Generation of Soil Solution Acid-Neutralizing Capacity by Addition of Dissolved Inorganic Carbon

A Spodosol B horizon (base saturation of 5.4%) collected at the Watershed Manipulation Project site at Lead Mountain, ME, was used to examine soil solution chemistry in response to increasing solution levels of dissolved inorganic carbon (DIC). Acid-neutralizing capacity (ANC), determined by Gran titration, increased from -5 to 163 μequiv L^-1 in response to increasing DIC, with a corresponding increase in base cations (Ca²⁺, Mg²⁺, K⁺, and Na⁺). For the negative ANC solutions, degassing increased solution pH (in equilibrium with atmospheric CO₂) slightly from 4.94 to 5.14, whereas solutions with positive ANC showed large pH shifts (e.g., ANC of 69, pH shift from 4.73 to 6.81). Under equilibrium assumptions and log K_A1 determined from 2.66pH - pA1, measured values for ANC, sum of cations, pH, and degassed pH were found to be in agreement with predictions from a chemical equilibrium model. Results illustrate the importance of pCO₂ levels and cation exchange from the solid phase in generating solution ANC and determining surface water pH. Environmental implications and limitations in the use of chemical equilibrium models are discussed.