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).