Electron transfer reactions and their role in soil carbon cycling

Electron transfer reactions are central to the transformation of energy in the environment and play an important role in biogeochemical element cycling. In soils, one of the main drivers of carbon cycling is the activity of organisms that utilize the energy stored in soil organic matter by extracting electrons from organic carbon and transferring them to various electron acceptors. Yet, our understanding of this process is incomplete and the response of the soil carbon pool to climate change remains one of the primary sources of uncertainty in projections of atmospheric carbon dioxide concentrations. Here, I discuss how we can track electron transfer reactions in soil and relate them to bioenergetic descriptors to elucidate controls on soil heterotrophic respiration. I will use two examples from my research to illustrate this: first, I show how to characterize the redox properties of solid phase electron acceptors on the basis of reaction thermodynamics. I focus on iron minerals which are abundant solid phase electron acceptors in many soils. Using mediated electrochemistry, I quantified differences in the reactivity and energetics of synthetic iron minerals and variations in mineral redox properties during microbial mineral reduction. Second, I demonstrate how we can assess effects of mineral redox reactivity on anaerobic microbial respiration in a redox-dynamic floodplain soil. To this end, I link the kinetics of electron transfer to electron acceptors to the rate of microbial carbon dioxide production in a series of soil incubations. These two examples provide inspiration on how to integrate the redox reactivities and energetics of electron acceptors into bioenergetic frameworks.