Pedosphere 28(5): 726--738, 2018
ISSN 1002-0160/CN 32-1315/P
©2018 Soil Science Society of China
Published by Elsevier B.V. and Science Press
Direct Measurement of CO2 Retention in Arable Soils with pH Above 6.5 During Barometric Process Separation Incubation
Hannah CONRADS1, Joachim INGWERSEN1, JU Xiaotang2, and Thilo STRECK1
1Institute of Soil Science and Land Evaluation, Biogeophysics Section, University of Hohenheim, Stuttgart 70599(Germany)
2College of Agricultural Resources and Environmental Sciences, China Agricultural University, Beijing 100094(China)
Corresponding Author:Hannah CONRADS
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 was re-established in the chamber. As expected, in soils with low pH (< 6.5), measured CO2 retention was low. CO2 retention patterns were sensitive to incubation temperature with tri-fold dissolution capacity at 5 ℃ compared to 25 ℃, but insensitive to variations in soil water content. The resulting soil-specific relationship between pCO2 and CO2,aq concentration allowed the quantification of CO2,aq concentration as a function of headspace pCO2.
Key Words:  carbonate equilibrium,CO2 dissolution capacity,CO2 partial pressure,microbial CO2 production,soil pH,sterilizationCO2-injection method,volume of headspace air,volume of soil solution
Citation: Conrads H, Ingwersen J, Ju X T, Streck T. 2018. Direct measurement of CO2 retention in arable soils with pH above 6.5 during barometric process separation incubation. Pedosphere. 28(5):726-738.
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