|REEDER, JEAN - Retired ARS Employee
Submitted to: Communications in Soil Science and Plant Analysis
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/17/2011
Publication Date: 3/27/2012
Citation: Sherrod, L.A., Hunter, W.J., Reeder, J., Ahuja, L.R. 2012. A rapid and cost effective method for soil carbon mineralization under static incubations. Communications in Soil Science and Plant Analysis. 43:6. 958-972. http://dx.doi.org/10.1080/00103624.2012.653031.
Interpretive Summary: Changes in soil organic C (SOC) due to changes in management are often undetected in measurements of the whole soil organic C pool since researchers are often looking for a relatively small change in a vary large pool. Sensitive indicators of management’s impact on SOC are found in the rapidly cycling C fraction that contains less than 5 % of the total soil organic C. This active C fraction consists of labile organic matter, microbial biomass and products with monthly to yearly turnover times. The most common method used to gauge microbial activity is via respiration, an approach that is comparable to C mineralization. Soil incubations with subsequent measurement of evolved carbon dioxide are a common approach used to estimate C mineralization rates. Routine analysis has been limited by equipment costs and more importantly sample run times. We present a new approach that makes use of a single-cell infrared gas analyzer (IRGA). Results of the IRGA method for measuring CO2 were not significantly different from the results from gas chromatography (GC) and from alkali absorption followed by titration. Our single-cell IRGA method for static laboratory C mineralization can read 90 samples per hour. Comparatively the GC sample run times are 26 per hour and the alkali absorption followed by titration sample run times are 10 per hour. The IRGA system also lends itself well to adding on an O2 analyzer to measure both CO2 and O2 in a single gas injection which would require a separate analysis if using an alkali absorption followed by titration. In addition to fast sample run times, the single-cell IRGA is approximately ¼ the cost of a dual channel IRGA or a GC but is more than adequate technology to measure CO2 from laboratory incubations. Equipment costs, simplicity and fast run times make the single cell IRGA method suitable for research and routine analysis of soil C mineralization.
Technical Abstract: Soil incubations with subsequent measurement of carbon dioxide (CO2) evolved are common soil assays to estimate C mineralization rates and active organic C. 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. We present a new approach that makes use of a single-cell infrared gas analyzer (IRGA). Objectives were to: (i) evaluate if the IRGA, GC and NaOH methods provide similar CO2 results over four incubation time periods (1, 3, 10 and 21 day); (ii) determine how well these methods correlate with each other; (iii) estimate the limit of quantitation (LOQ) for methods; and (iv) estimate the concentrations of CO2 and O2 in the chamber headspace that result in respiration suppression. To obtain a range of labile soil organic matter, soils were collected from a summit, side, and toeslope sequence within four cropping systems across two field replications (n=24). Results of the IRGA method for measuring CO2 were not significantly different from the results of the GC and NaOH methods by day 21. 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 GC had the lowest LOQ of 90 ppm CO2 followed by 115 and 600 ppm CO2 for IRGA and NaOH methods, respectively. Headspace concentrations of CO2 and O2 that produced respiratory suppression were 10.5% and 5.3 %, respectively. Equipment costs, simplicity and fast run times make the single cell IRGA method suitable for research and routine analysis of soil C mineralization.