Carbon Dioxide Degassing Effects on the pH of Spodosol Soil Solutions

Abstract

AbstractHigh levels of CO2 in soil air can influence the composition of the soil solution according to carbonate equilibrium relationships. Carbon dioxide can lower soil solution pH by reacting with water to form aqueous carbonic acid (H2CO*3; the asterisk denotes hydrated carbon dioxide plus carbonic acid). Measurements of soil CO2 from a subalpine Spodosol showed concentrations as high as 12 000 µL L−1 (1.2%). Standard methods used to collect soil solutions can degas CO2 from soil solutions, resulting in elevated solution pH or decreased dissolved inorganic C (DIC). The objectives of this study were to determine whether degassing affected solution pH when solutions were collected from an acidic soil using two different techniques. Both collection methods, low‐tension lysimetry and centrifugation with double‐bottomed cups, were found to cause degassing of CO2 from soil solutions; however, pH was affected only in solutions from the lower B horizon and C horizon. The pH of the solutions from the upper horizons was not affected because high concentrations of soluble organic acids maintain these solutions at a pH <4.5. Carbonate equilibrium relationships suggest that the effect of CO2 degassing on soil solution pH should be negligible for forest soils with a solution pH <4.5 with soil‐air CO2 concentrations up to 6000 µL L−1.