A Comprehensive Method for Fractionating Soil Organic Matter Not Protected and Protected from Decomposition by Physical and Chemical Mechanisms |
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| Authors: | César Plaza José M Fernández Engil I P Pereira Alfredo Polo |
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| Institution: | 1. Instituto de Ciencias Agrarias, Consejo Superior de Investigaciones Científicas, Madrid, Spain;2. Department of Earth and Environmental Science, University of Pennsylvania, Hayden Hall, Philadelphia, PA, USA;3. Department of Plant Sciences, University of California, Plant and Environmental Sciences Building, Davis, CA, USA |
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| Abstract: | The objective of this work was to describe a method for isolating meaningful and measurable soil organic matter (SOM) pools that differ in the mechanisms by which they are protected from decomposition. The proposed method is appropriate for soil C stabilization and sequestration studies. Unlike previous fractionation schemes, this procedure allows free SOM located between aggregates (unprotected C pool) and SOM occluded within both macroaggregates and microaggregates (C weakly and strongly protected by physical mechanisms, respectively) to be recovered separately, freed from the soil mineral matrix and the mineral‐associated SOM pool (C pool protected by chemical mechanisms) and thus well suited to advanced chemical characterization by 13C‐NMR. Briefly, free SOM is isolated by an initial density separation. Stable macroaggregates are broken up into stable microaggregates and intra‐macroaggregate SOM, which is then separated by density. Finally, intra‐microaggregate SOM is isolated from mineral‐associated SOM by a third density separation after ultrasonic disruption. The SOM dissolved during the fractionation procedure is also recovered. Results obtained on soil samples with contrasting textures suggested that clay content induces a decrease of the proportion of free organic C and an increase of mineral‐associated organic C content. Free SOM is characterized by a marked presence of undecayed organic material and biologically labile substances, such as carbohydrates and proteins. In contrast, SOM occluded within aggregates, especially within microaggregates, represents a more decomposed fraction, relatively enriched in unsubstituted‐aliphatic material, most probably lipid biopolymers. |
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| Keywords: | Aggregates C stabilization Nuclear magnetic resonance Physical fractionation Soil organic matter |
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