Direct Measurement of CO2 Retention in Arable Soils with pH Above 6.5 During Barometric Process Separation Incubation

Abstract The barometric process separation (BaPS) technique is a well-established incubation method to simultaneously measure gross nitrification and respiration rates in soil. Its application, however, is still critical in soils with pH above 6.5. Here, a substantial part of microbial CO 2 production is retained in soil solution (CO 2,aq ) due to shifts in the carbonate equilibrium. This may lead to substantial errors in gas balance calculation. Yet, utilization of the BaPS technique is only reliable if the critical term is adequately quantified. We present an easy, inexpensive, and direct method, the sterilization-CO 2 -injection (SCI) method, to measure CO 2 retention during soil incubation. Sterilized soil was incubated in the BaPS system, and defined volumes of CO 2 were injected to stepwise increase CO 2 partial pressure ( p CO 2 ) inside the chamber and to analyse the physicochemical equilibration process. Five exemplary agricultural soils from Northeast China and Southwest Germany were used for method establishment, presenting pH values between 4.4 and 7.6 and carbonate contents between 0% and 3.9%. We observed that in soils with pH > 6.5, 70%–90% of the injected CO 2 was taken up by the soil until the equilibrium inside the chamber was re-established. As expected, in soils with low pH ( 2 retention was low. CO 2 retention patterns were sensitive to incubation temperature with tri-fold dissolution capacity at 5 °C compared to 25 °C, but insensitive to variations in soil water content. The resulting soil-specific relationship between p CO 2 and CO 2,aq concentration allowed the quantification of CO 2,aq concentration as a function of headspace p CO 2 .