Plant responses to
atmospheric CO2 enrichment, particularly above- vs. belowground
allocation of net primary production (NPP), will affect soil organic matter
(SOM) dynamics and stocks. Here we
address mechanisms responsible for increased belowground C storage at the sweetgum FACE experiment at Oak Ridge National Laboratory. Plant
C fixed under elevated CO2 was traced into free and microaggregated particulate organic matter (POM) and silt
plus clay associated C using stable isotopes and alkaline CuO-extractable
biopolymers (i.e. lignin phenols and aliphatic biopolymer substituted fatty
acids-SFA). Ratios of syringyl
to vanillyl and cinnamyl to
vannillyl lignin phenols indicated that root-derived lignin was quantitatively more important
than foliar-derived lignin, and that lignin in all fractions has become more
root-like after 5 y of CO2 enrichment. This increase in root lignin
was also associated with 13C depletion of all soil fractions, as a
result of inputs of C fixed under the 13C-depleted elevated CO2,
as well as C accrual in all soil fractions but microaggregated
silt and clay. Sweetgum leaf litter and roots had distinct molecular
signatures in their relative abundances of octadecyl
and hexadecyl-SFA, allowing us to track their
relative contributions to SOM. In
contrast with lignin analyses, the SFA content of both coarse and fine POM was apparently
dominated by leaf cutin rather than root suberin. However, consistent with lignin analyses, both
fractions showed progressive enrichment with root input (suberin)
as a result of increased root growth in the elevated-CO2 treatment. These
results documenting enhanced root biopolymer retention lend further support to
the hypothesis that allocation patterns under elevated CO2 have
shifted towards greater delivery of C directly
to the soil matrix where the potential for soil C accrual is increased, and
confirm that changes in plant allocation patterns can have significant impacts
on SOM dynamics and stabilization.