Quantifying belowground C inputs
We quantified root production as the total root mass recovered within a
given core after two years. Fine roots of temperate trees typically
turn-over in one year (McCormack et al. 2015), and thus roots
recovered in the ingrowth cores likely resulted from one and not two
years of production. Consequently, we did not divide our estimates of
fine root production by two. However, we acknowledge our root production
values reflect the balance between root production and root turnover
over a two-year period. We quantified root-derived C accumulation into
each core using a two-pool mixing model following Panzacchi et al.
(2016). Broadly, as C3 rhizodeposits are incorporated
into the C4 soil core, the δ13C
signature of the C4 soil becomes more deplete over time,
i.e. the δ13C signature becomes more similar to that
of the C3 soil. This change in δ13C of
the ingrowth core soil can be used to calculate total root-derived C
inputs into the core.
First, the fraction of soil C derived from root inputs
(Frd; unitless) was calculated using a two-end member
mixing model:
Frd = (δ13Cingrowth- δ13Ccontrol) /
(δ13Croot –
δ13Ccontrol)
where δ13Cingrowth is the
δ13C of C4 soil collected from an
individual ingrowth cores after two years in the field and
δ13Ccontrol is the average
δ13C of C4 soil collected from two PVC
control cores from the same plot as the ingrowth core after two years in
the field. We estimated root δ13C for AM/ECM ‘mixed’
plots as the mean of site-specific AM and ECM δ13C
values. For any given plot, δ13Crootwas estimated as the mean δ13C for a given mycorrhizal
plot-type at a given site. The net root-derived C inputs
(Crd-net; g C m-2) into surface soils
(0-15 cm) was calculated as:
Crd-net = ⍴ * [C] * Frd * 75000
where ⍴ is the initial C4 soil bulk density (g
cm-3), [C] is the C content (%) of the core after
two years in the field, and 75000 is the conversion factor to transform
% C to g C m-2 to a depth of 15 cm. Bulk density
(1.21 g mL-1) was measured on the initial
C4 soil: sand mixture and is thus constant across all
plots. To estimate an annual net flux (for comparison to annual
aboveground net primary production), we divided root-derived C by the
number of years cores were in the field (i.e. two years at all
sites).
Several assumptions are worth noting with this approach. First, the
δ13C signature is assumed to be uniform throughout the
soil core, such that various C fractions (e.g. mineral-associated vs.
particulate C) of the C4 soil which may turnover at
different rates are assumed to have identical δ13C
values. While recent crop rotations between C4 and
C3 plants where the ingrowth C4 soil was
harvested could theoretically create heterogeneity in soil
δ13C, any such differences are likely small given the
long-term dominance of C4 corn production at the site.
Second, diffusion of dissolved organic C either laterally or vertically
through the ingrowth cores could also deplete δ13C
signatures, resulting in artificially high estimates of root-derived
soil C. However, vertical diffusion should be similar between ingrowth
and control cores, and previous estimates of δ13C
depletion within ingrowth cores due to lateral diffusion suggest the
effect is negligible (Phillips, R.P. unpublished data ).