each treatment (provided in SI table 1).