Simulating the accumulation of calcite in soils using the soil hydraulic model HYDRUS-1D
The distributions of calcite rich horizons within dryland soils are commonly used as paleoclimate proxies. Comprehensive conceptual and mathematical models of calcite accumulation in soils are required to accurately interpret and calibrate these proxies. A conceptual model for calcite accumulation is already well established: As water percolates through a soil, it dissolves minerals, such as calcite, transporting the soluble minerals downward. As soil water is removed by evaporation and transpiration, the water solution becomes supersaturated resulting in precipitation of calcite at depth. The impacts of dynamic plant growth and microbial respiration have not yet been simulated in numerical models for calcite accumulation but are likely important because of their influence on variables governing calcite solubility. The soil hydraulic modeling software, HYDRUS-1D, simulates water and solute transfer through a soil column, accounting for variations in all previously studied variables (temperature, water content, soil pCO₂) while additionally simulating vegetation-soil interactions. Five separate sensitivity studies were conducted to determine the importance for calcite accumulation of 1) soil texture, 2) plant growth, 3) plant phenology, 4) atmospheric CO₂ concentrations, and 5) the proximal variables that control calcite dissolution and precipitation: soil CO₂, soil water content, and soil temperature. In each modeling simulation, calcite was leached from the top several cm and redistributed deeper in the soil after 20 years. Soils with courser texture yield deeper (+20cm), more diffuse calcite horizons, as did simulations with bare soil compared to vegetated soil. The phenology of plant communities (late spring versus late summer growth) resulted in soil calcite accumulation at temperatures differing by at least 10°C. Changes in atmospheric CO₂ concentrations do not affect the soil calcite distribution. Variations in concentration of soil CO₂, rather than soil water content, have the greatest direct effect on calcite solubility. The most significant time periods of annual accumulation also corresponded with positive water fluxes resulting from high matric potential at the surface. Transpiration and evaporation moisture sinks caused solution to travel upward from higher to lower soil CO₂ concentrations, causing CO₂ de-gassing and calcite accumulation. This pathway describes a new qualitative mechanism for soil calcite formation and should be included in the conceptual model.