Did elevated atmospheric CO2 alter soil mineral weathering?:: an analysis of 5-year soil water chemistry data at Duke FACE study

A principal driver of biogeochemical weathering of the Earth's crust is soil CO2, produced mainly by plant roots and soil heterotrophs, a water-soluble gas that forms carbonic acid which reacts with soil minerals via cation exchange and mineral dissolution. We examined effects of elevated atmospheric CO2 (ambient + 200 ppmv) in a young pine forest on belowground carbonic acid chemistry of soil water. Soil water was collected every 2-3 weeks over a 5-year period from O horizons and at 15, 70, and 200 cm in mineral soils at the Duke free air CO2 enrichment facility located in a warm temperate climate in North Carolina, USA. Concentrations of major ions were volume-weighted and statistically analyzed using linear mixed-effects models. Experimental interest was placed on interactive effects of CO2 treatment and time: to test effects of gradually increasing CO2 in deep soil horizons where CO2 is highest in concentration, and to protect against inherent plot-to-plot differences in soil water chemistry being interpreted as responses to CO2 treatments. Although significant time-dependent interactive effects were reported for soil CO2, interactive effects were not significant for soil water constituents. These data, combined with limited pretreatment sampling of soil water chemistry and recently determined large heterogeneity in soil solid chemistry at this site, indicate that CO2-weathering response is smaller than the more-than-doubling of weathering reported previously and that increases in weathering are masked by in situ soil heterogeneity. Although the hypothesis that elevated CO2 increases cation leaching and weathering dissolution is supported in laboratory experiments and field studies, quantifying the stimulation of chemical weathering by elevated atmospheric CO2 remains to be tested rigorously in the field.