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

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.

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.

Impact of abiotic and biogeochemical processes on halogen concentrations (Cl, Br, F, I) in mineral soil along a climatic gradient

In contrast to earlier ideas that halogens behave inertly in soil, extensive biogeochemical cycling of fluorine (F), chlorine (Cl), bromine (Br) and iodine (I) has been shown for temperate forests. To further advance our understanding of halogen behaviour in soil beyond humid temperate forests, we sampled soil profiles in protected areas along the Chilean Coastal Cordillera, representing a pronounced climatic gradient spanning from arid to humid. Halogen concentrations in soil were analysed by combustion ion chromatography. Highest average total halogen concentrations occurred at the arid site (Cl, F: 4270 and 897 mg kg^-1) as well as the humid end of the climatic gradient (Br, I: 42.6 and 9.8 mg kg^-1). Vertical distribution patterns of halogens were most pronounced at the humid end of the gradient and became less distinct under drier climate. The climatic gradient demonstrates the important role of biotic processes (e.g. the halogenation of organic matter) on the retention of halogens in the soil. However, this climate-specific role may be overridden by mainly abiotic processes within a given climate zone (e.g. weathering, leaching, sorption to secondary soil minerals, evaporative enrichment), resulting in vertical relocation of halogens in the soil. Since some of these processes oppose each other, complex interactions and depth distributions of F, Cl, Br and I occur in the soil. In summary, our findings provide new insights into the fate of halogens in mineral soil of different climatic zones, which is important, for example, when radiohalogens are deposited on a large scale after nuclear accidents.

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.

Organohalide-Respiring Bacteria at the Heart of Anaerobic Metabolism in Arctic Wet Tundra Soils

Recent work revealed an active biological chlorine cycle in coastal Arctic tundra of northern Alaska. This raised the question of whether chlorine cycling was restricted to coastal areas or if these processes extended to inland tundra. The anaerobic process of organohalide respiration, carried out by specialized bacteria like Dehalococcoides, consumes hydrogen gas and acetate using halogenated organic compounds as terminal electron acceptors, potentially competing with methanogens that produce the greenhouse gas methane. We measured microbial community composition and soil chemistry along an ∼262-km coastal-inland transect to test for the potential of organohalide respiration across the Arctic Coastal Plain and studied the microbial community associated with Dehalococcoides to explore the ecology of this group and its potential to impact C cycling in the Arctic. Concentrations of brominated organic compounds declined sharply with distance from the coast, but the decrease in organic chlorine pools was more subtle. The relative abundances of Dehalococcoides were similar across the transect, except for being lower at the most inland site. Dehalococcoides correlated with other strictly anaerobic genera, plus some facultative ones, that had the genetic potential to provide essential resources (hydrogen, acetate, corrinoids, or organic chlorine). This community included iron reducers, sulfate reducers, syntrophic bacteria, acetogens, and methanogens, some of which might also compete with Dehalococcoides for hydrogen and acetate. Throughout the Arctic Coastal Plain, Dehalococcoides is associated with the dominant anaerobes that control fluxes of hydrogen, acetate, methane, and carbon dioxide. Depending on seasonal electron acceptor availability, organohalide-respiring bacteria could impact carbon cycling in Arctic wet tundra soils.IMPORTANCE Once considered relevant only in contaminated sites, it is now recognized that biological chlorine cycling is widespread in natural environments. However, linkages between chlorine cycling and other ecosystem processes are not well established. Species in the genus Dehalococcoides are highly specialized, using hydrogen, acetate, vitamin B₁₂-like compounds, and organic chlorine produced by the surrounding community. We studied which neighbors might produce these essential resources for Dehalococcoides species. We found that Dehalococcoides species are ubiquitous across the Arctic Coastal Plain and are closely associated with a network of microbes that produce or consume hydrogen or acetate, including the most abundant anaerobic bacteria and methanogenic archaea. We also found organic chlorine and microbes that can produce these compounds throughout the study area. Therefore, Dehalococcoides could control the balance between carbon dioxide and methane (a more potent greenhouse gas) when suitable organic chlorine compounds are available to drive hydrogen and acetate uptake.

Towards dynamic and process-based modelling of radionuclides cycling in terrestrial radioecology

Mathematical models are frequently used in terrestrial radioecology to interpret observations and to assess the detrimental impacts of radioactive releases to the environment. Conventional radioecological models are largely based on equilibrium and empirical relationships with reasonable data requirements, making them practical tools for long-term assessments. But conventional models may be inadequate to simulate radionuclide dynamics in terrestrial environments realistically. Specifically, the structure of such models seldom conforms to the physics of water flow and solute transport in soils. The equilibrium relationships may fail to predict seasonality in radionuclide transfer between environmental compartments; model transferability between sites is often hampered by its empirical nature. Numerous studies have highlighted the need to circumvent these limitations. In this paper, we introduce dynamic and process-based modelling to a conventional radioecological model by coupling an empirical plant module to a process-based soil module that simulates water flow, solute transport and root uptake in the soil column. Illustrative simulations are presented using the coupled model and stable chlorine cycling in a temperate Scots pine (Pinus sylvestris L.) stand as an example. The model satisfactorily reproduced soil moisture dynamics and the inventory of inorganic chlorine in the tree and forest floor compartments. The inventory of organic chlorine in the stand, however, was overestimated, indicating that processes pertinent to organochlorine cycling at the stand were missing from the model. The approach proposed in this paper is a step towards dynamic and process-based modelling in terrestrial radioecology and impact assessment. It can be particularly useful for modelling transfer of elements, such as redox-sensitive radionuclides, whose behaviour in soil-plant systems is moisture-dependent.

Vegetation canopy effects on total and dissolved Cl, Br, F and I concentrations in soil and their fate along the hydrological flow path

Although halogens are omnipresent in the environment, detailed understanding of processes involving chlorine (Cl), bromine (Br), fluorine (F) and iodine (I) in the terrestrial halogen cycle is still sparse. Our objectives were to (i) assess vertical depth profiles of total and water-extractable inorganic halogen concentrations (Cl_tot, Br_tot, F_tot, I_tot) in solid soil, (ii) test the effect of a tree canopy, and (iii) follow the fate of dissolved inorganic halogens along the hydrological flow path. More than 200 soil samples and ecosystem solutions (rainwater, soil solution, adit and creek water) collected in the Schwarzwald, SW Germany, were analyzed by combustion ion chromatography and ion chromatography for total and inorganic halogen concentrations. We found decreasing Cl_tot concentrations with increasing soil depth which were indicative of biological chlorination of organic matter and nutrient uplift, both associated with Cl accumulation in upper soil horizons. Vertical patterns of total Br, F and I were contrary to Cl_tot concentrations and were related significantly (positively) to pedogenic oxides, revealing their dependence on abiotic processes. The presence of a canopy at our study site resulted in significantly higher halogen concentrations in throughfall compared to rainfall and higher Br_tot concentrations in the organic layer. We attribute this difference to leaching from leaves and needles and wash-off of dry deposition. There were hardly any differences in halogen concentrations along the hydrological flow path except for significantly higher inorganic I concentrations in soil solution compared to rainfall due to equilibrium reactions between the soil solution and the solid soil phase. Highest inorganic F concentrations of up to 0.2 mg L^-1 were detected in creek water samples and may originate from the weathering of fluorite-bearing veins. Our study indicates halogen-specific processes underlying Cl, Br, I and F cycling in ecosystems.

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.

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.