Radiotracer evidence that the rhizosphere is a hot-spot for chlorination of soil organic matter

Aims The ubiquitous and extensive natural chlorination of organic matter in soils, leading to levels of chlorinated soil organic matter that often exceed the levels of chloride, remains mysterious in terms of its causes and regulation. While the composition of plant species and the availability of labile organic matter was recently shown to be important, the physical localization of chlorination in soils remains unclear but is a key for understanding regulation and patterns observed. Here we assess the relative importance of organic matter chlorination in (a) bulk soil, (b) the plant roots plus the rhizosphere zone surrounding the roots, and (c) above-ground plant biomass, in an experimental plant-soil system. Methods A radiotracer, Cl-36, was added to study translocation and transformations of Cl- and Cl-org in agricultural soil with and without wheat (Triticum vulgare) over 50 days. Results The specific chlorination rates (the fraction of the added Cl-36(-) converted to Cl-36(org) per day) in soil with plants was much higher (0.02 d(-1)) than without plants (0.0007 d(-1)) at peak growth (day 25). The plant root and rhizosphere showed much higher formation of Cl-36(org) than the bulk soil, suggesting that the rhizosphere is a hotspot for chlorination in the soil. In addition, the treatment with plants displayed a rapid and high plant uptake of Cl-. Conclusions Our results indicate that the rhizosphere harbour the most extensive in-situ chlorination process in soil and that root-soil interaction may be key for terrestrial chlorine cycling.