Two-dimensional modeling of CO2 mineral trapping through the oxalate-carbonate pathway: Influence of the root system model

Two-dimensional reactive transport models, one with a simplified root system and the other accounting for dynamically evolving root architecture, were constructed to examine the influence of model complexity on capturing the effect of soil-root dynamics relating to the Oxalate Carbonate Pathway (OCP) of the Iroko tree over 170 years. Oxidation of oxalate from fallen tree tissue by soil bacteria enables local soil pH increase, leading to the sequestration of atmospheric carbon in carbonate minerals (calcite) in the shallow soil surrounding the tree. Simulations of both root models corroborate previous one-dimensional models of the OCP focused on Ca and C mass balance, where high weathering rates of Ca-containing silicate minerals in bedrock, along with contributions from groundwater, provided sufficient Ca for precipitation of observed quantities of calcite. Both simulations demonstrate the development of a distinct high pH zone where oxalate is oxidized, Ca accumulates, and calcite precipitates (OCP zone); and a low pH zone where roots collect Ca, later returned to the top soil as calcium oxalate (Total Root Extent/TRE zone) via litterfall. While the extent of OCP zone development near the ground surface was very similar between simulations, differences in localized root water uptake between the two approaches resulted in variation in water and solute transport and influenced the geometry of the OCP zone at depth, with implications for calcite precipitation in the soil. Trends in CO2 and O2 partial pressures in the OCP zone were mirrored in the TRE zone, suggesting linkage between the two zones with regard to gas transport. Near the end of the tree's lifespan, results indicate that soil permeability decreases due to calcite precipitation may limit O2 ingress and availability in the shallow soil, while trapping CO2 released from the oxidation of organics in the shallow soil, with implications for the long-term sustainability of the OCP itself.