Soil solution pH measurements using in-line chambers with tension lysimeters

During soil water extraction, pH can change as a result of atmospheric gas exchange. The pH change is important for monitoring soil acidification and determination of mineralogic controls on the solution composition. As part of a global change programme in Greenland for monitoring long-term changes in Arctic soil solutions we observed that the pH of extracted soil solution s increased in the order of a half pH unit during traditional sampling and handling of the soil solution. CO2 degassing is often considered the most i mportant factor causing such a pH increase. Thus, traditional as well as in -line pH measurements were performed during the summers 1997 and 1998. The in-line method was designed to eliminate atmospheric contact with soil solu tions prior to pH measurements. The time-dependent pH error was quantified based on laboratory experiments with soil solution under controlled tempera tures and CO2 partial pressures. Equilibrium speciation modelling was used to predict pH values observed in the field and in the laboratory and the mo del was found to reproduce the observations well. We conclude that traditio nal pH measurements on extracted soil solutions in the pH range from 5 to 7 are not appropriate for detailed pH measurements due to errors associated with CO2 degassing. In-line measurements provide more accurate measurement necessary for detailed studies on soil acidification dynamics.