Chemical weathering of three Piedmont soils in North Carolina

Chemical weathering of soils and rocks has been studied with great interest because weathering dissolution provides many essential elements for life and because it is coupled with current elevation of atmospheric CO₂. One of the primary weathering reagents is carbonic acid generated by both plant-root and soil microbial respiration.; Weathering reactions were investigated in three contrasting soils in the Piedmont of North Carolina using open-system mass-transport functions and solid-phase chemistry, field-collected soil-water chemistry, and laboratory column-leaching experiments. The soils are Tarrus series from slate, Cecil series from granitic gneiss, and Enon series from diabase.; Estimated strains using geochemical mass-balance equation indicated that isovolumetric weathering may be the exception rather than the rule in saprolite formation. Desilication was the dominant pedogenic process. Si losses were about 39, 50, and 73% of total elemental molar losses in Enon, Tarrus, and Cecil profiles, respectively. Losses of base cations accounted for up to 50% of the total elemental loss in the Tarrus profile. Aluminum and iron were translocated from surfacial horizons and accumulated in secondary minerals at depth.; Effects of CO₂ on weathering were investigated by analyzing soil-water chemistry in the Duke FACE experiment in which above-ground CO ₂ were increased by 200 ppmv in elevated CO₂ plots over five years. Although volume-weighted mean concentrations of Ca2+, Mg2+, Si, and alkalinity were greater in waters collected at 200 cm in elevated CO₂ plots (p = 0.004 to 0.08), insignificant interactive effects of time and CO₂ treatment made the effects of CO₂ on weathering equivocal. Substantial within-soil variation and the lack of pretreatment data combined to make effects of CO₂ difficult to detect suggesting that more controlled laboratory experiments were in order.; Laboratory column-leaching experiments were conducted with saprolite from the three soils using 1, 10, and 100% CO₂. All three saprolites were substantially acidified by leaching with CO₂. Cation exchange was the main mechanism releasing cations to solution and comprised from 53 to 82% of total leached base-cations. Results suggest that both cation exchange and mineral dissolution will contribute additional cations to drainage waters if soil pCO₂ increases as atmospheric CO₂ continues to increase.