Electron flows across organo-microbe-mineral units and impact on soil carbon cycling

Soils stock a large fraction of the Earth’s total C. About 1500 Pg C are stocked in the top meter of soils. This is more than twice the amount of C stored in the atmosphere. Thus, understanding the role of soils in modulating climate change as well as maintaining essential soil functions requires a quantitative and mechanistic understanding of soil carbon cycling. Pedogenic minerals are recognized as primary regulators of OM dynamics. Organo-mineral associations involving aluminum and iron minerals are well known to stabilize or retain SOM. In recent years, however, Mn(III, IV) species, which are produced primarily by biological catalysis, have been suggested to be important players in the C cycle. Due to their nanoscale dimensions and high redox reactivity, Mn oxides can impact C cycling and lead to its stabilization (sorption processes) or its decomposition (hydrolytic or oxidative mechanisms. The current literature is sparse both with respect to the mechanism through which oxidized Mn species transform OM and the magnitude to which these processes are important in terms of SOM dynamics.The goal of this project is to obtain a mechanistic and quantitative description of the coupling between the C and Mn cycles. Our guiding hypothesis is that Mn(III, IV) species, including Mn oxide nanoparticles, are important modulators of the C cycle but their biogeochemical impact is constrained by the biotic processes responsible for their formation and the occurrence of suitable biogeochemical niches conducive to active and rapid redox cycling of Mn between reduced, aqueous Mn(II) and oxidized and reactive Mn(III, IV) species. This research project will feature three components. We will quantify the extent and mechanisms of SOM transformation by Mn(III, IV) species and assess the impact of these chemical reactions on the soil microbiome. Because the concentrations of Mn in soils are typically small, an efficient mechanism to recycle and re-generate these reactive species is needed. Thus, we will determine the controls on manganese biomineralization in pure and mixed cultures. Finally, the quantitative relevance of the mechanisms identified in simple model systems will be tested in microcosms and lab-on-a-chip devices where coupled elemental cycles emerge from the interactions between redox sensitive minerals, organic matter and microorganisms, all of which are spatially distributed. This project will deliver novel experimental approaches to elucidate the abiotic and coupled biotic-abiotic processes through which Mn(III, IV) species facilitate the transformation, storage or loss of OC from soil. The results from this research will advance our understanding of the terrestrial C cycle, soil functioning and climate change.