Response of soil mineral weathering to elevated carbon dioxide

Understanding the rates of weathering of soil minerals and the factors that may either enhance or inhibit these rates is a crucial part of understanding many processes from the watershed to the global scale. One potentially important factor in mineral weathering that is not yet well understood is the effect of elevated CO₂ concentrations on weathering rates. Here, the direct and indirect effects of elevated soil CO₂ are examined in field and laboratory-based studies, and the incorporation of the relationship between CO₂ and mineral weathering in soil chemistry models is critically evaluated.; At Mammoth Mountain, California, volcanic ash soil is exposed to naturally occurring high levels of CO₂ from a magmatic source. Comparative analyses of chemical and mineralogical characteristics of exposed and control soils suggest that decade-long exposure to elevated CO₂ concentrations has altered soil dissolution rates. Indirect effects of elevated soil CO ₂ at this site, including vegetation mortality and a decrease in pH, have significant potential to alter weathering rates. Laboratory dissolution studies on whole soils under varying conditions of pH, P_CO2, and concentrations of oxalate (chosen as a proxy for low-molecular-weight organic acids associated with vegetation in soils) were designed to assess both the direct and indirect effects of CO₂. The results of these experiments provide confirming evidence that CO₂ does not directly influence soil dissolution rates under acidic conditions. However, soil dissolution rates are sensitive to indirect effects of elevated CO₂, including changes in pH and organic acid concentration. The inclusion of a direct CO ₂ dependence in a widely used soil chemistry model, PROFILE, may be perpetuating confusion on this issue. Erroneous conclusions in future model applications could result if this relationship is not removed from the PROFILE model.; A significant and striking decrease in the specific surface area of the soil material was observed during all soil dissolution experiments. These observations call into question the informal convention of normalizing reported dissolution rates to the initial surface area. For effective comparison of weathering rates and identification of the factors influencing them, changes in surface area must be accounted for in reporting dissolution rates.