Organochlorine molecules (Cl org ) are surprisingly abundant in soils and frequently exceed chloride (Cl − ) levels. Despite the widespread abundance of Cl org and the common ability of microorganisms to produce Cl org , we lack fundamental knowledge about how overall chlorine cycling is regulated in forested ecosystems. Here we present data from a long-term reforestation experiment where native forest was cleared and replaced with five different tree species. Our results show that the abundance and residence times of Cl − and Cl org after 30 years were highly dependent on which tree species were planted on the nearby plots. Average Cl − and Cl org content in soil humus were higher, at experimental plots with coniferous trees than in those with deciduous trees. Plots with Norway spruce had the highest net accumulation of Cl − and Cl org over the experiment period, and showed a 10 and 4 times higher Cl − and Cl org storage (kg ha −1 ) in the biomass, respectively, and 7 and 9 times higher storage of Cl − and Cl org in the soil humus layer, compared to plots with oak. The results can explain why local soil chlorine levels are frequently independent of atmospheric deposition, and provide opportunities for improved modeling of chlorine distribution and cycling in terrestrial ecosystems.

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Chlorine (Cl) in the terrestrial environment is of interest from multiple perspectives, including the use of chloride as a tracer for water flow and contaminant transport, organochlorine pollutants, Cl cycling, radioactive waste (radioecology; 36Cl is of large concern) and plant science (Cl as essential element for living plants). During the past decades, there has been a rapid development towards improved understanding of the terrestrial Cl cycle. There is a ubiquitous and extensive natural chlorination of organic matter in terrestrial ecosystems where naturally formed chlorinated organic compounds (Clorg) in soil frequently exceed the abundance of chloride. Chloride dominates import and export from terrestrial ecosystems while soil Clorg and biomass Cl can dominate the standing stock Cl. This has important implications for Cl transport, as chloride will enter the Cl pools resulting in prolonged residence times. Clearly, these pools must be considered separately in future monitoring programs addressing Cl cycling. Moreover, there are indications that (1) large amounts of Cl can accumulate in biomass, in some cases representing the main Cl pool; (2) emissions of volatile organic chlorines could be a significant export pathway of Cl and (3) that there is a production of Clorg in tissues of, e.g. plants and animals and that Cl can accumulate as, e.g. chlorinated fatty acids in organisms. Yet, data focusing on ecosystem perspectives and combined spatiotemporal variability regarding various Cl pools are still scarce, and the processes and ecological roles of the extensive biological Cl cycling are still poorly understood.

Natural chlorination of organic matter is common in soils. The abundance of chlorinated organic compounds frequently exceeds chloride in surface soils, and the ability to chlorinate soil organic matter (SOM) appears widespread among microorganisms. Yet, the environmental control of chlorination is unclear. Laboratory incubations with Cl as a Cl tracer were performed to test how combinations of environmental factors, including levels of soil moisture, nitrate, chloride, and labile organic carbon, influenced chlorination of SOM from a boreal forest. Total chlorination was hampered by addition of nitrate or by nitrate in combination with water but enhanced by addition of chloride or most additions including labile organic matter (glucose and maltose). The greatest chlorination was observed after 15 days when nitrate and water were added together with labile organic matter. The effect that labile organic matter strongly stimulated the chlorination rates was confirmed by a second independent experiment showing higher stimulation at increased availability of labile organic matter. Our results highlight cause-effect links between chlorination and the studied environmental variables in podsol soil-with consistent stimulation by labile organic matter that did overrule the negative effects of nitrate.

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, 36 Cl, 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 36 Cl − converted to 36 Cl 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 36 Cl 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.