Simulating the Effect of Vegetation in Formation of Pedogenic Carbonate

The impacts of dynamic plant growth and microbial respiration have not been simulated in numerical models for calcite accumulation but are likely important because of their influence on variables governing calcite solubility. We simulated soil calcite dissolution and precipitation with HYDRUS-1D, which considers vegetation-soil interactions. We investigated vegetated vs. non-vegetated soil surfaces and the effect of plant phenology on seasonal calcite accumulation patterns in a silt loam soil. In each model run, calcite was leached from the shallow subsurface and redistributed deeper in the soil. Under identical boundary conditions, calcite accumulated in a narrower, more concentrated horizon (55% more calcite by mass) under a vegetated than a bare surface. The most significant periods of calcite accumulation corresponded with the onset of seasonal plant growth and root water uptake, when water was flowing up and focusing Ca ions in the root zone, resulting in the narrower, concentrated carbonate horizon. Therefore, accumulation of soil calcite occurred at temperatures 10 C higher for a simulation with summer plant growth {phenology based on C-4 grass blue grama [Bouteloua gracilis (Kunth) Lag. ex Griffiths]} than for a simulation with spring plant growth occurred in the spring (intended to approximate a C-3 plant community). Calcite was precipitated at or near thermodynamic equilibrium in the simulations, whereas previous empirical studies of calcic soils indicate the typical presence of calcite-supersaturated soil water and groundwater. More accurate, future simulations of CaCO3 accumulation should address this discrepancy, perhaps by incorporating a process that inhibits calcite precipitation, which might push calcite accumulation later in the year than simulated here.