ROOT-PROMOTED MINERAL WEATHERING: A REACTIVE TRANSPORT MODELING STUDY

Roots are considered as a driver for mineral weathering in terrestrial systems. Fundamental understanding of the impact of roots structure on in situ mineral weathering, however, is largely lacking due to the challenges of direct measurement and complex processing coupling in natural subsurface settings. In this work, we use a widely used Reactive Transport Model (RTM) CrunchFlow to examined the effects of root structure on mineral weathering rates and solute transport. The image data of root structures of herringbone and dichotomous systems with different branching patterns were read into the simulation domain. Results showed that mineral weathering (in terms of Ca, Mg, Al, Si, Fe, and K outflow rate, mol m-2 d-1) was enhanced by 3-6 times due to the lower soil pH induced by the excretion of organic acids and ligand (oxalate), although the presence of roots slowed down solute transport and by reducing the local permeability. Though soils with the dichotomous system had lower pH than those with herringbone system, the mineral weathering was slower in dichotomous system than in herringbone system, indicating the physical transport (limited by the reduced local permeability) played a more dominant role than the chemical reaction (promoted by the lower pH) in determining mineral weathering rate. In general, roots could enhance mineral weathering rate by excretion of organic acids and ligand; however, different root structures exhibited varying effects on mineral weathering rates depending both on promoted chemical reactions and limited solute transport. This work provide valuable insights on how roots affect mineral weathering and soil chemistry.