A Rapid and Cost Effective Method for Soil Carbon Mineralization Under Static Incubations.

Soil incubations with subsequent determination of carbon dioxide (CO₂) are common soil assays used to estimate C mineralization rates and active organic C pools.  Two common methods used to detect CO₂ in laboratory incubations are gas chromatography (GC) and alkali absorption followed by titration (NaOH).  These methods have various strengths and weaknesses that are realized in cost of equipment and analysis time.  A new approach makes use of an infrared gas analyzer (IRGA) with a simple single path analysis.  This is significantly less costly than a dual channel system.  The objectives of this study were (i) evaluate if the IRGA, GC and NaOH methods for CO₂ determination obtain the same concentrations over 4 time periods (1, 3, 10 and 21 day); (ii) how well do these methods correlate to each other across incubation times; (iii) estimate the limit of quantitation (LOQ); and (iv) estimate the concentration of CO₂ and O₂ gases in the chamber headspace that shows respiration suppression.  Soil samples were collected from along a catena sequence from 4 cropping systems within 2 field replications (n=24) to obtain a range of labile organic matter.  The IRGA method had similar results as GC and NaOH methods with no significant differences by day 21 of the incubation.  The GC had the lowest LOQ of 90 followed by 115 and 600 ppm CO₂ for IRGA and NaOH methods respectively.  Correlations of GC vs. NaOH, GC vs. IRGA, and NaOH vs. IRGA over the 21 days had R² values of 0.93, 0.94, and 0.92 respectively.  The headspace CO₂ and O₂ that showed respiratory suppression was 9.1% and 10.9 % respectively.  The single cell IRGA system is a rapid and economic estimation of C mineralization and active soil organic C pools.