Abstract
Abstract. Dissolved inorganic carbon (DIC) fluxes across the vadose zone are influenced by a complex interplay of biological, chemical and physical factors. A novel soil mesocosm system was evaluated as a tool for providing information on the mechanisms behind DIC percolation to the groundwater from unplanted soil. Carbon dioxide partial pressure (pCO2), alkalinity, soil moisture and temperature were measured with depth and time, and DIC in the percolate was quantified using a sodium hydroxide trap. Results showed good reproducibility between two replicate mesocosms. The pCO2 varied between 0.2 and 1.1%, and the alkalinity was 0.1–0.6 meq L−1. The measured cumulative effluent DIC flux over the 78-day experimental period was 185–196 mg L−1 m−2 and in the same range as estimates derived from pCO2 and alkalinity in samples extracted from the side of the mesocosm column and the drainage flux. Our results indicate that the mesocosm system is a promising tool for studying DIC percolation fluxes and other biogeochemical transport processes in unsaturated environments.
Highlights
The global flux of carbon dioxide (CO2) from the soil to the groundwater as dissolved inorganic carbon (DIC) is estimated at 0.2 Gt carbon (C) yr−1 and is much less than the upward flux of CO2 from the soil to the atmosphere of 59– 76.5 Gt C yr−1 (Kessler and Harvey, 2001; Raich and Potter, 1995; Houghton, 2007)
This suggests that DIC transport to aquifers, in agreement with theory (Appelo and Postma, 2005), can be described by soil gas pressure of CO2 (pCO2), soil water alkalinity and drainage flux, and underlines the gas tightness and reliability of the applied mesocosm system
Our results suggest that the mesocosm system is well suited for investigation of the effect of different agricultural practices such as liming, fertilization, irrigation or cropping on the DIC percolation flux
Summary
The global flux of carbon dioxide (CO2) from the soil to the groundwater as dissolved inorganic carbon (DIC) is estimated at 0.2 Gt carbon (C) yr−1 and is much less than the upward flux of CO2 from the soil to the atmosphere of 59– 76.5 Gt C yr−1 (Kessler and Harvey, 2001; Raich and Potter, 1995; Houghton, 2007). A better understanding of the processes controlling DIC formation and transport to aquifers can be obtained from measurements at field conditions or from studies under controlled conditions. Soil column studies under controlled conditions in the laboratory may be less realistic, but provide potential for a detailed study in a homogeneous environment and may thereby offer better process understanding. Process understanding from mesocosm experiments may be doublechecked through subsequent modeling studies for which homogenous and controlled conditions provide the ideal study frame. A simple and economical soil mesocosm system consisting of carefully filled, homogenized, sieved soil was evaluated for its capability of producing reliable DIC percolation fluxes to aquifers beneath unplanted soil. We compare DIC fluxes obtained from direct measurements with DIC fluxes indirectly determined via measurements of pCO2, pore water alkalinity and drainage flux
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