CO2 fluxes to aquifers beneath cropland:merging measurements and modeling

Carbon dioxide (CO2) fluxes in the vadose zone are influenced by a complex interplay of biological, chemical and physical factors. To determine the controls behind dissolved inorganic carbon (DIC) percolation to aquifers, CO2 fluxes in planted soil mesocosms were described with the SOILCO2 model formulation (Šimůnek et al., 1993) that was implemented into the HP1 module of the Hydrus 1D software package. Water flow, cation exchange, and supersaturation for amorphous aluminum hydroxide were modeled. The model provided a good fit to measured water content and outflow time series. Also, the measured DIC efflux, CO2 partial pressure (pCO2), and CO2 efflux were simulated well throughout most of the experimental period. However, alkalinity was significantly overestimated, indicating additional acidity production in the mesocosms. CO2 fluxes were strongly influenced by a higher root growth in the mesocosms as compared to the field, which caused steep increases in pCO2. The model showed that the high pCO2 triggered weathering of calcite, leading to increases in alkalinity. DIC percolation lacked accompanying increases, indicating an overestimation of the DIC percolation estimated from measured pCO2, alkalinity, and water flux.