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Monitoring of chlorine in the forest ecosystem – its cycling and effects

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Detailed Scientific Program

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Introduction

Chloride, the best known compound of chlorine, originating mainly from the ocean, has been considered chemically an inert compound; and its role in the forest ecosystem thus insignificant. Chlorinated organic compounds have been regarded as purely of anthropogenic origin. However, it has been found that the role of chlorine in nature is active and complex. Chloride is rinsed out from the atmosphere by rainfall or its aerosols are directly caught by vegetation. Cl- deposition fluctuates between tens and hundreds of kg/ha/year depending on the geographical situation, i.e. distance from the sea, concentration of salt in it, wind direction, altitude etc. Chloride does not behave in the chlorine cycle passively; it is not conservative at all, but reacts with soil organic matter (SOM). This reaction is mainly mediated by enzymes and to a lesser extent in an abiotic way, and so free Cl- is bound relatively rapidly. The content of bound chlorine is usually higher than 200 mg Clorg/kg forest soil dw (FH horizon), both in the CR and in Scandinavia.
Our previous study on the role of chloroacetic acids (CAA) in environmental processes showed that these phytotoxic compounds affect coniferous trees. At the same time CAA are intermediates during degradation of SOM in the forest ecosystem, where litter and humus represent the main reservoir of carbon. It was found by the application of carbon 14 labeled di- and trichloroacetic acid (DCA and TCA respectively), that these CAA are in the soil rapidly degraded to CO2 microbially, supposedly by fungi and soil bacteria. CAA formation in soil was also confirmed by use of the radioactive isotope chlorine 36. The action of microorganisms, mainly of fungi, is an important factor in the above mentioned processes. Soil microorganisms and their extracellular enzymes are capable of converting chloride into hypochloric acid or chlorine radicals, which chlorinate SOM. The formed DCA, TCA and chloroform can be easily detected. It is also known that other chlorinated compounds, e.g. humic substances formed in the soil, are present in the run-off from the forest ecosystem. Chlorinated aromatic compounds belonging to adsorbable organohalogens (AOX) and originating from lignin are also expected. AOX released into communal and industrial waste waters are not considered here as they belong to anthropogenic pollution. AOX content is proportional to the total organic carbon (TOC) and chloride content and influenced by pH. The formation of chlorinated compounds in the forest soil is undoubted, but the proportions of individual compounds and their structures and properties are little known. Also little known is the influence of chloride present in road salt on the vegetation and water, where its effects are the most severe. Chlorination leads to the formation of compounds which are more easily dissolved in water and also more easily degradable by microorganisms or by their enzymes, and sometimes by abiotic processes. In this way chlorination contributes to mineralization of SOM and to loss of carbon from the forest ecosystem, thus participating also in the carbon cycle. Run-off water from the forest ecosystem reduces the quality of drinking water sources. The role of chlorine can therefore be considered as one of the SOM degrading agents in the forest ecosystem – a large terrestrial reservoir of carbon – and chlorination as a source of a large number of chlorinated compounds, including volatile chlorinated hydrocarbons (VOCl), and also as one of the processes influencing global climate change.
The aim of the project is to investigate chlorination processes in the forest ecosystem – a sink of carbon and chlorine – and to evaluate their role as a so far unknown or little known contribution to mineralization of carbon in the geographic situations of the Czech Republic as a continental and of Norway as a coastal country. The project will utilize the network of intensively monitored ICP Forests Level II plots to monitor chloride in precipitation and in the soil, TOX in forest soil, AOX and DOC (dissolved organic carbon) in soil solution, and possibly also in run-off. To investigate the formation and degradation processes of the above mentioned chlorinated compounds in detail, model experiments will be conducted using common analytical and radiochemical trace techniques (radioactively labeled compounds, mainly 36Cl-chloride). The data acquired from the monitoring will be modeled by the DyDOC model (developed originally to follow the formation, transport and retention of organic carbon in forest soil) to investigate the dynamics of organically bound chlorine in the forest soil. The influence of microorganisms will be examined by molecular genetic methods. For the implementation of the project, a team of experts has been created with the participation of a Norwegian research group.

Background and justification of the project

Chloride has been considered as an inert compound chemically (e.g. Neal and Rosier 1990), and has even been used as a “tracer” to follow streams of water in ecosystems. On the other hand, chlorinated organic compounds (Clorg) have generally been regarded as xenobiotics; it has been presumed that chlorine does not participate in biological processes and is only present naturally in the environment as chloride (Öberg 1998, 2002). However, it is now known that this is not the case: more than 2000 natural chlorinated compounds have been described (Gribble 2003). In the forest ecosystems of the Czech Republic, Clorg has not so far been monitored, except for some preliminary determinations for orientation. Its determination utilizes standard methods for the determination of AOX, representing a mixture of heterogeneous chlorinated compounds (Asplund et al. 1994).
Rather than being inert or of conservative nature, chloride participates in a complex biochemical cycle including the formation, leaching, degradation and volatilization of Clorg. Absorption, formation and degradation of AOX in the soil horizons containing litter and organic matter have great ecological significance as they may be correlated with microbial degradation of more stable forms of SOM. Litter and humic substances represent an important part of stable SOM coming mainly from lignic structures (Piccolo 2001). According to an approach using numeric models, Europe with its climatic conditions is one of the more important parts of the world for sequestration of bioorganic carbon as woody litter and in the humus subsequently formed (Foley 1994). It is necessary to emphasize here that there is more carbon in the pedosphere than in the biosphere (Krull et al. 2003). The biodegradation of litter in the forest ecosystem is the result of the activity of many living organisms, including fungi, bacteria, actinomycetae, animals etc., which ensure among other things the mixing of litter with soil particles and exudates, its physical disintegration, transformation and final biodegradation (Morris and Paul 2003). Biodegradation itself represents a way for biota to obtain energy for further growth. The energy is released largely by oxidation processes, as a result of which the organic forms of carbon are mineralized and carbon itself released in the form of carbon dioxide, which is then recycled by photosynthesis (e.g. Finzi et al. 2001). Soil microflora (mainly saprophytic fungi) produce various enzymes which are involved in the biodegradation of organic compounds. Most of the enzymes are hydrolytic, causing depolymerization of high molecular weight compounds, and enzymes catalyzing the oxidation of organic compounds.
If chlorination in the carbon cycle of forest ecosystems is significant, then the role of chlorine must be re-evaluated also in connection with the global carbon cycle. The formation of breakdown compounds and their final mineralization (and the subsequent CO2 release into the atmosphere) lead to carbon loss from the entire forest ecosystem (e.g. Schlesinger 1999, Piccolo et al., 2004). In this way, chlorine participates in the carbon cycle and the connection with the increase of atmospheric CO2 concentration and thus with climate change is obvious (e.g. Adams and Piovesan 2005).
Soil microbial processes result in the production not only of CO2 but also of dissolved organic carbon (DOC) (Marschner and Kalbitz 2003, Kalbitz et al. 2000, 2003, Clarke et al. 2005, 2007). DOC is on the one hand included in the run-off from the forest ecosystem, and on the other hand penetrates into the soil’s mineral horizons, where it may be removed from solution and in this way increase sequestration of carbon. A model of this dynamic process has been elaborated (Michalzik et al. 2003). Although this model did not specifically include chlorine or its effects, it can be used to explore the dynamics of the formation, transport and storage of Clorg as part of SOM and DOC in both Czech and Norwegian forest ecosystems.
In the 1990s it was found that a great number of chlorinated compounds of natural origin are present in the natural environment. It was not quite clear at that time that the breakdown processes of organic compounds in the forest ecosystem proceed with the participation of chlorine as well as microorganisms (Asplund et al. 1989, 1994, Öberg and Groen 1998). Nevertheless, these processes were intensively studied and the results found were published many times, mainly by the Swedish group (Asplund et al. 1989, 1993a, 1994, Asplund 1995, Hjelm et al. 1995, 1999, Öberg 1998, 2002, 2003, Öberg et al. 1996, 1997, 1998, Öberg & Groen 1998, Haselmann et al. 2000, Johansson et al. 2003). For studies of the behavior of chlorine in soil, its isotopes were used. Hoekstra et al. (1999b) used 37Cl to prove chlorination of humic substances in forest soil and formation of TCA and chloroform. The radioisotope 36Cl was used rather exceptionally (Silk et al. 1997, Lee et al. 2001) to study its binding to SOM, i.e. the reactivity of chlorine in soil. Silk et al. (1997) found that chlorinated dibenzodioxins are also formed by chlorination in soil. These studies, however, were rather exceptional.
Preliminary laboratory experiments using 36Cl were conducted with the participation of our group (Bastviken et al. 2007). According to our knowledge, the role of SOM chlorination in the soil has never been studied in detail under continental climatic conditions. The geographic position of the Czech Republic offers this opportunity. It would also make a very useful comparison with a coastal country like Norway, where deposition of chloride is very high in the coastal regions – among the highest in Europe, even reaching hundreds of kg/ha/year. In this respect one can look at the roadside forest stands in the Czech Republic, where winter road salting is executed, as an extreme case of high chloride content.
It has long been known that microorganisms can convert chloride into organically bound chlorine (Shaw and Hager, 1959) and that halo- or chloroperoxidases are the responsible enzymes (Morris and Hager 1966, Hager et al. 1966). Moreover, Keppler et al. (2000) also described the abiotic chlorination of SOM leading to aliphatic haloderivatives, while Fahimi et al. (2003) showed the abiotic chlorination of SOM leading to chloroacetete formation. The latter process was successfully explained as Fenton’s reaction. It seems that the basis of both microbial/enzymatic and abiotic chlorination of SOM is the presence of hypochlorous acid or of the chlorine radical coming from the reaction of chloride with hydroxyl radicals present in the soil (Matucha et al. 2003a,b and 2007a).
It is evident that we find chlorination of organic compounds such as lignin (Johansson et al. 2000) and fulvic acids (Niedan et al. 2000), and the formation of for example chlorinated hydrocarbons and CAA in the forest ecosystem (Haiber et al. 1996, Fahimi et al. 2003, Pracht et al. 2001, Matucha et al. 2003b, Matucha et al. 2004, Laturnus et al. 2005, Laturnus and Matucha 2007), where chloroperoxidases are present at the same time (Asplund et al. 1993b, Laturnus et al. 1995). It is possible to reason that enzymatically or biotically formed chlorinated compounds might be intermediates during the degradation of SOM. It seems that fungi play an essential role in these processes (de Jong and Field 1997, Verhagen et al. 1998). Microbial/enzymatic and abiotic chlorination also lead to the formation of compounds or structures which are more soluble in water and more easily degraded, and thus affect DOC formation.
Among the less studied products of soil chlorination are volatile methyl chloride and chloroform (Svensson et al. 2007a,b, de Jong and Field 1997, Winterton 2000, Hamilton et al. 2003). Their formation from senescent plant material, as well as the C-Cl bond formation (Myneni 2002), has been proved. Atmospheric methyl chloride damages the stratospheric ozone layer (Lovelock 1975, Harper et al. 1990, Harper and Kennedy 1986). Winterton (2000) refers to chlorination in plants as well, and this has also been suggested by our group with formation of VOCl in mosses and ferns (Laturnus and Matucha, 2007). The formation of chloroform has been observed many times (Hoekstra et al. 2001, Haselmann et al. 2000 and 2002, Laturnus et al. 2000 and 2002, Svensson et al. 2007a,b). Its penetration into soil water can be expected; moreover, it was detected in discharge from forest ecosystems in concentrations of tens of ng/L (Svensson et al. 2007a). The presence of chloroform in discharge from forest ecosystems has implications for drinking water quality.
CAAs are part of Clorg (Winterton 2000). TCA, one of the phytotoxic CAAs and one of the most well-known products of SOM chlorination, was studied by our group in connection with its effects on and contribution to forest decline as a secondary air pollutant (Matucha et al. 2001, Matucha and Uhlířová 2002, Uhlířová et al. 1995, 1996, 2001, 2002, Coufal et al. 2003). Radioactively labeled [1,2-14C]TCA and DCA were synthesized according to Bubner et al. (2001) and used for study of the behavior of CAA in the Norway spruce/soil system (Forczek et al. 2001, Matucha et al. 2003a,b a 2004, 2005, 2006a, Schröder et al. 2003). How TCA is distributed in spruce (taken up by roots, translocated by the transpiration stream and accumulated in the needles) (Forczek et al. 2001, 2004), absorbed in litter residues, and degraded microbially in the soil depending on soil humidity, temperature and concentration (Schröder et al. 2003, Matucha et al. 2003a a 2007b) was elucidated. We also confirmed the formation of TCA and DCA in forest soil (Matucha et al. 2007a,b) and further elucidated the role of chloroacetic acids in the environment, focusing on the Norway spruce/soil system (Matucha et al. 2003b, Matucha et al. 2005). The effect of TCA was investigated also on the ultracellular level in needles, where it decreases chloroplast size, the visual damage symptoms - chlorosis, necrosis and needle loss - appearing later (Forczek et al. 2003). An even more pronounced effect of TCA was found after its formation by biooxidation from tetrachloroethylene in the chloroplasts, causing damage directly at the place of formation in the photosynthetic apparatus; tetrachloroethylene is, unlike TCA, easily taken up through the needle cuticle (Weissflog et al. 2007, Forczek et al. 2008). The stress response in spruce can be observed in the changes of the content of photosynthetic pigments (Sutinen et al. 1995) or by the increased enzyme activity of detoxifying pathways – peroxidases, GST etc. (Dickey et al. 2004, Schröder et al. 1997). Microbial degradation of TCA in needles and their microbial colonization from soil was also found with [1,2-14C]TCA (Forczek et al. 2004).
In conclusion, we may understand chlorine as a degradation agent in the decay of SOM, a big terrestrial reservoir of carbon (Krull et al. 2003). Chlorination as one of the degradation processes in SOM mineralization, i.e. formation of CO2, is a factor related to global changes in climate. The project will involve monitoring of chloride in soil, AOX/TOX in soil, soil solution, precipitation and AOX and DOC in run-off. For detailed studies of the processes of formation and degradation of chlorinated substances in the laboratory, model experiments using conventional analytical and radiotracer techniques are suggested, using radioanalytical methods and radioactively labeled compounds, some of which were used in the previous studies but suitably modified as appropriate. From the field of monitoring, the long-term experience from the European forest monitoring program ICP Forests of both the Czech partner (FGMRI) and the Norwegian partner (Norwegian Forest and Landscape Institute, Aas) will be very helpful.

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Abbreviations

AOX – adsorbable organically bound halogen
CAA – chloroacetic acids
Clorg – organically bound chlorine
DOC – dissolved organic carbon
DCA – dichloroacetic acid
dw – dry weight
OCl – organically bound chlorine
SOM – soil organic matter
TCA – trichloroacetic acid
TOX – total organically bound halogen
TX – total chlorine
VOCl – volatile chlorinated hydrocarbons

Monitoring of chlorine in forest ecosystems Forestry and Game Management Research Institute v.v.i.

Monitoring, ensuring of data for partners and subcontractors, field and technical work, sampling and sample processing, ensuring of laboratory work and analysing, research, data evaluation, cooperation on model development.

Laboratory chlorination experimentsInstitute of Experimental Botany, Academy of Sciences of the Czech Republic v.v.i.

Monitoring of chloroform and AOX, laboratory research of chlorination processes, evaluation of data obtained from monitoring and laboratory research, preparing of reports and manuscripts.

Modeling of the dynamics of organic chlorine Norwegian Forest and Landscape Institute
Data validation, modelling of the dynamics of organic chlorine in forest soils and waters and its relationship with organic carbon.

Management and publicity This activity involves management, coordination of activities, financial management and administration work of the project
and activities linked with publicity of the project (publications in scientific journals, information on website, and final workshop).
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