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Chloride retention in forest soil by microbial uptake and by natural chlorination of organic matter |
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| Authors: | David Bastviken Frida Thomsen Susanne Karlsson George Shaw Gunilla Öberg |
| Institution: | a Department of Geology and Geochemistry, Stockholm University, SE-106 91 Stockholm, Sweden b Centre for Climate Science and Policy Research, The Tema Institute, Linköping University, SE-601 74 Norrköping, Sweden c Department of Water and Environmental Studies, Linköping University, SE-581 83 Linköping, Sweden d Division of Agricultural & Environmental Sciences, University of Nottingham, University Park, Nottingham NG7 2RD, UK e Isotope Laboratory, Institute of Experimental Botany, Ac. Sci. Czech Rep., Videnska 1083, CZ-14220 Prague 4, Czech Republic f Institute for Resources, Environment and Sustainability, University of British Columbia, 428-2202 Main Mall Vancouver, BC, Canada V6T 1Z4 |
| Abstract: | Inorganic chlorine (i.e. chloride; Clin) is generally considered inert in soil and is often used as a tracer of soil and ground water movements. However, recent studies indicate that substantial retention or release of Clin can occur in soil, but the rates and processes responsible under different environmental conditions are largely unknown. We performed 36Cl tracer experiments which indicated that short-term microbial uptake and release of Clin, in combination with more long-term natural formation of chlorinated organic matter (Clorg), caused Clin imbalances in coniferous forest soil. Extensive microbial uptake and release of Clin occurred over short time scales, and were probably associated with changes in environmental conditions. Up to 24% of the initially available Clin within pore water was retained by microbial uptake within a week in our experiments, but most of this Clin was released to the pore water again within a month, probably associated with decreasing microbial populations. The natural formation of Clorg resulted in a net immobilization of 4% of the initial pore water Clin over four months. If this rate is representative for the area where soil was collected, Clorg formation would correspond to a conversion of 25% of the yearly wet deposition of Clin. The study illustrates the potential of two Clin retaining processes in addition to those previously addressed elsewhere (e.g. uptake of chloride by vegetation). Hence, several processes operating at different time scales and with different regulation mechanisms can cause Clin imbalances in soil. Altogether, the results of the present study (1) provide evidence that Clin cannot be assumed to be inert in soil, (2) show that microbial exchange can regulate pore water Clin concentrations and (3) confirm the controversial idea of substantial natural chlorination of soil organic matter. |
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