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

Abstract

We sought to determine the effect of elevated atmospheric CO2 on mineral weathering reactions in midlatitude carbonate‐bearing forest soils of differing nutrient availability. Increased plant growth and soil respiration under elevated atmospheric CO2 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 CO2 and soil N. Soil solution chemistry and soil gas PCO2 profiles beneath aspen were collected from planting (1997) to harvest (1999). Carbonate mineral weathering products (Ca2+, Mg2+, HCO3−) dominated solutions, which were saturated with respect to calcite. Soil PCO2 values at 25 cm depth were 41% higher in high N soils, but CO2 treatment was not significant. An ANOVA model tested treatment effects on spring 1998 solution chemistry. CO2 treatment had a significant effect on DIC, which was 12% higher in elevated than ambient CO2 chambers. Little effect of CO2 treatment was observed in low N soils. In high N soils, solutions had higher concentrations of carbonate weathering products (DIC, 15%; HCO3−, 27%; Ca2+, 3%, not significant; Mg2+, 5%, not significant). Soil N availability had a significant, positive, effect on mean concentrations of Ca2+, Mg2+, K+, Na+, NO3−, SO42−, and DOC. The soil N treatment difference in solutes may result from differences in PCO2 and, additionally, NO3− from organic matter decomposition. Our results suggest that increased carbonate weathering may occur under increased atmospheric CO2 and in fertile soils.