Soil organic matter extraction using water at high temperature and elevated pressure

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Göttlein, Axel ; Schwesig, David ; Haumaier, Ludwig ; Blasek, Roland ; Ilgen, Gunter:
Soil organic matter extraction using water at high temperature and elevated pressure.
In: Mitteilungen der Deutschen Bodenkundlichen Gesellschaft. Bd. 87 (1998) . - S. 277-278.
ISSN 0343-107x

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Abstract

High amounts of soil organic matter are extracted when using salt solutions or organic solvents, but extracts obtained by these solutions do not represent organic matter soluble under natural conditions, and tend to alter the chemical characteristics of the extracted material. The main drawback of water as a natural extractant is the small yield of organic matter. Objectives of our study were to evaluate the usefulness of high-temperature water extraction at elevated pressure for soil organic matter, and to evaluate the conditions under which the highest yield of chemically unaltered material can be obtained as compared to conventional extraction methods. Samples from a Cambic Podzol in the Fichtelgebirge (NE Bavaria, Germany) were extracted with water at 50, 100, 150 and 200°C under a pressure of 10 MPa, using an Accelerated Solvent Extractor (ASE200, Dionex Co., Sunnyvale, USA). These extracts were compared to extracts obtained by conventional extraction methods using distilled water, NaOH and Na4P2O7 solutions. Yields of soil organic matter extracted by the ASE increased with temperature for all horizons and were up to twenty-fold higher than yields of the conventional water extracts, but in most cases did not compete with NaOH and Na4P2O7 extracts. NaOH and Na4P2O7 extracts had a high ash content, and yields of these extracts were not this high when calculated as percentage of total soil carbon. C/H and C/N ratios were nearly constant and seemed to be no reliable indicators for the chemical quality of extracted organic matter. UV spectra as well as 1H and 13C NMR spectra revealed a high similarity between conventional water extracts and ASE extracts up to 150°C, whereas the 200°C extracts resembled the NaOH and Na4P2O7 extracts. The latter had higher proportions of aromatic and alkyl structures, whereas O-alkyl structures were clearly diminished. We conclude that high-temperature water extraction using an ASE is a suitable tool to obtain high yields of soil organic matter with only little alteration of its chemical characteristics as compared to a conventional water extract. Additional advantages of ASE extraction are the small amounts of water (about 20 mL) and soil (10 to 20 g) needed for an extraction, and the good reproducibility (RSD 5 to 10 %). The optimal temperature for getting highest yields of soil organic matter is between 100 and 150 °C. At higher temperatures thermal degradation occurs leading to extracts enriched in aromatic and depleted in O-alkyl-C structures, strongly resembling the NaOH and Na4P2O7 extracts.