Enhancing soil carbon sequestration may enable the bioenergy industry to achieve carbon neutrality. To realize this potential, we must develop a predictive understanding of how management and feedstock decisions impact carbon (C) stabilization in soil organic matter (SOM). However, there remains uncertainty in how interactions between plant traits (i.e. litter chemistry, rhizosphere exudation) and microbial traits (i.e. carbon use efficiency (CUE), turnover) drive SOM formation for bioenergy feedstocks. To address this uncertainty, we traced the fate of 13C-labeled feedstock litter from traditional maize and miscanthus x giganteus feedstocks into soil microbial biomass, respiration, and SOM pools in lab microcosms. We also added root exudate solutions to half of these microcosms to simulate plant exchange of labile C with soil microbes for nutrients. This allowed us to test plant-microbial interactions controls over SOM formation both indirectly through litter-chemistry dependent microbial decomposition and directly through exudation-promoted microbial decomposition. Microbial necromass is thought to preferentially form mineral associated SOM over particulate SOM. Thus, we hypothesized that low carbon to nitrogen ratio (C:N) maize litter decomposes faster, produces more microbial necromass, and forms more mineral associated SOM than high C:N miscanthus litter. Miscanthus is thought to rely more strongly on microbially-fixed nutrients rather than heavy fertilization. We hypothesized that soil microbes in miscanthus systems have a greater capability to use root exudates to drive biomass production and mineral associated SOM formation than maize microbes.
Results/Conclusions
In support of our first hypothesis, maize litter initially decomposed very rapidly, prompting greater respiration losses of carbon compared to miscanthus litter (56± 2.5% compared to 40±4.2% of litter C). In the last four weeks, maize litter decomposition slowed as litter C was immobilized in microbial biomass and mineral associated SOM. By contrast, miscanthus litter C continued to decompose at a consistent rate by less efficient microbes with more of the litter being recovered in particulate SOM forms. Supporting our second hypothesis, exudate carbon additions promoted decomposition of mineral-associated SOM in maize but increased the formation of mineral-associated SOM in miscanthus. Collectively, these results indicate that there is an important interaction between litter chemistry and root traits that controls the formation and decomposition of mineral associated SOM in bioenergy systems.