Chloride retention in forest soil by microbial uptake and by natural chlorination of organic matter

Inorganic chlorine (i.e. chloride; Cl in) 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 Cl in 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 Cl in, in combination with more long-term natural formation of chlorinated organic matter (Cl org), caused Cl in imbalances in coniferous forest soil. Extensive microbial uptake and release of Cl in occurred over short time scales, and were probably associated with changes in environmental conditions. Up to 24% of the initially available Cl in within pore water was retained by microbial uptake within a week in our experiments, but most of this Cl in was released to the pore water again within a month, probably associated with decreasing microbial populations. The natural formation of Cl org resulted in a net immobilization of 4% of the initial pore water Cl in over four months. If this rate is representative for the area where soil was collected, Cl org formation would correspond to a conversion of 25% of the yearly wet deposition of Cl in. The study illustrates the potential of two Cl in 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 Cl in imbalances in soil. Altogether, the results of the present study (1) provide evidence that Cl in cannot be assumed to be inert in soil, (2) show that microbial exchange can regulate pore water Cl in concentrations and (3) confirm the controversial idea of substantial natural chlorination of soil organic matter.