Associate Professor University of Illinois at Urbana Champaign, United States
Background/Question/Methods
Background: Plants sequester carbon in the soil as they grow via plant litter deposition, exudation, and other rhizodeposits. However, these exudates and added nutrients fuel microbial activity and increase the rates of organic matter decomposition. This rhizosphere priming effect (RPE) acts in opposition to belowground C sequestration. In bioenergy grasses such as Miscanthus x giganteus, belowground soil organic carbon (SOC) dynamics are of particular concern. With deeper roots than shallow ground covers currently in place, M. x giganteus has access to deeper pools of soil. Evaluating the costs and benefits of planting Miscanthus, particularly on nutrient-poor farmland, is crucial to its success as a bioenergy feedstock.
Question: How does planting Miscanthus x. giganteus impact the RPE across the soil profile?
Methods: I measured the magnitude of this RPE through a series of soil incubations. Initially, I confirmed that RPE was occurring both in surface and deep soils by adding glucose to M. x giganteus soil. I also measured the inorganic and organic pools of phosphorus, a potential driver of RPE. Currently, I am utilizing 13C-labeled glucose to measure RPE across the soil profile in both M. x giganteus and the adjacent unmanaged alley.
Results/Conclusions
Results: According to preliminary data, the addition of glucose induces a RPE representing a doubling in heterotrophic soil respiration (p = 0.0180). Additionally, overall respiration differed in shallow soil when compared to deeper soil (p = 0.0383). Through an observational assessment of P levels in both M. x giganteus and the unmanaged alley I found that P levels in M. x giganteus were 27% lower in the top 20 cm of soil. Differences in inorganic P were confined to the top 10 cm, while differences in organic P were clearer across the top 20 cm. From 20 cm down to 100 cm, overall P levels were lower but similar across both M. x giganteus and the alley.
Conclusions: This evidence of both a positive RPE induced by labile C and differences in P availability across the soil profile provided assurance that the deeper-rooted M. x giganteus is changing SOC dynamics across the soil profile. To elucidate the nature of these changes, I am leveraging 13C stable isotope approaches to determine the exact magnitude of this RPE across several soil depths in these soils. A better understanding of SOC will allow us to develop more sustainable approaches to growing bioenergy feedstocks.
