The Impacts of Biochar and Compost On Microbial Extracellular Electron Transfer Processes As Shown By Studies On Soil Microbial Fuel Cells.

Microbial anaerobic respiration is most frequently associated with aquatic or sediment environments. However, anoxic processes are relevant even in non-flooded systems such as soil aggregates. During extracellular respiration, electrons may be transferred to soil minerals - especially iron, given the abundance of this element in the earth’s crust. The process may also be coupled to oxidation or reduction of mediators – redox-active substances that may act as intermediate electron acceptors or donors - such as humic substances and charcoal or biochar (charcoal applied to soil). Extracellular electron transfer is also involved when microbes utilize reduced minerals or mediators while respiring on nitrate. Therefore, EET in soil is associated with electron transfer to and from electron acceptors and donors, respectively. Biochar is of particular interest to us due to its potential applications in waste management and C sequestration in soil. In this presentation, we report on how we are using soil microbial fuel cells (SMFCs) to study complex processes that influence EET in soil. SMFCs are relatively easy to construct and monitor and can function as a microbial trap to facilitate the study of the microbiology of the system. We are investigating the possibility that microbially-mediated processes occurring in wet soil aggregates can be dissected by studying the regions on and surrounding both the anode and cathode of SMFCs. Factors that may influence EET and whose study is facilitated by investigating a surrogate system include soil type, C and N dynamics as well as the quality and quantity of soil amendments such as compost and biochar. In this presentation, we focus on the effects of soil amendments (biochar, compost and pumice) on the performance, physicochemistry and microbiology of SMFCs. We are using next-generation DNA sequencing to accomplish microbial analysis while pure culture and mixed microbial consortia derived from SMFCs are being used to investigate how biochar and humic substances influence the cycling of iron in soil. Conditions that favor anaerobic respiration are also frequently associated with emissions of greenhouse gases methane and nitrous oxide. Results of this study will potentially impact management of these gases especially in agricultural systems. The microbiology associated with EET is also potentially associated with carbon and nitrogen fixation, which will have implications for building up sustainable soil microbial biomass – a key driver that may have led to the enduring fertility of ancient Anthrosols such as the Amazon Dark Earths.