The Tema Institute Campus Norrköping
Bachelor of Science Thesis, Environmental Science Programme, 2011
Elias Broman & Maria Hägglund
A study of dechlorination of organic
matter in forest soil using
36
Cl as a
Rapporttyp Report category Licentiatavhandling Examensarbete AB-uppsats C-uppsats D-uppsats Övrig rapport ________________ Språk Language Svenska/Swedish Engelska/English ________________ Titel Title
A study of dechlorination of organic matter in forest soil using 36Cl as a tracer
Författare Author
Elias Broman & Maria Hägglund
Sammanfattning
Abstract
During the Fukushima Daiichi power plant incident sea water was used in an attempt to cool reactor Unit 3. Since sea water contains an excessive amount of chloride, 36Cl has likely been formed and spread in the environment. Because of the long residence time and the presumed high mobility in water there is an increased interest to learn more about the biogeochemical cycle of chlorine from a radiation risk assessment perspective. Chlorine occurs in inorganic form as chloride (Clin) or bound to organic matter as organic chlorine (Clorg) and is commonly found in
the environment due to both anthropogenic and natural processes. Though there are still uncertainties regarding all of the components of the chlorine cycle in soil, the chlorination of organic matter has been exemplified by research. The reverse process, Clorg mineralizing into Clin, has
however not been thoroughly investigated. For this study the objective was to observe at what rate Clorg mineralizes into Clin, this by using 36Cl as
a tracer in forest soil. 36Cl was added to the soil and 36Clorg was formed over a period of approximately 100 days. After chlorination the samples
were incubated in different conditions and the amount of 36Cl
org was observed over a period of time (180 days). The result showed no evident
dechlorination during the experiment period which indicates that Clorg can be stable in the organic horizon in forest soil.
ISBN _____________________________________________________ ISRN LIU-TEMA/MV-C—11/07--SE _________________________________________________________________ ISSN _________________________________________________________________ Serietitel och serienummer
Title of series, numbering
Handledare Tutor
David Bastviken
Nyckelord
Keywords
Chlorine, Dechlorination, 36Cl, forest soil, organic matter, carbon cycle
Datum
Date 2011-06-10
URL för elektronisk version
http://www.ep.liu.se/index.sv.html
Institution, Avdelning
Department, Division
Tema vatten i natur och samhälle, Miljövetarprogrammet
Department of Water and Environmental Studies, Environmental Science Programme
2
Abstract
During the Fukushima Daiichi power plant incident sea water was used in an attempt to cool reactor Unit 3. Since sea water contains an excessive amount of chloride, 36Cl has likely been formed and spread in the environment. Because of the long residence time and the presumed high mobility in water there is an increased interest to learn more about the biogeochemical cycle of chlorine from a radiation risk assessment perspective. Chlorine occurs in inorganic form as chloride (Clin) or bound to organic matter as organic chlorine (Clorg) and is commonly found in the environment due to both anthropogenic and natural processes. Though there are still uncertainties regarding all of the components of the chlorine cycle in soil, the
chlorination of organic matter has been exemplified by research. The reverse process, Clorg mineralizing into Clin, has however not been thoroughly investigated. For this study the objective was to observe at what rate Clorg mineralizes into Clin, this by using 36Cl as a tracer in forest soil. 36Cl was added to the soil and 36Clorg was formed over a period of approximately 100 days. After chlorination the samples were incubated in different conditions and the amount of 36Clorg was observed over a period of time (180 days). The result showed no
evident dechlorination during the experiment period which indicates that Clorg can be stable in the organic horizon in forest soil.
3
Acknowledgements
We would like to thank our tutor David Bastviken for the opportunity to participate in this research project and for all the support and helpful comments during this process. We would also like to thank Henrik Reyier and Paul-Olivier Redon for practical assistance in the laboratory and helpful conversations. And finally we are grateful to Lena Lundman, Susanne Karlsson, Malin Gustavsson and Teresia Svensson who have taken time to assist us with this project.
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Table of contents
Introduction ... 5
Materials and methods ... 7
Soil collection ... 7
Experimental setup ... 7
Soil extractions ... 9
Combustion of soil-bound 36Cl org ... 10
Liquid scintillation counting (LSC) ... 10
Statistical analysis ... 11
Results ... 12
Discussion ... 14
Conclusions ... 16
5
Introduction
In March 11th 2011 an earthquake occurred east of Honshu, Japan near the coast. As a consequence of this a tsunami hit the eastern mainland of Japan and cut the power supply of the nuclear power plant Fukushima Daiichi. A few days later, in an attempt to cool reactor Unit 3, helicopters scooped up seawater and released it over the plant (IAEA 2011). Seawater contains an excessive amount of chloride (Bigg 2003) and when in contact with an
environment producing free neutrons non-radioactive chloride can be transformed into 36Cl (Cook & Herczeg 2000). 36Cl is a β emitter with a half-life of 300 000 years (Thierfeldt & Deckert 1995). Because of the long residence time and the presumed high mobility in water there is an increased interest to learn more about the biogeochemical cycle of chlorine from a radiation risk assessment perspective.
Chlorine occurs in inorganic form as chloride (Clin) or bound to organic matter as organic chlorine (Clorg) and is commonly found in the environment due to both anthropogenic and natural processes (Winterton 2000). This also applies to soil where Clorg is more abundant than Clin (e.g., Svensson 2006). A study by Svensson et al. (2007) confirmed that in the observed catchment Clorg was the most common form of chlorine found in the soil. Similarly the total storage of Clin and Clorg were measured to be 30 times larger than the input and output of Clin. Similar results were observed in other research (e.g., Johansson et al. 2003). Clin has been observed to be double that of Clorg in forest soil leachate (Öberg & Sandén 2005). Natural processes in the topsoil can form Clorg which eventually leaches to the deeper layers where most of it is retained or mineralized. The amount of Clorg decreases with the depth of the soil and a similar decrease was found for organic carbon (Öberg 1998; Öberg & Sandén 2005). Johansson et al. (2003) showed that there was a positive correlation between organic carbon and Clorg for forest soils in southern Sweden.
Clin has for a long time been presumed to be inert in soil with littleinfluence from vegetation or other processes (Schlesinger 1997) and has therefore been used to trace water movement (e.g., Lockwood et al. 1995; Kirchner et al. 2000). However extensive research conducted during the last decade have shown that Clin is in fact reactive in the ecosystem and takes part in a complex biogeochemical cycle (Figure 1) (Winterton 2000; Myneni 2002; Öberg 2002; Lovett et al. 2005; Öberg & Sandén 2005; Öberg et al. 2005, Svensson 2006, 2007; Bastviken et al. 2007; Bastviken et al. 2009; Rohlenová et al. 2009; Svensson et al. 2010).
There are several sources of soil chlorine (Figure 1) and atmospheric deposition is one source of Clin in soil and it is transported to the soil through wet deposition and dry deposition (Eriksson 1960). Sea salt and road salt are other sources of Clin (Blomqvist 2001; Löfgren 2001) and weathering can also be a contribution (Lovett et al. 2005). One substantial
anthropogenic source of Clorg in soil is point sources, such as usage of chlorine compounds in paper bleaching (Stringer & Johnston 2001). Precipitation has been found to contain Clorg (Laniewski et al. 1999), although the origin of Clorg in precipitation is not fully understood. Öberg et al. (1998) found Clorg in spruce forest throughfall in Denmark. However the result of the study indicated that dry deposition likely was not the cause of this, rather Clorg came from
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internal sources. The understanding of Clorg in dry deposition is still not thoroughly comprehended.
FIGURE 1. Simplified overview of the chlorine cycle in soil
The arrows represents in and out flows of chlorine in the soil system. For more details see the text.
Though there are still uncertainties regarding all of the components of the chlorine cycle in soil (Svensson et al. 2010), chlorination of organic matter has been exemplified by several studies (e.g., Hjelm et al. 1995; Hjelm 1996; Bastviken et al. 2006; Bastviken et al. 2007; Svensson 2007). The cause of this is not fully explained but microbiological processes are an important factor in the chlorination of organic matter (Lovett et al. 2005; Rohlenová et al. 2009; Svensson et al. 2010). Clin in pore water was, for example, observed to be regulated by microbiological activity (Bastviken et al. 2007). Research regarding temperature sensitivity by Bastviken et al. (2009) further established the importance of the microbiological processes for the chlorination. The optimal temperature for the chlorination was concluded to be 20 °C during oxic conditions but chlorination occurred in the range of 4-40 °C. Chlorination was also observed at 50 °C and during anoxic conditions which indicates that abiotic processes can cause chlorination even though biotic chlorination typically is predominant. Chlorination by abiotic processes has also been observed in earlier studies by Fahimi et al. (2003) and Keppler et al. (2000). The reversed process, i.e. dechlorination of organic matter in soil, has however not been exemplified through research. Though studies of PCB congeners in sediments has shown a dechlorination processes due to microbiological activity (e.g., Tiedje et al. 1993; Natarajan et al. 1998; Chen et al. 2001), which was found to occur at 12°C in an anoxic condition (Tiedje et al 1993). Previous research conducted by Bastviken (2006) has confirmed that Clorg can mineralize into Clin in soil. This by using lysimeters to measure soil leachate containing Clin. The rate of this mineralization has though not been thoroughly investigated.
Cl
orgCl
inSoil
Wet and dry deposition of Clorg and Clin
Leaching
Formation
Mineralization
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The objective of this study was to investigate at what rate Clorg in forest soil can be released as Clin, using 36Cl as a tracer. The effect of temperature and oxygen on the dechlorination was observed. Half of the samples had an addition of glucose and maltose but the effect this had on the dechlorination phase could not be thoroughly observed. This because the addition was mainly added to stimulate the chlorination process (to increase chlorination of organic matter) which would give a more distinct result. The results of Bastviken (2009) regarding
temperature and oxic/anoxic conditions with an optimal temperature of 20°C in an oxic condition for chlorination of organic matter, came to influence the hypotheses for this study:
The amount of 36Clorg decreases with time during the mineralization which indicates dechlorination.
higher temperature will increase the mineralization rate
and oxic condition will increase the mineralization rate of 36Clorg.
Materials and methods
Soil collection
The soil used in this experiment was collected in September of 2008 in the south of Östergötland county, close to the locality of Horn in Kinda municipality. The area is characterized by plane forest land, moraine and with a texture of fine sand. The ground
vegetation consists of thin-leaved grass types. The sample was taken from the organic horizon (O horizon) after litter and grass had been removed. Approximately 2-3 kg of forest soil was collected in a plastic bag (polyethylene) and immediately transported to the laboratory where it was stored at 4°C awaiting further analysis.
Experimental setup
In January of 2010 approximately 2 g of the soil (fresh weight) was added to 50 ml
polypropylene centrifuge tubes (Sarstedt, Germany), to a total of 144 tubes. The water content of the soil was estimated to be 36.4 %. To each centrifuge tube 0.3 ml 36Clin (Amersham Biotech; 0.59 MBq mg Cl-1) diluted in Milli-Q water was added. For each tube the added 36
Clin was equivalent to 400 000 disintegrations per minute (dpm; 60 dpm corresponds to 1 Bq). The added 36Clin was equivalent to a mass of 11.06 µg Clin. In half of the centrifuge tubes approximately 0.11 g glucose and 0.11 g maltose was added, to stimulate 36Clorg formation based on previous unpublished results. The soil in each tube was then mixed using a syringe needle to homogenize the soil. The centrifuge tubes were left open in 20°C under a fan until the soil reached its original fresh weight after the 36Clin addition. To ensure evenly drying throughout the whole sample a cross was made in the soil with a syringe needle. Six samples were then frozen (three replicates with glucose and maltose and three without) to determine initial amount of 36Clorg before chlorination.
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The samples were then incubated to allow chlorination of the soil organic matter with the isotope labeled chloride for approximately 100 days in 20°C. To maintain oxic conditions in the samples an aquarium pump was used to regularly pump air through the tubes. To maintain air humidity and soil moisture the air was first pumped through a tube with water before reaching the samples. After the chlorination phase and before the start of the experiment phase focusing on dechlorination it was necessary to remove excessive 36Clin which had not been incorporated into the organic matter. To do this soil extractions were performed according to the method described below (see Soil extractions). The only differences were that Milli-Q water was used for the first three extractions and for the final extraction 10 ml of soil water was used, this to restore the ion balance in the samples. This to ensure a natural environment for the microorganisms regarding salt and water balance. The soil water was prepared by filtering Milli-Q water through fresh unprepared soil. After the extractions a syringe needle was used to make a cross in the soil for each sample as described above and dried in 20°C under a fan.
Before the start of the dechlorination phase of the experiment the samples were divided into series with different conditions such as temperature, glucose and maltose and oxic/anoxic treatment (Table 1).
TABLE 1. Different treatments and conditions for the dechlorination phase.
Treatment Addition Number of samples
Oxic 4°C 18
Oxic 4°C Glucose and maltose 18
Oxic 10°C 18
Oxic 10°C Glucose and maltose 18
Oxic 20°C 15a
Oxic 20°C Glucose and maltose 15
Anoxic 20°C 18
Anoxic 20°C Glucose and maltose 18
a
Oxic 20°C treatments had three samples less and no withdrawal at 180 days occurred for these treatments.
A majority of the samples were incubated in an oxic condition (with an aquatic pump as described above). Samples incubated in an anoxic condition were stored in a glovebox containing N2. The samples in the glovebox were kept moist using the same principle as described for the oxic treatments but instead of using air a flow of N2was used.Before the start of the incubation 24 samples, three for each treatment, were withdrawn and frozen for further analysis to determine 36Clorg.After 5, 15, 60, 120 and 180days of incubation three replicates for each treatment were withdrawn and frozen until further analysis. An overview of the steps in the experiment is displayed in Figure 2.
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FIGURE 2. Overview of the experiment
Forest soil was collected and later prepared with 36Clin for the chlorination phase. The chlorination was
performed to form 36Clorg this to be able to use 36
Cl as a tracer. The samples were prepared for the dechlorination phase by extraction to remove excess 36Clin. The dechlorination phase lasted up to 180 days and afterwards soil
extractions were performed to remove possible occurrence of 36Clin. The soil-bound 36Clorg was then combusted
using an AOX instrument to trap released 36Clin. Samples were then analyzed using liquid scintillation counting
which yielded the result in dpm which corresponds to radiation activity. For a more comprehensive description of the steps see the text.
Soil extractions
The routine of the soil extractions was modified from Bastviken (2007) to extract 36Clin. The soil extractions wereperformed to remove possible occurrence of 36Clin, to not interfere with further analyses. The frozen samples were thawed in 20°C for 15-20 minutes, after the
thawing 20 ml Milli-Q water was added. An end-over-end shaker was then used to tumble the samples for 30 minutes to ensure that the soil and liquid were mixed. Once tumbled the samples were centrifuged at 6000 g for 10 minutes to separate the solid and liquid phase to
facilitate the extractions. The supernatant was extracted by pipet to new centrifugetubes (extraction 1) which were frozen until the following day. The soil was dried in 60°C and later stored in 20°C. The following day the soil was grinded and then 5 ml Milli-Q water was added to the samples. The soil was sonicated (Bandelin electronic) for 45 seconds at 50% intensity. The sonication destroys cells and 36Clin in thecells are released. Afterwards 15 ml Milli-Q water was added to the soil and the samples were tumbled and then centrifuged again. The supernatant was extracted as described above (extraction 2) and pooled with extraction 1. For the third extraction 20 ml 0,01 M KCl was added to the soil, the theory is that the chloride
Soil Collection
Experiment setup
Chlorination phase
Dechlorination phase
Combustion of soil-bound 36Clorg Soil extractions
Liquid scintillation counting (LSC)
Extraction 1: 20 ml Milli-Q water
Extraction 2: Grinding, sonication and 20 ml Milli-Q water Extraction 3: 20 ml KCl
Extraction 4: 20 ml Milli-Q water
10
in KCl will exchange with the 36Clin possibly being bound to the surface of particles. The samples were tumbled and centrifuged again, the supernatant was extracted (extraction 3) to new centrifuge tubes. For the final extraction 20 ml Milli-Q water was added to the soil, they were then tumbled and centrifuged. The supernatant was extracted (extraction 4) and pooled with extraction 3. All of the extracts were frozen for further analysis. The soil was then dried in 60°C and later stored in 20°C until further analysis.
During the soil extractions when approximately 20 ml Milli-Q water or KCl was added before centrifugation, sometimes an additional volume had to be added. This to ensure a balance between samples in the centrifuge with a difference less than ± 0.1 g. The Milli-Q water and KCl were mainly used to extract 36Clin from the soil, therefore an addition to the volume should not be of major importance for the results. To give more detail to this study analyses of the soil extracts could have been performed. Due to time limitations such analyses could not be conducted as part of this work.
Combustion of soil-bound 36Cl org
The method used to prepare the soil-bound 36Clorg for further analysis was previously
described by Bastviken et al. (2007) and Bastviken et al. (2009). For each sample 0.15-0.18 g soil was combusted in 1000°C for 20 minutes using an Adsorbable Organic Halogens (AOX) instrument (Euroglas). This causes the 36Clorg to become gas, following a gas flow of oxygen the 36Cl is led through H2SO4. The reaction with H2SO4 forms HCl containing 36Cl. This acid is then led into two series connected scintillation vials prepared with 10 ml 0.1 M NaOH, this causes a reaction that traps the 36Cl in the vials. The connection of the vials in a series was used to observe the amount of 36Cl that did not got trapped in the first vial. Blank samples were combusted to determine background radiation, this by combusting an empty sample cup. The scintillation vials were then stored in 4°C until further analysis. This technique of using an alkaline solution to trap organohalogens was described by Laniewski et al. (1999) and later adapted by Bastviken et al. (2007).
Some of the blank samples that were produced during combustion showed relatively high amounts of radioactivity compared to expected values. This however had no major impacts on the results, since when tested with expected blank values the variation between samples and statistical tests showed similar results compared to the original results.
Liquid scintillation counting (LSC)
To perform an analysis of the scintillation vials containing 36Cl 10 ml scintillation cocktail (Ultima Gold XR, Chemical instruments AB) was added to each vial. The scintillation cocktail causes the β radiation emitted by the 36Cl to appear as scintillations of light (Ghosal & Srivastava 2009). The vials, together with blank samples from the combustion of soil-bound 36Clorg, were then analyzed with a Beckman LS6500 scintillation counter which detects the amount of flashes per time unit which corresponds to radiation intensity. The blank samples were used to quantify background radiation which was used to calculate the dpm for each sample. For the calculation the total amount of soil in the centrifuge tubes were taken
11
into account. These dpm values, in comparison to the initial dpm values from the added isotope, gave a percentage of recovered 36Clorg.
Statistical analysis
A bivariate correlation test was conducted to observe if there was a correlation between the proportion of the added 36Clin as 36Clorg and days of incubation.Because the data was not normally distributed the nonparametric Kendall’s Tau-b correlation test was used (Wheater & Cook 2000).
To test for significant differences between oxic and anoxic condition Mann-Whitney U nonparametric test was performed. Kruskal-Wallis nonparametric test was performed to test for significant differences between temperatures (Wheater & Cook 2000). Table 2 displays the setup for the statistical analyses. For all statistical analyses the conventional significant level of 5 % was used.
TABLE 2. Setup for statistical analyses
Kendall’s Tau-b Mann-Whitney U Kruskal-Wallis
Correlation for all treatments regarding proportion of 36Clin as 36
Clorg and days of incubation.
Test for significant difference between oxic and anoxic condition.
Test for significant differences between temperatures, 4, 10 and 20°C.
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Results
The proportions of recovered 36Clorg from the total amount of added 36Clin for oxic treatments are displayed in Figure 3 and anoxic treatments are shown in Figure 4. The percentage is calculated using the initial 400 000 dpm for each centrifuge tube and the dpm value retrieved from the LSC analysis. The graphs indicate that no distinguishable dechlorination of 36Clorg had occurred and the results of the Kendall’s Tau-b correlation tests verified this with no significant results (Table 3). Instead the abundance of 36Clorg appears to be stable over time (although highly variable in some cases). The Mann-Whitney U tests yielded no significant difference between oxic and anoxic condition (P-value = 0.668). Similarly no significant difference between temperatures were observed with the Kruskal-Wallis test (P-value = 0.085). These statistical results indicate that temperature and oxic/anoxic condition had no major impact on the dechlorination process.
TABLE 3. Results from Kendall’s Tau-b correlation test
Treatment Addition P-value
Oxic 4°C 0.419
Oxic 4°C Glucose and maltose 0.788
Oxic 10°C 0.564
Oxic 10°C Glucose and maltose 0.847
Oxic 20°C 0.761
Oxic 20°C Glucose and maltose 0.105
Anoxic 20°C 0.847
Anoxic 20°C Glucose and maltose 0.419
Treatments with an addition of glucose and maltose displayed larger variation (Figure 3b, 3d, 3f; Figure 4b) compared to treatments without. For all treatments the distribution of the replicates were irregular and did not follow an obvious pattern. Some of the treatments had mean values at certain days of incubation above the initial mean value at day 0. Figure 3f has an outlier at 78 % for 120 days of incubation.
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FIGURE 3. Percentage of recovered 36Clorg of the total added isotope for oxic treatments.
The graphs show all of the oxic treatments for the temperatures 4, 10 and 20°C. The diamonds denote the three replicates. The interpolation line represents the mean at each withdrawal. For some graphs a mean value is above the initial mean at day 0. Treatments with added glucose and maltose (right side) shows larger variation and an irregular pattern compared to treatments without glucose and maltose (left side). Note that at 120 days incubated graph f has an outlier at 78 % which is not displayed in the graph but still part of the graph. Graph e and f each have three replicates less than the other treatments, with 120 days of incubation as the last withdrawal.
a b
c d
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FIGURE 4. Percentage of recovered 36Clorg of the total added isotope for anoxic treatments.
The graphs show the anoxic treatments at the temperature 20°C. The diamonds denote the three replicates. The interpolation line represents the mean at each withdrawal. The treatment with added glucose and maltose show larger variation between the replicates than the treatment without glucose and maltose.
Discussion
The results have shown that the hypotheses could not be verified because no significant dechlorination could be established (Table 3). This means that 36Clorg could not be observed to decrease with time. Neither did temperature and oxic/anoxic conditions have an effect on the dechlorination. The study of Bastviken et al. (2009) established an optimum temperature for chlorination of soil organic matter at 20°C and the dechlorination was thereforeassumed to be favored by similar temperatures. Because no optimum temperature or oxic/anoxic condition for a possible dechlorination could be established this could be an indication that a
microbiological dechlorination of soil organic matter is not occurring at a significant degree. It is therefore possible that the process of dechlorination of soil organic matter is functioning differently than that of chlorination.
Treatments that were added with glucose and maltose at the start of the experiment showed higher initial values of 36Clorg. This confirms that microbiological processes for chlorination of soil organic matter were stimulated by an addition of glucose and maltose, as described in unpublished results. The higher amount of 36Clorg in these samples seems to have had an effect on the dechlorination phase of the experiment, such as larger variation between the replicates as seen in Figure 3 and 4. This variation could possibly be a result of a larger amount of microbiological processes during the dechlorination phase, due to stimulated microbiological growth by addition of glucose and maltose during the chlorination phase. The high variability in the result of this study is presumably difficult to elude due to the dynamic structure of microbiological systems. The results of this study indicate that in the O horizon
microbiological impact does not seem to be as important in mineralization of Clorg to Clin
15
compared to formation of Clorg. It is possible that microorganisms in deeper soil layers controls the mineralization of Clorg, but further research is needed to establish this.
Even though variation was observed the abundance of 36Clorg appears to be stable over time in the O horizon, and may therefore explain the larger amounts of Clorg found in forest soil compared to Clin (Johansson et al 2003; Svensson 2007). This because Clorg would have to be transported to deeper soil layers before being dechlorinated. The results of this study could also strengthen earlier observations discussed by Öberg & Sandén (2005) that Clorg is
mineralized in the deeper layers. Another way to interpret the results is that no dechlorination occurs in forest soil but considering previous research this seems unlikely. For example Öberg & Sandén (2005) observed that Clin was double that of Clorg inthe soil leachate which
indicates a dechlorination of soil organic matter. Research by Bastviken (2006) using lysimeters also confirmed that Clorg was mineralized into Clin. The possibility that Clorg is mineralized in deeper soil layers are further strengthened by research that showed that Clorg is more common than Clin in soil (e.g., Johansson et al. 2003; Svensson 2007), compared to the observation by Öberg & Sanden (2005) that soil leachate had higher amounts of Clin than that of Clorg. A possible explanation of this dechlorination process of organic matter in deeper soil layers could be conditions that are favored by other types of microorganisms, such as an anoxic condition and/or a lower temperature.
If Clorg is stable in the O horizon, as indicated by the results, this could potentiallymean that degradation of organic matter would be slowed down when bound to chloride. This could cause an increased carbon sink in the soil when chloride enters the system. This theory of an increased carbon sink is supported by research showing a positive correlation between organic carbon and Clorg in forest soil (e.g., Johansson et al 2003).
Due to the events that occurred at the Fukushima Daiichi power plant these results can be of interest when studying the 36Cl cycle in the affected soil. If 36Clorg is stable in the organic horizon this could imply that the 36Clorg is not mineralized easily in the O horizon. This would increase the surface soil residence time for the 36Cl possibly present, before it is eventually leached to deeper layers in the soil. In deeper layers the emitted radiation would be less harmful to life above the O horizon. The soil could therefore work as a possible sink for 36
Clin.
To further verify the results from this study an analyses of the soil extracts could be
performed. This to determine the amount of 36Clin in the extracts and to be able to compare this to the variation observed in the results. For the future, research regarding the importance of soil depth for mineralization of Clorg could be of interest. New experiments using the same strategy as for this study could be performed on forest soil in different horizons, such as the A, E and B horizon. Because previous research has observed a microbiological dechlorination of PCB congeners in sediments at 12°C in an anoxic condition (e.g., Tiedje et al. 1993), future experiments regarding dechlorination of soil organic matter could focus on anoxic conditions in different temperatures.
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Conclusions
No evident dechlorination could be observed.
Samples that had high initial amounts of 36Clorg showed high variation.
17
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