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

Abstract

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 CO2 production is retained in soil solution (CO2,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-CO2-injection (SCI) method, to measure CO2 retention during soil incubation. Sterilized soil was incubated in the BaPS system, and defined volumes of CO2 were injected to stepwise increase CO2 partial pressure (pCO2) 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 CO2 was taken up by the soil until the equilibrium inside the chamber was re-established. As expected, in soils with low pH (