The extent to which roots and mycorrhizal fungi facilitate SOM
stabilization or destabilization in agricultural ecosystems may be
modulated by fertilization. For example, some N-limited plants can
dynamically shift C allocation belowground to root exudation and
mycorrhizal symbionts to stimulate microbial decomposition in the
rhizosphere and increase N acquisition (Brzostek et al., 2014; Kane et
al., 2022). When N limitation is alleviated by fertilization, plants can
also reduce belowground C allocation, suppressing SOM decomposition
(Eastman et al., 2021; Frey et al., 2014). In the absence of roots and
tightly coupled root-microbe interactions, the degree to which
fertilization alters SOM cycling depends upon the activity of microbial
decomposers. In contrast to plants, soil microbes are primarily
understood to be energy, or carbon, limited (Soong et al., 2020). As
such, organic fertilizer that contains C and N can prime microbial
activity and decomposition relative to inorganic N fertilizer (Cui et
al., 2022; Ndung’u et al., 2021). However, uncertainty remains in the
extent to which the priming of microbial activity leads to net soil C
losses by enhancing decomposition or net C gains by promoting the
production of microbial necromass that can form more stable SOM.
Collectively, the effect of fertilization on SOM stabilization depends
upon the strength of plant-microbe interactions and the form of
fertilizer applied, but the magnitude of this effect is uncertain.
Given the uncertainty above, our objectives were to: 1)
determine how living roots and symbiotic fungi influence litter
decomposition and SOM stabilization in distinct SOM pools and 2) assess
how SOM stabilization processes are altered by fertilization. For the
first objective, we assayed the net effect of the opposing hypotheses
illustrated in Figure 1. For the second objective, we tested two
hypotheses: (1) the effect of living roots on SOM stabilization would be
strongest in unfertilized soil and (2) organic fertilizer would
accelerate microbial decomposition and SOM cycling to a greater extent
than inorganic fertilizer. To meet our objectives, we measured the
effects of living roots and fungi on new SOM formation from isotopically
enriched litter over one growing season. We incubated litter inputs in
soil cores that were open to roots and fungal ingrowth, that excluded
roots but were open to fungal ingrowth, or that excluded both roots and
fungi to quantify the effect of living roots and fungi on new SOM
formation (SI Fig. 1 ). We installed ingrowth cores inMiscanthus x giganteus (herein miscanthus) plots with different
nutrient treatments to investigate the effect of soil N and C
availability on how roots, mycorrhizal fungi, and saprotrophic microbes
drive the transfer of litter C and N into light POM, heavy POM, and
MAOM. We used the bioenergy feedstock crop miscanthus as a study system
because it produces extensive root systems to overcome nutrient
limitation (Dohleman & Long, 2009; Heaton et al., 2008) and because
miscanthus agriculture typically increases SOM levels (Harris et al.,
2015). Further, because bioenergy offers the potential to become a C
neutral or C negative alternative to fossil fuels, it is particularly
critical to investigate what drives SOM accumulation in these ecosystems
(Hanssen et al., 2020).
We show that miscanthus roots increased litter decomposition but did not
lead to a net C loss because roots enhanced the incorporation of litter
C into more stable, heavy POM. Roots also selectively mobilized litter N
from both POM pools without additional C release. As such, roots can
promote the net retention of more stable C while still enhancing N
mining. These root effects did not depend on fertilization. However,
organic fertilization enhanced microbial decomposition of litter without
increasing litter stabilization in MAOM. Regardless of treatment, the
rapid stabilization and destabilization of MAOM in our ingrowth cores
supports recent theories that MAOM may cycle dynamically.