Effects of CO₂ and nutrient availability on mineral weathering in controlled tree growth experiments

Abstract

We sought to determine the effect of elevated atmospheric CO 2 on mineral weathering reactions in midlatitude carbonate‐bearing forest soils of differing nutrient availability. Increased plant growth and soil respiration under elevated atmospheric CO 2 suggest increased rates of carbon cycling, which may affect mineral weathering. A randomized complete block experiment was conducted, where aspen and maple saplings were grown in open top chambers under two levels of atmospheric CO 2 and soil N. Soil solution chemistry and soil gas PCO 2 profiles beneath aspen were collected from planting (1997) to harvest (1999). Carbonate mineral weathering products (Ca 2+ , Mg 2+ , HCO 3 − ) dominated solutions, which were saturated with respect to calcite. Soil PCO 2 values at 25 cm depth were 41% higher in high N soils, but CO 2 treatment was not significant. An ANOVA model tested treatment effects on spring 1998 solution chemistry. CO 2 treatment had a significant effect on DIC, which was 12% higher in elevated than ambient CO 2 chambers. Little effect of CO 2 treatment was observed in low N soils. In high N soils, solutions had higher concentrations of carbonate weathering products (DIC, 15%; HCO 3 − , 27%; Ca 2+ , 3%, not significant; Mg 2+ , 5%, not significant). Soil N availability had a significant, positive, effect on mean concentrations of Ca 2+ , Mg 2+ , K + , Na + , NO 3 − , SO 4 2− , and DOC. The soil N treatment difference in solutes may result from differences in PCO 2 and, additionally, NO 3 − from organic matter decomposition. Our results suggest that increased carbonate weathering may occur under increased atmospheric CO 2 and in fertile soils.