Reactive transport modeling the oxalate carbonate pathway to assess carbon sequestration by the Iroko tree

A reactive transport (RT) model of the oxalate-carbonate pathway (OCP) of the Iroko tree in the Ivory Coast, Africa, was constructed to evaluate rates and amounts of biogenically-induced calcite formation. Previous field and laboratory works indicate that the OCP mediates biomineralization via calcium uptake by roots and formation of calcium oxalate crystals in plant tissues. This leads to litter fall containing calcium oxalate particles, and then oxalate release and oxidation in the soil, resulting in the formation of calcite in soils where no calcite was previously present. Any carbon stored as calcite in soil, would appear to be a viable form of carbon capture over and beyond the life cycle of the tree. The RT model focused on quantitatively evaluating the fluxes and fate of calcium and carbon in the different components of the OCP process, to assess how much carbon may be stored in soils over the life cycle of a mature tree (170 years). To calculate a feasible mass balance, literature regarding calcium input from dust, precipitation, bedrock, and other potential sources on the Ivory Coast, was combined with average precipitation rates, soil porosity, uptake rates, litter fall rates, litter decomposition rates, and known values for the kinetics of the relevant oxidation-reduction reactions. Initial conditions (soil pH, soil pCO2, pO2, and soil water content) were set based off literature values regarding tropical ferralitic soils near the Ivory Coast. A 1-D model representing a vertical soil profile was constructed using the RT code MIN3P-THCm. Initial model results indicate that soil pH rose from an initial acidic value (4.7) to more alkaline, over 170 model years. The alkaline soil did allow calcite formation, particularly near the surface where pH was the most basic, and where calcium oxalate had explicitly been present. Levels of calcite formation dropped where calcium oxalate had been absent, despite the presence of other calcium sources in the bulk soil. Subsequent 2-D models are being constructed to further assess the spatial extent and influence of the pH change, to be coupled with a root architecture model to investigate the influence of solute uptake, and the overall role of the root system on biomineralization and the OCP of the Iroko tree