Carbon dioxide degassing effects on soil solutions collected by centrifugation

Extraction by centrifugation is a common method of obtaining soil solutions for chemical analysis. Productive pastures in New Zealand commonly contain 70 to 140 times the atmospheric CO2 concentrations in the soil atmosphere and are thus very susceptible to artifacts associated with CO2 degassing. While the effects of CO2 degassing from soil solutions isolated from the solid-phase have been documented, the effects of CO2 degassing of soil samples before soil solution extraction have not been reported previously. This study of CO2 degassing from noncalcareous New Zealand pasture soils specifically examines the effects of soil collection methods and sample storage time on the alteration of soil solution chemistry. Soil solutions extracted from bulk soil samples and soil collected directly into centrifuge tubes lost from 86 to 92% of their total dissolved carbonates after 1 to 2 h and 99% of their total dissolved carbonates after 24 h. Degassing did not affect solution pH since the degassing occurred before solution extraction from the solid-phase, allowing cation exchange reactions to buffer solution pH. The primary alteration to soil solution chemistry was a reduction in HCO3 concentrations, from 0.67 mM in the in situ soil to 0.085 and 0.008 mM after 1-2 h and 24 h of soil storage, respectively. Since HCO3 is the dominant anion in these soil solutions, cation concentrations must similarly be underestimated because of the loss of the HCO3 anion. These findings differ from previous studies that document a pH increase and no ionic strength change caused by CO2 degassing of isolated soil solutions; CO2 degassing of soil samples before extraction resulted in a large decrease in the ionic strength but no change in solution pH. Failure to recognize this artifact could seriously affect the interpretation of data resulting from collection and analysis of soil solutions extracted by centrifugation.