Abstract:
Biochar is the product of thermal degradation of organic matter in the absence of oxygen (pyrolysis), and is distinguished from charcoal by its use as a soil amendment. Biochar as a soil amendment is capable of improving soil fertility and mitigating climate change, which is related to soil microbial composition shifts. Biochar has been demonstrated to be a redox-active and conductive carbon matrix. Biochar has been used as an electron shuttle influencing dissimilatory Fe(III) reduction by mediating electron transfer via surface redox functional groups (i.e. its ability to take up and donate electrons via redox-active functional groups, thus functioning as geobattery) between microorganisms and Fe(III) minerals. Additionally, biochar as conductive material has been suggested to contribute to electron transfer from electron-donating microorganisms (e.g. Geobacter spp.) to electron-accepting microorganisms (e.g. Methanosarcina) via its conductive carbon matrix (functioning as geoconductor). This electron transfer mechanism is involved in a conductive-materials interspecies electron transfer (CIET) process contributing to methanogenesis, especially in anoxic environments such as paddy soils. Electron shuttling mechanisms have been proved to stimulate electron transfer between Fe(III)-reducing bacteria and Fe(III) minerals. An increasing number of observations, however, have questioned the geobattery function of biochar stimulating microbial Fe(III) reduction because of an inhibition effect of biochar (at low concentration) on microbial Fe(III) reduction as it was observed recently. To this end, this thesis determined the rates and extent of microbial Fe(III) reduction by Shewanella oneidensis MR-1 in cell suspension experiments amended with different ratios of wood-derived biochar to ferrihydrite (g biochar/mol Fe) and different biochar particle sizes. Moreover, this thesis also has shown the extent of aggregation of cells, biochar and ferrihydrite at different biochar:Fh ratios and has investigated the fate of electrons from substrate oxidation flow to microbial Fe(III) reduction and could be stored in biochar based on thermodynamically calculations. This thesis has explicitly illustrated the contribution of biochar as geobattery and geoconductor to microbial Fe(III) reduction, which depends on the extent of aggregation of cells, biochar and Fe(III) minerals, and biochar particle sizes (Chapter 3). When biochar was applied to paddy soil, biochar as soil amendment alters the soil microbial community and mitigates methane emissions. Biochar can participate in biogeochemical electron transfer processes due to its function as geobattery) and its function as geoconductor. Each of these two mechanisms has been separately demonstrated to play a role in biogeochemical iron cycling and greenhouse gas formation. Yet, little is known about the coupling of both biochar electron transfer mechanisms, despite the fact that naturally occurring electron transfer through biochar is expected to rely on both geobattery and geoconductor mechanisms simultaneously. Here we conducted anoxic microcosm incubations to investigate how biochar influences electron transfer in a paddy soil and affects the indigenous soil microbial community. We found that the coupled function of biochar as geobattery and geoconductor simultaneously promoted the rates of microbial Fe(III) reduction and methanogenesis by 2.1- and 2.3-fold, respectively, with smaller biochar particles leading to higher rates of Fe(III) reduction and methanogenesis than larger particles. In contrast, the redox-active model compound anthraquinone-2,6-disulfonate (AQDS), which functions solely as geobattery, only stimulated iron reduction in our microcosms. While the biochar geobattery mechanism dominated microbial Fe(III) mineral reduction, the stimulation of methanogenesis was likely a result of the conductive-particles interspecies electron transfer caused by biochar functioning as geoconductor. Microbial community analysis supported this hypothesis by showing that the addition of biochar stimulated the syntrophic activity of acetate-oxidizing Geobacteraceae taxa and methane-producing Methanosarcina taxa and an obvious increase in copy numbers of 16S rRNA genes specific for Geobacter spp. and methyl-coenzyme M reductase subunit alpha (mcrA) gene. In summary, our results demonstrated that a coupled effect of biochar functioning both as geobattery and geoconductor affect soil microbial metabolisms by facilitating electron transfer either from cells to minerals or cells to cells, influencing methane emission in a paddy soil (Chapter 4). Taken together, the results presented in this thesis reveled that coupled function of biochar as geobattery and geoconductor play an important role either in microbial Fe(III) reduction or methanogenesis. Extent of aggregation of cells-biochar and biochar-Fe(III) minerals influence the electron transfer mechanisms via biochar from microorganisms to microorganisms or from microorganisms to Fe(III) minerals. These new findings improve our understanding about the role of biochar in electron transfer and highlight the importance of biochar as soil amendment in dissimilarity Fe(III) reduction and methanogenesis.
Biochar