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The occurence and origin of organic chlorine in soil
08/2008 Research over the past 10 years has revealed that all organic matter in soil may contain organically bound chlorine, and that chlorine is a natural major consitutent of organic matter that hitherto has been overlooked. Measurements in soil from Sweden, The Netherlands, Hawaii, Australia, etc have shown that organic chlorine in organic matter is generally present in concentrations of 0.2-3 mg/g C. The major fraction (generally >90%) appears to be incorporated into the high-molecular weight soil humus, which shows a low environmental mobility. Substances with a lower molecular weight have a potentially larger environmental impact, as a result of increased mobility and/or bioavailability. Chlorinated fulvic acids and other water-soluble compounds are leached by water downwards in the soil profile, and are subsequently transported to groundwater, rivers and lakes. Since these compounds contain chlorine, the transport of organic matter from soil to water contributes to the organic chlorine content in such waters. Concentrations up to 200 mg Clorg/L (measured as AOX) have been found in unpolluted stagnant waters, while in running waters concentrations of 10-20 mg Clorg/L can be considered to be normal natural background values. As a rule of the thumb, the natural concentration of organic chlorine follows the concentration of organic matter, i.e. humus rich brownish water generally show higher values than clear-water systems. The organic chlorine in soil was originally suggested to be of anthropogenic origin, resulting from atmospheric transport and deposition of man-made chlorinated compounds. However, the total atmospheric deposition of organic chlorine in remote areas can only explain a small fraction of the organic chlorine found in soil. Furthermore, it has been shown that soil constituents which originate from the period before industrialisation also contain organic chlorine. Very little is known of the biogeochemical cycling (formation, mineralisation, leaching etc) of chlorinated organic matter in soil. Recent studies at Linköping University have revealed that the net-formation in spruce forest soil is closely related to degradation of organic matter. The ecological role of this formation is so far unknown, but recent findings at Linköping University that the amount of organically bound halogens in soil increases with decreasing pH, that production is hampered by conventional forest fertilization (NPK) and seems to be related to lignin degradation, in combination with studies which suggest that production of organochlorine is a common feature among white-rot fungi, makes it tempting to suggest a relationship between lignin degradation and production of organohalogens. Such a relation may result from an enzymatically catalyzed formation of reactive halogen species as outlined below. It is well known from the pulp and paper industry that addition of reactive chlorine to pulp causes degradation of lignin, and it is also known that this process is accompanied by an undesired increase in the organohalogen content. The suggested relationship between natural formation of organically bound chlorine and lignin degradation could be the result of active participation of enzymatically produced reactive chlorine in lignin degradation followed by the production of organically bound halogens. Recent studies at Linköping University support this hypothesis as our results indicate that addition of reactive chlorine enables microbial degradation of lignin. Nonetheless, it remains to be determined whether white-rot fungi actually produce exo-enzymes capable of catalyzing the formation of reactive halogen species and if such reactive species are generally involved in lignin degradation. |
