Organic matter chlorination rates in different boreal soils: the role of soil organic matter content

Natural Disinfection-like Process Unveiled in Soil Microenvironments by Enzyme-Catalyzed Chlorination

Substantial natural chlorination processes are a growing concern in diverse terrestrial ecosystems, occurring through abiotic redox reactions or biological enzymatic reactions. Among these, exoenzymatically mediated chlorination is suggested to be an important pathway for producing organochlorines and converting chloride ions (Cl-) to reactive chlorine species (RCS) in the presence of reactive oxygen species like hydrogen peroxide (H2O2). However, the role of natural enzymatic chlorination in antibacterial activity occurring in soil microenvironments remains unexplored. Here, we conceptualized that heme-containing chloroperoxidase (CPO)-catalyzed chlorination functions as a naturally occurring disinfection process in soils. Combining antimicrobial experiments and microfluidic chip-based fluorescence imaging, we showed that the enzymatic chlorination process exhibited significantly enhanced antibacterial activity against Escherichia coli and Bacillus subtilis compared to H2O2. This enhancement was primarily attributed to in situ-formed RCS. Based on semiquantitative imaging of RCS distribution using a fluorescence probe, the effective distance of this antibacterial effect was estimated to be approximately 2 mm. Ultrahigh-resolution mass spectrometry analysis showed over 97% similarity between chlorine-containing formulas from CPO-catalyzed chlorination and abiotic chlorination (by sodium hypochlorite) of model dissolved organic matter, indicating a natural source of disinfection byproduct analogues. Our findings unveil a novel natural disinfection process in soils mediated by indigenous enzymes, which effectively links chlorine-carbon interactions and reactive species dynamics.

Driving factors influencing spatiotemporal variation of natural organic chlorine in Shennongjia forest soil

Studying the geochemical behavior of chlorine is the basis of understanding the chlorine cycle in nature. To explore the spatiotemporal distribution of natural organic chlorine (Clorg), L layer (litter fall), F-H layer (humification zone), topsoil layer (0-20 cm), and deep soil layer (20-40 cm) samples were collected from 18 sampling sites at different altitudes (851-2918 m) in Shennongjia Forest in May, August, and December. Clorg content was analyzed, and the Clorg stocks were calculated. The major factors affecting the distribution of Clorg were explored. The results revealed that the sum of Clorg content in four layers varied from 7.958 to 184.686 mg/kg, and the highest value was observed in August. Clorg accounted for 46%-77% of total chlorine, with the highest mean ratio in soil layer (0-20 cm). Clorg content exhibited the following trend: F-H layer > L layer > topsoil layer (0-20 cm) > deep soil layer (20-40 cm). The seasonal patterns of Clorg in soil layers were different from that in L and F-H layers, which were mainly controlled by the content and humification degree of organic matter. Clorg storage was much higher in soil layers (61-246 kg/ha) than those in F-H layer (1.1-7.1 kg/ha) and in L layer (0.1-0.8 kg/ha) because of the large thickness of the soil layers. Overall, the Clorg content exhibited an increasing trend with altitude, except at an altitude of approximately 1800 m. Clorg content in L and F-H layers varied more obviously with altitude than that in soil layers. When inorganic chlorine (Clin) was not a limiting factor for the chlorination process, Clorg content in L and F-H layers was significantly affected by climate and organic matter controlled by altitude, while Clorg content in soil layers was also mediated by metal ions and pH, and soil particle size. This study could provide a scientific basis for assessing the chlorine cycle in nature.

36Cl, a new tool to assess soil carbon dynamics

Soil organic carbon is one of the largest surface pools of carbon that humans can manage in order to partially mitigate annual anthropogenic CO2 emissions. A significant element to assess soil sequestration potential is the carbon age, which is evaluated by modelling or experimentally using carbon isotopes. Results, however, are not consistent. The 14C derived approach seems to overestimate by a factor of 6-10 the average carbon age in soils estimated by modeling and 13C approaches and thus the sequestration potential. A fully independent method is needed. The cosmogenic chlorine nuclide, 36Cl, is a potential alternative. 36Cl is a naturally occurring cosmogenic radionuclide with a production that increased by three orders of magnitude during nuclear bomb tests. Part of this production is retained by soil organic matter in organochloride form and hence acts as a tracer of the fate of soil organic carbon. We here quantify the fraction and the duration of 36Cl retained in the soil and we show that retention time increases with depth from 20 to 322 years, in agreement with both modelling and 13C-derived estimates. This work demonstrates that 36Cl retention duration can be a proxy for the age of soil organic carbon.

Chlorine Distribution in Soil and Vegetation in Boreal Habitats along a Moisture Gradient from Upland Forest to Lake Margin Wetlands

The assumed dominance of chloride (Cl^-) in terrestrial ecosystems is challenged by observations of extensive formation of organically bound Cl (Cl_org), resulting in large soil Cl storage and internal cycling. Yet, little is known about the spatial distribution of Cl in ecosystems. We quantified patterns of Cl distribution in different habitats along a boreal hillslope moisture gradient ranging from relatively dry upland coniferous forests to wet discharge areas dominated by alder. We confirmed that dry habitats are important for Cl storage but found that Cl pools tended to be larger in moist and wet habitats. The storage of Cl_org was less important in wet habitats, suggesting a shift in the balance between soil chlorination and dechlorination rates. Cl concentrations in the herb layer vegetation were high in wet and moist sites attributed to a shift in plant species composition, indicating plant community-dependent ecosystem Cl cycling. Mass-balance calculations showed that internal Cl cycling increased overall ecosystem Cl residence times at all sites and that plant uptake rates of Cl^- were particularly high at wet sites. Our results indicate that habitat characteristics including plant communities and hydrology are key for understanding Cl cycling in the environment.

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

Driving effects and transfer prediction of heavy metal(loid)s in contaminated courtyard gardens using redundancy analysis and multilayer perceptron

The distribution and migration of heavy metal(loid)s in the soil-vegetable systems of courtyard gardens near mining areas have rarely been investigated, leading to potential food safety risks for residents. Moreover, the existing research is mainly focused on the total content of heavy metal(loid)s (tMetals) rather than the bioavailable contents (aMetals). In this study, 26 and 28 pairs of soil and vegetable samples were collected from the courtyard gardens near the Realgar mine in Baiyun Town and the lead-zinc (Pb-Zn) mine in Shuikoushan Town, respectively. The tMetal and aMetal of cadmium (Cd), mercury (Hg), arsenic (As), Pb, chromium (Cr), nickel (Ni), copper (Cu), Zn, manganese (Mn), iron (Fe), and calcium (Ca) in the samples were analyzed in this study. The results showed that courtyard gardens were polluted by various heavy metal(loid)s at varying degrees. The bioavailabilities of different metals varied significantly, among which Cd has the highest bioavailability (> 30%). In the transfer process of heavy metal(loid)s, the transfer rate (Tf) was ranked as soil-roots (1.50) > stems-leaves (1.07) > roots-stems (0.46) > stems-fruits (0.33). Redundancy analysis was used to evaluate the driving effects, and the results revealed that aCa, aZn, and aFe in soil could inhibit the absorption of aCd by plant roots. Soil organic matter was the inhibiting factor regarding the transfer of aAs and aCu, whereas it was also the promoting factor for transferring aPb, aNi, and aCr. Furthermore, the multilayer perceptron (MLP) could effectively predict the Tf of heavy metal(loid)s based on the aMetal. The R² values of the MLP were ranked as follows: 0.91 for As, 0.88 for Zn, 0.85 for Hg, 0.83 for Cu, 0.79 for Cr, 0.66 for Cd, 0.65 for Pb, and 0.52 for Ni. This study emphasizes the aMetal-based ecological characteristics and prediction ability. The study results are significant for guiding residents to strategize appropriate crop planting and ensure the safe production and consumption of vegetables.

Freshwater Salinization Syndrome Alters Retention and Release of 'Chemical Cocktails' along Flowpaths: from Stormwater Management to Urban Streams

We investigate impacts of Freshwater Salinization Syndrome (FSS) on mobilization of salts, nutrients, and metals in urban streams and stormwater BMPs by analyzing original data on concentrations and fluxes of salts, nutrients, and metals from 7 urban watersheds in the Mid-Atlantic U.S. and synthesizing literature data. We also explore future critical research needs through a survey of practitioners and scientists. Our original data show: (1) sharp pulses in concentrations of salt ions and metals in urban streams directly following both road salt events and stream restoration construction (e.g., similar to the way concentrations increase during other soil disturbance activities); (2) sharp declines in pH (acidification) in response to road salt applications due to mobilization of H+ from soil exchange sites by Na+; (3) sharp increases in organic matter from microbial and algal sources (based on fluorescence spectroscopy) in response to road salt applications likely due to lysing cells and/or changes in solubility; (4) significant retention (~30-40%) of Na+ in stormwater BMP sediments and floodplains in response to salinization; (5) increased ion exchange and mobilization of diverse salt ions (Na+, Ca2+, K+, Mg2+), nutrients (N, P), and trace metals (Cu, Sr) from stormwater BMPs and restored streams in response to FSS; (6) downstream increasing loads of Cl-, SO4 2-, Br-, F-, and I- along flowpaths through urban streams, and P release from urban stormwater BMPs in response to salinization, and (7) a significant annual reduction (> 50%) in Na+ concentrations in an urban stream when road salt applications were dramatically reduced, which suggests potential for ecosystem recovery. We compared our original results to published metrics of contaminant retention and release across a broad range of stormwater management BMPs from North America and Europe. Overall, urban streams and stormwater management BMPs consistently retain Na+ and Cl- but mobilize multiple contaminants based on salt types and salinity levels. Finally, we present our top 10 research questions regarding FSS impacts on urban streams and stormwater management BMPs. Reducing diverse 'chemical cocktails' of contaminants mobilized by freshwater salinization is now a priority for effectively and holistically restoring urban waters.

Chlorination of soil organic matter: The role of humus type and land use

The levels of natural organic chlorine (Cl_org) typically exceed levels of chloride in most soils and is therefore clearly of high importance for continental chlorine cycling. The high spatial variability raises questions on soil organic matter (SOM) chlorination rates among topsoils with different types of organic matter. We measured Cl_org formation rates along depth profiles in six French temperate soils with similar Cl deposition using ³⁶Cl tracer experiments. Three forest sites with different humus types and soils from grassland and arable land were studied. The highest specific chlorination rates (fraction of chlorine pool transformed to Cl_org per time unit) among the forest soils were found in the humus layers. Comparing the forest sites, specific chlorination was highest in mull-type humus, characterized by high microbial activity and fast degradation of the organic matter. Considering non-humus soil layers, grassland and forest soils had similar specific chlorination rates in the uppermost layer (0-10 cm below humus layer). Below this depth the specific chlorination rate decreased slightly in forests, and drastically in the grassland soil. The agricultural soil exhibited the lowest specific chlorination rates, similar along the depth profile. Across all sites, specific chlorination rates were correlated with soil moisture and in combination with the patterns on organic matter types, the results suggest an extensive Cl cycling where humus types and soil moisture provided best conditions for microbial activity. Cl_org accumulation and theoretical residence times were not clearly linked to chlorination rates. This indicates intensive Cl cycling between organic and inorganic forms in forest humus layers, regulated by humic matter reactivity and soil moisture, while long-term Cl_org accumulation seems more linked with overall deep soil organic carbon stabilization. Thus, humus types and factors affecting soil carbon storage, including vegetation land use, could be used as indicators of potential Cl_org formation and accumulation in soils.

Chlorine cycling and the fate of Cl in terrestrial environments

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.

A field study to predict Cd bioaccumulation in a soil-wheat system: Application of a geochemical model

An accurate model to predict Cd accumulation in crops based on soil properties would facilitate evaluations of soil quality and the potential risk posed by metals. However, given the heterogeneity of soil, such models are difficult to establish on large regional scales. This study for the first time examined the applicability of a multi-surface speciation model (MSM) in predicting Cd accumulation in wheat at a regional field scale, based on 140 soil-wheat paired samples collected from a 205-km² field. The MSM resulted in a better correlation between Cd accumulation in wheat grain (R² = 0.75) and roots (R² = 0.74) than obtained with chemical extraction methods (total Cd in soil, 0.01 M CaCl₂, and 0.43 M HNO₃). In addition, while the performance of the MSM was comparable to that of a traditional multiple regression model, a parameter-fitting process was not required. The predictive ability of the MSM was further used to assess and predict the soil Cd risk and to develop a soil Cd sensitivity map to better localize areas of greatest sensitivity to Cd contamination. The results showed that the MSM can serve as a useful tool for regional soil risk assessments and thus in the development of soil protection measures.

Assessing the recycling of chlorine and its long-lived 36Cl isotope in terrestrial ecosystems through dynamic modeling

It is unclear to what extent chlorine (Cl) and its long-lived isotope ³⁶Cl are recycled in different terrestrial environments in response to time-variable inputs. A new version of a dynamic compartment model was developed to examine the transformation and transfer processes influencing the partitioning and persistence of both Cl and ³⁶Cl in forest ecosystems. The model's performance was evaluated by comparing simulations and field observations of scenarios of stable Cl atmospheric deposition and of global ³⁶Cl fallout. The model reproduced Cl storage in soil reasonably well, despite wide heterogeneity in environmental conditions and atmospheric deposits. Sensitivity analysis confirmed that the natural production of organochlorine in soil plays a major role in Cl build-up and affects long-term Cl dynamics. The timeframe required for the soil organochlorine pool to reach equilibrium in a steady-state system was several thousands of years. Interestingly, root uptake flux, a predominant pathway of the inorganic cycle, was found to affect both inorganic and organic pools in soil, highlighting the importance of plant-soil interactions in Cl dynamics. Model outputs agreed well with local ³⁶Cl measurements, and demonstrated that 90% of the ³⁶Cl found in soil may have come from bomb-test fallout. The pattern of estimated ³⁶Cl/Cl ratios showed that soil ³⁶Cl was not in equilibrium with ³⁶Cl levels in rain input in the post-bomb period. Complete recovery of a natural isotopic ratio in drainage water will need a time close to the residence time of organic ³⁶Cl in soil: i.e., 800 years. A simple dynamic model concept was found to be suitable to illustrate the plant-soil interactions combining both the inorganic and organic Cl cycles acting over different time scales.

Behavior of 36Cl in agricultural soil-plant systems: A review of transfer processes and modelling approaches

This article aims to review up-to-date knowledge and data acquired on ³⁶Cl transfers to terrestrial soil-plant systems, evaluate the existing modelling approaches and identify priorities for future model improvements. This update has revealed the existence of fairly recent studies, whose results could be used for improving the modelling approaches which have been developed over the last decade. The priority areas include the consideration of the dry deposition process and the transfer of both gaseous and aerosol ³⁶Cl to plants. The consideration of secondary processes such as the synthesis/mineralization of organochlorines and plant biomass litterfall is not recognized as a priority issue when assessing the impact of gaseous discharges. It was also identified that additional experimental studies had to be conducted to improve the understanding of the processes governing stable Cl and ³⁶Cl dynamics in other terrestrial ecosystems (field crops, vegetables, grass) than forest environments on which most of the reported knowledge and data are reviewed.

Influence of Multiple Environmental Factors on Organic Matter Chlorination in Podsol Soil

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 ³⁶Cl 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.

The trans-continental distributions of pentachlorophenol and pentachloroanisole in pine needles indicate separate origins

The production and use of pentachlorophenol (PCP) was recently prohibited/restricted by the Stockholm Convention on persistent organic pollutants (POPs), but environmental data are few and of varying quality. We here present the first extensive dataset of the continent-wide (Eurasia and Canada) occurrence of PCP and its methylation product pentachloroanisole (PCA) in the environment, specifically in pine needles. The highest concentrations of PCP were found close to expected point sources, while PCA chiefly shows a northern and/or coastal distribution not correlating with PCP distribution. Although long-range transport and environmental methylation of PCP or formation from other precursors cannot be excluded, the distribution patterns suggest that such processes may not be the only source of PCA to remote regions and unknown sources should be sought. We suggest that natural sources, e.g., chlorination of organic matter in Boreal forest soils enhanced by chloride deposition from marine sources, should be investigated as a possible partial explanation of the observed distributions. The results show that neither PCA nor total PCP (ΣPCP = PCP + PCA) should be used to approximate the concentrations of PCP; PCP and PCA must be determined and quantified separately to understand their occurrence and fate in the environment. The background work shows that the accumulation of airborne POPs in plants is a complex process. The variations in life cycles and physiological adaptations have to be taken into account when using plants to evaluate the concentrations of POPs in remote areas.

Arctic and Subarctic Natural Soils Emit Chloroform and Brominated Analogues by Alkaline Hydrolysis of Trihaloacetyl Compounds

There has been increasing recognition of the occurrence of natural, halogenated organic compounds in marine and terrestrial environments. Chloroform is an example of a halogenated organic compound with natural formation as its primary source. Chloroform emission from soil has been reported from diverse Arctic, temperate, and (sub)tropical ecosystems. The terrestrial environment is a significant source to the atmosphere, but little is known about the formation pathway of chloroform in soil. Here, we present evidence that chloroform is formed through the hydrolysis of trichloroacetyl compounds in natural, organic-rich soils. In situ emissions of chloroform from soil in nine Arctic and subarctic ecosystems were linked to soil trichloroacetyl turnover. The residence time from formation of the trichloroacetyl compounds in soil to the release of chloroform to the atmosphere varied between 1 and 116 active months in unfrozen topsoil, depending on soil pH. Nonspecific halogenation that leads to trihaloacetyl formation does not discriminate between chloride and bromide, and brominated analogues were formed alongside chloroform. Soil may therefore be a previously unrecognized, natural source of brominated haloforms. The formation pathway of haloforms through trihaloacetyl compounds can most likely be extended to other ecosystems with organic topsoils.

A metagenomic-based survey of microbial (de)halogenation potential in a German forest soil

In soils halogens (fluorine, chlorine, bromine, iodine) are cycled through the transformation of inorganic halides into organohalogen compounds and vice versa. There is evidence that these reactions are microbially driven but the key enzymes and groups of microorganisms involved are largely unknown. Our aim was to uncover the diversity, abundance and distribution of genes encoding for halogenating and dehalogenating enzymes in a German forest soil by shotgun metagenomic sequencing. Metagenomic libraries of three soil horizons revealed the presence of genera known to be involved in halogenation and dehalogenation processes such as Bradyrhizobium or Pseudomonas. We detected a so far unknown diversity of genes encoding for (de)halogenating enzymes in the soil metagenome including specific and unspecific halogenases as well as metabolic and cometabolic dehalogenases. Genes for non-heme, no-metal chloroperoxidases and haloalkane dehalogenases were the most abundant halogenase and dehalogenase genes, respectively. The high diversity and abundance of (de)halogenating enzymes suggests a strong microbial contribution to natural halogen cycling. This was also confirmed in microcosm experiments in which we quantified the biotic formation of chloroform and bromoform. Knowledge on microorganisms and genes that catalyze (de)halogenation reactions is critical because they are highly relevant to industrial biotechnologies and bioremediation applications.

Chlorination and dechlorination rates in a forest soil - A combined modelling and experimental approach

Much of the total pool of chlorine (Cl) in soil consists of naturally produced organic chlorine (Clorg). The chlorination of bulk organic matter at substantial rates has been experimentally confirmed in various soil types. The subsequent fates of Clorg are important for ecosystem Cl cycling and residence times. As most previous research into dechlorination in soils has examined either single substances or specific groups of compounds, we lack information about overall bulk dechlorination rates. Here we assessed bulk organic matter chlorination and dechlorination rates in coniferous forest soil based on a radiotracer experiment conducted under various environmental conditions (additional water, labile organic matter, and ammonium nitrate). Experiment results were used to develop a model to estimate specific chlorination (i.e., fraction of Cl(-) transformed to Clorg per time unit) and specific dechlorination (i.e., fraction of Clorg transformed to Cl(-) per time unit) rates. The results indicate that chlorination and dechlorination occurred simultaneously under all tested environmental conditions. Specific chlorination rates ranged from 0.0005 to 0.01 d(-1) and were hampered by nitrogen fertilization but were otherwise similar among the treatments. Specific dechlorination rates were 0.01-0.03d(-1) and were similar among all treatments. This study finds that soil Clorg levels result from a dynamic equilibrium between the chlorination and rapid dechlorination of some Clorg compounds, while another Clorg pool is dechlorinated more slowly. Altogether, this study demonstrates a highly active Cl cycling in soils.

Hydrogeologic Processes Impacting Storage, Fate, and Transport of Chloride from Road Salt in Urban Riparian Aquifers

Detrimental effects of road salt runoff on urban streams are compounded by its facilitated routing via storm drains, ditches, and flood channels. Elevated in-stream salinity may also result from seasonal storage and discharge of chloride in groundwater, and previous work has hypothesized that groundwater discharge to streams may have the effect of diluting stream chloride concentrations in winter and enriching them in summer. However, the hydrogeological processes controlling these patterns have not been thoroughly investigated. Our research focuses on an urban stream and floodplain system in Syracuse, NY, to understand how groundwater and surface water exchange impacts chloride storage, fate, and transport. We created a 3D groundwater flow and solute transport model of the floodplain, calibrated to the distributions of floodplain hydraulic heads and groundwater fluxes to the stream throughout the reach. We used a sensitivity analysis to calibrate and evaluate the influence of model parameters, and compared model outputs to field observations. The main source mechanism of chloride to the floodplain aquifer was high-concentration, overbank flood events in winter that directly recharged groundwater. The modeled residence time and storage capacity of the aquifer indicate that restoration projects designed to promote floodplain reconnection and the frequency of overbank flooding in winter have the potential to temporarily store chloride in groundwater, buffer surface water concentrations, and reduce stream concentrations following periods of road salting.

Experimental evidence of large changes in terrestrial chlorine cycling following altered tree species composition

Organochlorine molecules (Clorg) are surprisingly abundant in soils and frequently exceed chloride (Cl(-)) levels. Despite the widespread abundance of Clorg and the common ability of microorganisms to produce Clorg, 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 Clorg after 30 years were highly dependent on which tree species were planted on the nearby plots. Average Cl(-) and Clorg 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 Clorg over the experiment period, and showed a 10 and 4 times higher Cl(-) and Clorg storage (kg ha(-1)) in the biomass, respectively, and 7 and 9 times higher storage of Cl(-) and Clorg 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.

Chlorine isotope effects and composition of naturally produced organochlorines from chloroperoxidases, flavin-dependent halogenases, and in forest soil

The use of stable chlorine isotopic signatures (δ(37)Cl) of organochlorine compounds has been suggested as a tool to determine both their origins and transformations in the environment. Here we investigated the δ(37)Cl fractionation of two important pathways for enzymatic natural halogenation: chlorination by chloroperoxidase (CPO) and flavin-dependent halogenases (FDH). Phenolic products of CPO were highly (37)Cl depleted (δ(37)Cl = -12.6 ± 0.9‰); significantly more depleted than all known industrially produced organochlorine compounds (δ(37)Cl = -7 to +6‰). In contrast, four FDH products did not exhibit any observable isotopic shifts (δ(37)Cl = -0.3 ± 0.6‰). We attributed the different isotopic effect to the distinctly different chlorination mechanisms employed by the two enzymes. Furthermore, the δ(37)Cl in bulk organochlorines extracted from boreal forest soils were only slightly depleted in (37)Cl relative to inorganic Cl. In contrast to previous suggestions that CPO plays a key role in production of soil organochlorines, this observation points to the additional involvement of either other chlorination pathways, or that dechlorination of naturally produced organochlorines can neutralize δ(37)Cl shifts caused by CPO chlorination. Overall, this study demonstrates that chlorine isotopic signatures are highly useful to understand sources and cycling of organochlorines in nature. Furthermore, this study presents δ(37)Cl values of FDH products as well of bulk organochlorines extracted from pristine forest soil for the first time.