Effect of common bean (Phaseolus vulgaris) on apatite weathering under elevated CO2

Abstract

Plants accelerate chemical weathering of minerals through the production of carbonic acid during root respiration, root exudation, and uptake of ions. These plant processes are increased by elevated concentrations of atmospheric CO2 and may in turn affect mineral weathering. We seek to understand the effect of predicted atmospheric CO2 concentrations on plant-mediated soil mineral weathering. Phaseolus vulgaris (common bean) was grown in flow-through microcosms consisting of a mixture of 85% quartz and 15% apatite sands (by weight). Nutrient release from apatite was measured using plants grown under two atmospheric conditions: ambient CO2 (~400 ppm) and elevated CO2 (~1000 ppm). To sustain plant growth, a nutrient solution without calcium (Ca) or phosphorous (P) was supplied to the plants using peristaltic pumps. Unplanted microcosms receiving the same nutrient solution served as abiotic controls in both CO2 treatments. Using atomic absorption spectroscopy and colorimetry, Ca and P concentrations of the leachate and plant tissue were measured as proxies for apatite dissolution. Plants were harvested every 2 weeks during the 8-week experiment to understand how Ca and P release in the two CO2 treatments changed over time. After 8 weeks, P. vulgaris grown in elevated CO2 had a greater root to shoot ratio than plants grown in ambient CO2. Plants grown in elevated CO2 released more P than plants grown in ambient conditions, possibly due to greater demands for mineral nutrients. P was preferentially released by the plants and there were no significant differences in Ca released by plants. Both elevated and ambient planted microcosms had a lower pH than abiotic controls, likely due to root respiration, nutrient uptake and exudation of organic acids. Organic acids were not identified in the leachate likely due to their low concentrations and rapid breakdown. Plants released as much as 7 times more Ca than abiotic controls, regardless of CO2 concentrations. Biotic experiments released over 200 times more P than abiotic experiments. Our results show increased atmospheric CO2 can determine rhizosphere processes, which in turn affect weathering products. Thus, mineral nutrient fluxes and C sequestration in inorganic pools are likely to increase as CO2 concentrations rise.