Discussion:
Collectively, this work identifies how roots and soil microbes drive SOM
loss and formation in miscanthus systems that can promote soil C
sequestration and support plant productivity. Root ingrowth did not
promote a net litter C loss from soil (Fig. 2 ) despite
increased light POM decomposition due to the enhanced transfer of C into
heavy POM (Fig. 3 ). Notably, we document the potential for
roots to mobilize litter-derived N from POM without priming litter C
loss (Fig. 2 , Fig. 4 ). We also identified that
microbial nutrient or carbon limitation may alter how microbes grow and
decompose litter-derived SOM, with more litter decomposition and less
MAOM formation from litter in organically fertilized soils (Fig.5 ).
It appears that miscanthus roots can mine N from litter without
stimulating corresponding litter C losses (Fig. 2 ) and can
increase the C:N of litter-derived light and heavy POM (Fig.4 ). This raises the question of how miscanthus accesses N from
decomposing litter without priming C losses that are commonly observed
in other ecosystems (Cheng et al., 2014; Zhu et al., 2014). One
plausible mechanism may be that miscanthus roots engineer their
rhizosphere microbiome composition or function to preferentially
decompose N-rich litter compounds like proteins, potentially by
stimulating proteolytic enzyme production (Brzostek & Finzi, 2011).
While the specific mechanism remains uncertain, preferential N mining
from litter has important implications for miscanthus sustainability
(e.g., the propensity of miscanthus to be high yielding and build soil
C). The resulting increase in remaining litter C:N may make new
litter-derived SOM even more resistant to further decomposition. In
addition, there has been a long-standing question of how miscanthus can
maintain relatively high yields with limited N inputs (Cadoux et al.,
2012). Previous research has posited that high nutrient use efficiency
(Beale & Long, 1997) or the promotion of N-fixing symbionts (Davis et
al., 2010) sustains N nutrition by miscanthus. Overall, our results
suggest that miscanthus may also meet its N nutrition by effectively
mining N from litter and SOM.
Our research suggests that roots can actively support the transfer of
litter derived C into more protected forms. We observed that the priming
of litter decomposition from light POM was balanced by litter C
incorporation in heavy POM (Fig. 3 ). The composition of heavy
POM is not as well-characterized as light POM or MAOM, but this pool is
commonly assumed to be composed of stable soil macro- or
micro-aggregates (Lavallee et al., 2020). Aggregate occluded SOM is
largely formed through root and mycorrhizal symbiont activity (Rillig &
Mummey, 2006) and often consists of partially decomposed plant and
microbial organic matter fragments. This pool has a higher activation
energy for decomposition than low C:N compounds like those in MAOM
(Williams et al., 2018) and is more protected from decomposers than free
light POM (Keiluweit et al., 2017; Kögel-Knabner et al., 2008). As such,
there is an opportunity to build soil carbon in high C:N, heavy POM
rather than lower C:N MAOM. The N requirements of low C:N SOM retention
have often been cited as a criticism to efforts to use soil C management
to mitigate global change (Schlesinger & Amundson, 2019). Future
research efforts that investigate how roots can build new, persistent,
and high C:N SOM could help realize the potential of soil C
sequestration to combat climate change.
We found that the organic fertilizer treatments had the greatest
microbial biomass and litter-derived light POM decomposition, in support
of our second fertilization hypothesis, but less litter C and N were
incorporated into MAOM (Fig. 5 , SI Figs. 4, 5 ). On one
hand, differences between fertilization treatments could arise from a
shift in the microbial community structure or function with organic
fertilization (Pan et al., 2014). However, other research at the site
has found no significant effects of nutrient treatment on microbial
diversity or mycorrhizal abundance between treatments (Kane et al. 2023,in review ). On the other hand, C vs. N limitation over microbial
decomposition can regulate the rate and efficiency of SOM cycling
(Averill & Waring, 2018; Schimel & Weintraub, 2003). As organic
fertilization deposits both C and N, our observations could be explained
by the alleviation of C limitation and induction of N limitation. In
support, we observed a reduction in nitrification rates with organic
fertilization relative to unfertilized plots (SI Fig. 6 ) and
other research found that organic fertilization increases plot-scale
microbial respiration (Kane et al., 2023, in review ). Here,
microbial decomposers could increase decomposition and growth while
respiring excess C and immobilizing N in living biomass rather than
forming more microbially-derived MAOM (Schimel & Weintraub, 2003).
While our experiment identified several important ways living roots and
soil microbes control litter decomposition and SOM formation, some
mechanisms may not have been fully captured. Our experiment was designed
to separate the effects of roots vs. mycorrhizal fungi on litter C and N
transformations, but our data only identifies a root effect despite the
presence of mycorrhizal fungal symbionts (SI Fig. 7 ). The lack
of differences between fungal ingrowth and total exclusion cores could
be linked to the greater dependence of AM plants on root than hyphal
foraging for nutrient uptake (Chen et al., 2016). As such, our
experiment may not have isolated fungal effects on litter decomposition
and SOM formation. Future efforts should quantify mycorrhizal fungal
ingrowth to better investigate the contribution of symbiotic fungi to
root-driven SOM transformations. In addition, our observations that
fertilization did not impact root biomass (SI Fig 4a ) and that
there was no significant interaction between fertilization and ingrowth
treatments (SI Table 2 ) do not support our first fertilization
hypothesis that roots would have the greatest effect in unfertilized
soils. While miscanthus root systems do not always respond to
fertilization treatments(Amougou et al., 2011), this pattern may have
been driven by the stand age of miscanthus in our experiment. These
plots were in the third year of growth whereas older, more nutrient
limited stands exhibit greater differences in root C allocation and N
acquisition (Kantola et al., 2022). As such, future efforts to
investigate how nutrient availability alters living root impacts on SOM
formation should leverage ecosystems with longer-term fertilization
history. Despite these limitations, our data has identified several
important mechanisms of SOM formation in situ and provides the
foundation for future efforts to study how living roots and fungi alter
SOM dynamics with more sophisticated measurements, under different
environmental conditions, or across different ecosystems and
plant-microbe interactions.
This work has expanded our mechanistic understanding of how living roots
shape ecosystem processes in agricultural systems. Our finding that
miscanthus roots can simultaneously prime N release from litter without
an additional C release and transfer C into a more persistent form of
SOM has important implications for the sustainability of bioenergy
production as well as the viability of restorative agricultural to
offset carbon emissions. Overall, our work suggests that living roots
can selectively mine N while sequestering soil C. This knowledge can
help improve the predictive understanding of SOM cycling that is
critical to meeting the goals of restorative agriculture.