Ingrowth core construction and installation
Ingrowth core treatments included root and fungal ingrowth, root
exclusion and fungal ingrowth, and root and fungal exclusion (SI Fig.1 ). Each ingrowth core was constructed with 10cm long, 4.5cm
diameter rigid plastic 5mm mesh tubing. The top 2.5cm of each core was
inserted into 5cm long PVC collars and attached with elastic sealant.
Mesh bases were sewn onto each core with 12lb nylon fishing line and
each core was wrapped with mesh that was glued on with 100% silicon
adhesive. Root and fungal ingrowth cores were constructed with 1.5mm
polyacrylic mesh that allowed fine root ingrowth. Root exclusion cores
were constructed with 50um nylon mesh that was too fine for root
ingrowth but allowed hyphal ingrowth (Phillips et al., 2012). Root and
fungal exclusion 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.1 ).
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 10cm were collected from each future
ingrowth core location and were brought back to the lab where they were
sieved to 2mm 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 53um diameter in a 9:1 soil:sand ratio. Isotopically
enriched corn leaf litter, generated as described in Ridgeway et al.,
2022, was used as the substrate in each ingrowth core. This litter had a
%C of 41.7% (±0.17%), C:N of 18.8 (±0.64), δ 13C of
7020 (±49), and δ 15N of 34,800 (±310) and was dried
and coarsely ground. Each core was filled with corresponding soil, and
250 mg labeled litter was gently mixed in to the top 2cm.
Within 5 days of initial soil collection, the assembled cores were
transported to the field location where they were installed into the top
10cm of soil in each corresponding treatment plot (SI Fig. 2a ).
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. 2b ). 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).