Using elevated CO2 to increase the biomass of a Sorghum vulgare × Sorghum vulgare var. sudanense hybrid and Trifolium pratense L. and to trigger hyperaccumulation of cesium

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Abstract

The most important challenge to use phytoremediation is how to improve its efficiency by increasing the accumulation of metals in plants, or by improving key plant biological traits that should enhance metal uptake. In this paper, we used open-top chambers to investigate the effects of elevated CO2 (860 μL L−1) on biomass and Cs uptake by a Sorghum vulgare × Sorghum vulgare var. sudanense hybrid and Trifolium pratense L. growing on soils spiked with various levels of cesium (0, 300, 1500 and 3000 mg Cs kg−1). The results showed that elevated CO2 not only increased aboveground biomass of the Sorghum and Trifolium species by 32–111%, and by 8–11%, respectively, compared to the ambient CO2 treatment, but also caused more accumulation of Cs by Sorghum species (up to 73%) than Trifolium species (up to 43%). It was speculated that the increase in biomass and the improvement in Cs accumulation ability at elevated CO2 could be related to lowered soil pH values, and changes in number and kind of microorganisms in the rhizospheres of the two tested species. This is the first report of a link among elevated CO2, increased biomass and hyperaccumulation of Cs by Sorghum and Trifolium species.

Introduction

With the development of the nuclear industry, soil contamination with radionuclides, especially with strontium and cesium, is receiving more attention [1], [2], [3]. A number of technologies have been developed for treating radionuclide contaminated soils, including excavation, soil washing, leaching with chelating agents, flocculation and reverse osmosis–ultrafiltration [4], [5]. These methods are not suitable, however, for treating large-scale contamination of soils with low-doses of radionuclides, due to their high-cost, and difficulty of implementation at the sites. By comparison, phytoremediation technologies have several advantages over the above mentioned more conventional approaches, such as in situ, environmental friendly, cost-effective, and aesthetically pleasing. Phytoremediation also has some disadvantages like toxicity of pollutants to the plants, risks posed to consumers of plants, taking a long time, dependence on season and on root systems of remediating plants. Disposal of contaminated parts of the plants after harvest is an unsolved problem [6], [7], especially for remedial plant materials containing radioactive isotopes. Despite these, more attention has been given to fundamental aspects of this technology in recent years, including screening of more hyperaccumulator plants (being capable of accumulating potentially phytotoxic elements to concentrations more than 100 times than those found in nonaccumulators [8], [9]), plant growth enhancing mechanisms, structural and functional changes in rhizosphere microbial communities in contaminated environments, as well as plant–bacteria interactions during phytoremediation [10], [11]. To increase the feasibility of this technology, we need to improve its efficiency by increasing the accumulation of radionuclides in plants, or by modifying the plant's biological traits (growth rate, the growth cycle, etc.) [12]. Our previous studies showed that elevated CO2 not only increased Indian mustard (Brassica juncea L. Czern.) and sunflower (Helianthus annuus L.) biomass, but also triggered hyperaccumulation of copper by the two plants [13]. This caused us to speculate that CO2 enrichment might increase the biomass of plants growing on radionuclide contaminated soils and cause increased accumulation of the radionuclide contaminants from the soils into the plants, in a similar way. However, to our knowledge, there has been little research conducted in this area.

Ongoing combustion of fossil fuels leads to increases in the atmospheric carbon dioxide concentration, contributing to global warming [14]. It is known that elevated CO2 enhances plant growth on non-contaminated soils in terms of plant biomass [15], [16], [17], water and nutrient use efficiency [18], [19], photosynthesis rate and intensity [20], [21], [22], and rhizosphere microecological environment characteristics [23], [24]. A survey of the literature indicates that few studies have investigated the effect of elevated carbon dioxide on plant uptake of pollutants from contaminated environments, especially environments contaminated with radionuclides [25]. Since the behavior of radionuclides in the environment follows that of stable elements and plants do not discriminate between stable and radioactive Cs isotopes, the understanding of the fate of stable Cs in the environment could guide us to understand the behavior of radioactive isotopes [26], [27], [28], and allow us to predict the long-term transfer of radioactive isotopes in plant–soil system [29]. In this paper, we used open-top chambers (OTC) to investigate the effects of elevated CO2 (860 μL L−1) on the growth and development of a Sorghum vulgare × Sorghum vulgare var. sudanense hybrid and Trifolium pratense, their associated uptake of stable Cs, and important rhizosphere characteristics of the two tested species, including microorganism populations and pH values. The main objective of the current work was to assess the possibility of using elevated CO2 as a gas fertilizer to increase the biomass of the S. vulgare × S. vulgare var. sudanense hybrid and T. pratense, and cause more accumulation of Cs by the two species, making them more effective as phytoremediation agents. To our knowledge, this is the first report on the utilization of elevated CO2 as a gas fertilizer to induce phytoextraction of cesium from contaminated soils.

Section snippets

Tested plant species

S. vulgare × S. vulgare var. sudanense hybrids provide high-quality forage with high biomass, are tolerant to drought and heat, and have potential for phytoremediation because they are easy to cultivate and produce large amounts of biomass. T. pratense is also high-quality forage grown widely in northern and central parts of China. These species were selected for this study because the Sorghum species is a C4 plant and Trifolium species is a C3 plant. Seeds were obtained from Beijing Feng-Nen

Plant growth

Fig. 1, Fig. 2 show the height and dry weight of the shoots and roots of the S. vulgare × S. vulgare var. sudanense hybrid and T. pratense growing on soils contaminated with various levels of Cs at ambient and elevated CO2. The height and shoot and root dry weight of the Sorghum and Trifolium species generally decreased with increasing Cs-spiked concentrations at either ambient or elevated CO2, while, after germination, T. pratense seedlings did not grow at all on soils spiked with 3000 mg Cs kg−1,

Effects of elevated CO2 on plant height and biomass

Previous studies revealed the effects of elevated CO2 on the growth and development of plants [18], [19], [21], [32]. The most noticeable effect is that CO2 enrichment increases plant biomass and yields. Poorter [33] reported an average biomass increase of 41% for C3 plants, 22% for C4 plants and 15% for CAM species, so that the growth stimulation for C3 species was substantially larger than for C4 plants. After investigating the effect of elevated CO2 on the short grass prairie, Morgan et al.

Conclusions

The results obtained in our study suggest that elevated CO2 not only resulted in an increase of aboveground biomass of the S. vulgare × S. vulgare var. sudanense hybrid and T. pratense by 32–111%, and by 8–11%, respectively, compared to the ambient CO2, but also enhanced its Cs resistance when the growth was conducted in Cs artificially contaminated soil and under elevated CO2. The effect of elevated CO2 on Cs uptake by the Sorghum and Trifolium species was evident with the maximum increase

Acknowledgements

The work was financially supported from the National Science Foundation of China (Grant No. 40773078). The authors acknowledge the partial funding from Central Public Research Institutes Basic Funds for Research and Development. We also want to thank Dr. Paul N. Williams from University of Aberdeen, U.K. for critically reviewing the initial manuscript, and the anonymous reviewers for their constructive suggestions.

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