Article pubs.acs.org/est
Improved Characterization of Soil Organic Matter by Thermal Analysis Using CO2/H2O Evolved Gas Analysis José M. Fernández,† Clément Peltre,‡ Joseph M. Craine,§ and Alain F. Plante*,‡ †
Instituto de Ciencias Agrarias, Consejo Superior de Investigaciones Científicas, Serrano 115 dpdo., Madrid, 28006, Spain Department of Earth & Environmental Science, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6316, United States § Division of Biology, Kansas State University, Manhattan, Kansas 66506, United States ‡
S Supporting Information *
ABSTRACT: Simultaneous thermal analysis [i.e., thermogravimetry (TG) and differential scanning calorimetry (DSC)] is frequently used in materials science applications and is increasingly being used to study soil organic matter (SOM) stability. Yet, important questions remain, especially with respect to how the soil mineral matrix affects TG-DSC results, which could confound the interpretation of relationships between thermal and biogeochemical SOM stability. The objective of this study was to explore the viability of using infrared gas analyzer (IRGA) based CO2/H2O evolved gas analysis (EGA) as a supplement or alternative to TG-DSC to improve the characterization of SOM. Here, we subjected reference samples and a set of 28 diverse soil samples from across the U.S. to TG-DSC coupled with IRGAbased EGA. The results showed the technical validity of coupling TG-DSC and CO2-EGA, with more than 80% of the theoretically evolved CO2−C recovered during pure cellulose and CaCO3 analysis. CO2-EGA and DSC thermal profiles were highly similar, with correlation coefficients generally >0.90. Additionally, CO2/H2O-EGA proved useful to improve the accuracy of baseline correction, detect the presence of CaCO3 in soils, and identify SOM oxidative reactions normally hidden in DSC analysis by simultaneous endothermic reactions of soil minerals. Overall, this study demonstrated that IRGA-based CO2/H2OEGA constitutes a valuable complement to conventional TG-DSC analysis for SOM characterization.
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INTRODUCTION Soil organic matter (SOM) plays a key role in the global C cycle. There is approximately three times more C stored in soils than in the atmosphere, and natural soil CO2 emissions from respiration are nearly seven times larger than anthropogenic emissions.1 Any alteration in the soil C balance represents a significant potential feedback to climate change,2 which is largely dependent on the biogeochemical stability SOM. Here we define SOM biogeochemical stability as the set of properties from which SOM derives resistance to microbial and enzymatic biodegradation and long residence time in soil.3 Many approaches have been used to characterize SOM stability, including physical or chemical fractionation, spectroscopic analyses such as Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectrometry, and pyrolysis coupled with gas chromatography and mass spectrometry (py-GC/MS).4,5 Among these techniques, thermal analysis has shown the potential to advance the characterization of the complete continuum of SOM stability in whole soil samples by integrating chemical composition and organo-mineral association through measurements of energy content.6 However, several technical issues still need to be resolved before thermal analysis can be applied as a routine method for SOM analysis. Thermal techniques have been increasingly used for SOM characterization over the past decade.7−10 In thermal analysis, © 2012 American Chemical Society
material properties are measured during the course of a heating program under a controlled atmosphere. Typical simultaneous thermal analysis involves measuring sample mass loss as a function of temperature (i.e., thermogravimetry, TG) and either temperature difference between the analyzed sample and a reference (i.e., differential thermal analysis, DTA) or heat flow rate to/from the sample (i.e., heat flux differential scanning calorimetry, DSC). Thermal analysis is advantageous for SOM characterization as it is relatively fast, highly reproducible, and requires minimal sample preparation.9 Despite the increasing use of thermal analysis in the study of SOM stability, important questions remain concerning the effects of the soil mineral matrix on TG-DSC results. Both the organic and mineral components of soil contribute to observed thermal reactions, and in some cases these reactions may overlap. This phenomenon has made the interpretation of the relationships between thermal and biogeochemical SOM stability difficult because it can mask some of the reactions related with the more stable fraction of the SOM, making them impossible to be quantified or identified.10 Received: Revised: Accepted: Published: 8921
April 6, 2012 July 6, 2012 July 18, 2012 July 18, 2012 dx.doi.org/10.1021/es301375d | Environ. Sci. Technol. 2012, 46, 8921−8927
Environmental Science & Technology
Article
concentrations of the Ultra-Zero air flowing through the system were baseline corrected to zero using the IRGA software. CO2-EGA data were converted from partial pressure (ppm CO2) to carbon mass (μg C), and the cumulative amount of CO2−C evolved was determined using the path length between the instruments and measured gas flow rates. Samples Analyzed. Pure fibrous cellulose (Sigma C-6663) and amorphous calcium carbonate powder (CaCO3, Fisher Chemical C-64) were used as reference materials to assess the technical feasibility of coupling the STA and IRGA. Although they exist in soils, the objective was to select materials generating well constrained thermal reactions, rather than materials representative of soil constituents. Pure cellulose was used to illustrate a typical reaction of thermal oxidation/ combustion of organic C, and pure CaCO3 was used to show a typical reaction of thermal decomposition of inorganic C. Subsequently, surface (0−10 cm) mineral soils from a previous study16 were used to test the feasibility of using CO2-EGA for the characterization of SOM. These samples represent a diverse range of soils from 28 sites across North America ranging from Alaska to Puerto Rico. Soil properties ranged from 5 to 63% fine soil (
