Effects of low-level radioactive soil contamination and sterilization on the degradation of radiolabeled wheat straw

https://doi.org/10.1016/j.jenvrad.2011.12.018Get rights and content

Abstract

After the explosion of reactor 4 in the nuclear power plant near Chernobyl, huge agricultural areas became contaminated with radionuclides. In this study, we want to elucidate whether 137Cs and 90Sr affect microorganisms and their community structure and functions in agricultural soil. For this purpose, the mineralization of radiolabeled wheat straw was examined in lab-scale microcosms. Native soils and autoclaved and reinoculated soils were incubated for 70 days at 20 °C. After incubation, the microbial community structure was compared via 16S and 18S rDNA denaturing gradient gel electrophoresis (DGGE). The radioactive contamination with 137Cs and 90Sr was found to have little effect on community structure and no effect on the straw mineralization. The autoclaving and reinoculation of soil had a strong influence on the mineralization and the community structure. Additionally we analyzed the effect of soil treatment on mineralization and community composition. It can be concluded that other environmental factors (such as changing content of dissolved organic carbon) are much stronger regulating factors in the mineralization of wheat straw and that low-level radiation only plays a minor role.

Highlights

► We observed the impact of contamination with Cs-137 and Sr-90 on soil functions. ► Microbial community was altered slightly. ► Mineralization of wheat straw was not affected. ► Microbes growing on applied straw compete for nutrients with soil microbes.

Introduction

The explosion of reactor 4 in Chernobyl on 26 April 1986 and recent activities in Japan have shown that the usage of nuclear power involves large-scale risks. While the ecological implications for Europe have not been so immense to date, the landscape around the reactors became highly contaminated. Cultivating the land and living in these areas may cause health risks to the human population (Dederichs et al., 2009).

Within the framework of post-Chernobyl programs, the pathways of radionuclides from soil to plants and into the human food chain were thoroughly examined (Bilo et al., 1993, Nisbet and Woodman, 2000). However, the effects of radioactive contamination on the soil micro-flora and its functions still remain widely unknown. Numerous studies observed the radiation effects on microorganisms but most of these studies used high radiation doses of up to several kGy or observed the effects on single microbial strains (McNamara et al., 2007, Pitonzo et al., 1999b). Due to these experiments, which usually involved sterilization of the soils, the lethal doses for several isolated strains are well known. Gamma-irradiation doses around 10 kGy are referred to as sub-sterilizing doses. This level of radiation has a lethal effect on most of the actinomycetes, fungi and invertebrates. The majority of bacteria is eliminated at 20 kGy and doses higher than 70 kGy kill radio-resistant bacteria (McNamara et al., 2003). Only a few studies have observed microbial behavior under low-level irradiation, e.g. Gochenaur and Woodwell (1974) and Jones et al. (2004). To our knowledge, studies conducted with contamination such as in the Chernobyl zone have not been performed to date. The radioactive contamination in agricultural fields around Chernobyl ranging from 400 to 20,000 kBq m−2 may play an important role from an ecological point of view, since such levels of contamination are more likely to appear and they affect larger areas than contaminations in the range of several kGy. In the case of Chernobyl, more than 300 km² were contaminated with 90Sr at a level higher than 400 kBq m−2 (Kashparov et al., 2001). Large parts of these areas had been used agriculturally, and today smaller parts are in use again by remigrated people (Dederichs et al., 2009).

Microbial communities have the capability to influence various ecosystem properties. Microorganisms are responsible for organic matter mineralization, humus formation in soil and the recycling of nutrients. Due to the capability of soil microbes to degrade numerous pollutants, they furthermore play an important role in protecting groundwater and improving soil quality. Since remediation of such huge areas is extremely expensive and the natural migration of several radionuclides is low (Konopleva et al., 2009, Thiry and Myttenaere, 1993, Zhu and Shaw, 2000), it is important to know how radionuclide contamination affects the soil micro-flora, as this in turn impacts directly on the soil quality.

In our study, we focused on the effects of soil contamination with 137Cs and 90Sr on the bacterial and fungal community structure and the degradation of wheat straw. The amount of radioactive contamination in soil was based on contamination in the zone around Chernobyl. The formal threshold for the exclusion zone is 1.48 MBq m² (40 Ci km²) for 137Cs. Transferred to soil mass this is 11.4 Bq g−1 (assumed parameters: soil density: 1.3 g cm−3, soil depth of homogeneous radionuclide distribution: 0.1 m). The highest values for 137Cs in the 10 km zone around the reactor in Chernobyl are around 40 MBq m−2, which conforms to 307 Bq g−1 (personal communication with Mr. Valery Kashparov, 08.12.2009) when equal soil parameters were assumed. Similar values up to 629 Bq g−1 were reported by Romanovskaya et al. (1998). The artificial contamination in the microcosms set up for our experiment began at 692 Bq g−1 and ranged up to 15,033 Bq g−1.

The objectives of this study were: i) to observe the impact of low-level radioactive contamination on the degradation of 14C-labeled wheat straw in soil and ii) to evaluate potential microbial community alteration caused by the applied radioactivity.

Section snippets

Experimental soil

The study was performed with an orthic luvisol field soil from Merzenhausen, Germany. The underlying sediments were quaternary sediments, mostly consisting of fluvial deposits from the Rhine/Maas River and the RurRiver system, covered by Pleistocene and Holocene eolian sediments. The used soil from the ploughing layer (0–0.35 m) consisted of 3% sand, 79% silt, and 18% clay. The pH was 7.0 (measured in 0.01 M CaCl2, the cation exchange capacity 12.0 cmol kg−1 (determined from exchange with a NH4

Degradation of 14C-labeled wheat straw

The mineralization was observed by measuring the 14CO2 emitted from the 14C-labeled wheat straw. As presented in Fig. 1a and b, the similarity of the curves from soils treated with 137Cs and 90Sr is remarkable. Almost no differences were observed between the two differently treated soils. The lower and higher contaminated soils, as well as the control soils without the application of radionuclides exhibited equal mineralization rates. However, significant differences in the mineralization

Radiation effects

Prior to incubation, the microcosms were contaminated artificially with various concentrations of caesium nitrate and strontium nitrate. The cumulative doses over 70 days were in the range of 0.2–5.5 Gy, respectively (Table 1). The radioactivity was applied in accordance with hotspots occurring in 10 km zone around the reactor in Chernobyl. Compared to other experiments (Davis et al., 1956, Eriksen and Emborg, 1978, McNamara et al., 2003, McNamara et al., 2007), the applied radioactivity was

Conclusions

We applied soil microcosms with various concentrations of 137Cs and 90Sr according to the radioactivity of these radionuclides in the 10 km zone around Chernobyl. The aim was to observe the impact of the radioactive contamination on the degradation of 14C-labeled wheat straw and on the microbial community structure. Slight population shifts were detected in the 16S and 18S rDNA DGGE gels; but the wheat straw degradation was not affected. When soil samples were sterilized and reinoculated with

Acknowledgements

This work was funded by the German Federal Ministry of Education and Research (BMBF) under contract number 02NUK002E, KVSF. We would like to thank Eberhard Kuemmerle for calculating the dose rates. Furthermore, we would like to thank Sascha Sokolowsky for preparing the DGGE gels, Sirgit Kummer and Werner Mittelstaedt for their help in the laboratory, Wolfgang Tappe and Nicolai Jablonowski for fruitful discussions and Janet Carter-Sigglow and Hazel Burlet for linguistic revision.

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