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Chloride is often considered as the main chlorine form in soils. However, recent studies show that chlorine is mostly present in soils as naturally produced organically bound molecules. The relative contribution of biotic, including microbial, and abiotic processes to formation of organically bound chlorine remains poorly understood. We performed a37Cl spiking batch experiment with a forest soil incubated under abiotic and biotic conditions over two time periods to simultaneously monitor the formation of organically bound chlorine from natural and tracer chlorine. To compare biotic and abiotic conditions without biased effect of sterilization technique for abiotic control, the soil was irradiated and reinoculated or not with soil microflora. Fifteen days after microbial inoculation, the natural non-extractable organic chlorine content in the inoculated soil was significantly higher than in the sterile soil, showing that microbial activity contributed to formation of organically bound chlorine. However, no significant difference was noted between the two incubation periods. The same trend was noted for tracer chlorine, yet without a significant difference. The present study shows that chlorination is mediated by microbial activity, but there is also some indication of abiotic formation of organically bound chlorine, with a non-extractable organic tracer chlorine formation of about 6% just after spiking in abiotic conditions.
•Inorganic 37Cl was used to trace organically bound chlorine formation in soil incubation.•Irradiated forest soil was inoculated or not with soil microflora.•Organically bound chlorine was higher in inoculated than in sterile soil.•Abiotic processes also contribute to organic chlorine formation.