Phytoremediation of cadmium contaminated soils by Amaranthus Hypochondriacus L.: The effects of soil properties highlighting cation exchange capacity
Introduction
Heavy metals contamination in agricultural soils raised global concerns due to the nonnegligible risk on crop safety and human health (Järup and Åkesson, 2009; Mao et al., 2019; Zhuang et al., 2009). A recent nationwide survey of soil contamination in China revealed that 19.4% of the investigated agricultural soils were contaminated by heavy metals, with cadmium (Cd) contamination ranking first (7%) (The Ministry of Environmental Protection and The Ministry of Land and Resources, 2014). In southern China, especially north Guangdong province, heavy metal contamination in farmland was worse than the national average by reason of the staggered distribution of farmlands and nonferrous metal mines. Cd can easily get into the food chain and thereby threaten public health due to its relatively high mobility and toxicity (Zhuang et al., 2014). To lower its environmental risk, there is an urgent need to remedy heavy metal polluted farmland (Bai et al., 2019; Wang et al., 2014; Yin et al., 2016).
Phytoremediation is a cost-effective and eco-friendly method to restore Cd-contaminated soils, which refers to using Cd (hyper)accumulators to take up Cd from soil and thereby reduce the content of Cd in soils. Several plants have been identified as Cd hyperaccumulators for the remediation of Cd-contaminated soils (Li et al., 2018; Sun et al., 2008; Yang et al., 2004). These plants can accumulate a relatively high concentration of Cd in their tissues, which was important to ensure the efficiency of phytoremediation. However, it should be noted that the metal uptake capacity of a (hyper)accumulator could be varied at different soils, indicating that some soil properties may play an important role in regulating the efficiency of phytoremediation (Rosenfeld et al., 2018; Wang et al., 2021; Wei et al., 2006). Therefore, it is important to ascertain the primary soil properties affecting Cd accumulation of a given (hyper)accumulator in various Cd-contaminated soils for efficient phytoremediation.
The total amount of Cd accumulation in plants, as a vital parameter to evaluate the efficiency of phytoremediation, is determined by both plant's Cd concentration and biomass. There might exist a trade-off between plant Cd uptake and plant growth, since excessive Cd in plants would be harmful to plant growth. Therefore, a good coordinate relation between plant growth and plant Cd uptake could be beneficial to the success of phytoextraction. It was reported that both plant growth and Cd uptake were closely related to soil properties, like soil pH (Ali et al., 2020; Kindtler et al., 2019), CEC (Ata-Ul-Karim et al., 2020; Hinesly et al., 1982), organic matter (He and Singh, 1993; Zhao et al., 2020) and available Cd (Wu et al., 2018; Xiao et al., 2018). In southern China, prolonged weathering and leaching under humid tropical and subtropical climate together with significant soil acidification exacerbated the loss of soil basic cations (mainly Ca and Mg) in widespread agricultural soils (Duchesne and Houle, 2006; Guo et al., 2010; Shi et al., 2010). Hence, relatively low CEC status featured importantly on the characteristic of the typical agricultural soil in southern China (Huang et al., 2019; Wei et al., 2020). However, whether and how the soil CEC level regulates the growth and Cd uptake of (hyper)accumulating plants in Cd-contaminated agricultural soil is not well understood.
Grain amaranth (Amaranthus Hypochondriacus L.) was identified as a capable Cd accumulator in our previous study (Li et al., 2010). With the merits of fast-growing, high biomass, easy to cultivate, and strong environmental adaptability (Li et al., 2012), it has been extensively studied to remedy Cd-contaminated agricultural soils (Li et al., 2019; Sun et al., 2020; Tai et al., 2018; Yu et al., 2020). However, owing to the difference in soil properties, the performances of grain amaranth in metal uptake capacity varied widely in previous studies. It was demonstrated that phytoremediation by grain amaranth could be affected by soil pH, soil NPK nutrients, and Cd bioavailability (Huang et al., 2020; Li et al., 2013; Wang et al., 2019; Xie et al., 2020). For instance, Huang et al. (2020) found that grain amaranth accumulated more Cd in acidic soils than in alkaline soils. Li et al. (2013) reported that the application of NPK compound fertilizer increased soil nutrients and substantially promoted grain amaranth growth, resulting in a large increment of Cd accumulation. Wang et al. (2019) and Yu et al. (2020) added chelating agents or low molecular weight organic acids in soils to improve soil Cd availability, which effectively enhanced the phytoremediation efficiency of grain amaranth. Regrettably, little is known about how the phytoremediation efficiency of grain amaranth changes with different soil CEC statuses. In this study, grain amaranth was grown in six long-term Cd-contaminated agricultural soils collected from southern China, with the intention to compare their phytoremediation efficiency at soils with different properties. Therefore, the objectives of this study were to (1) evaluate the suitability of grain amaranth to restore a range of long-term Cd-contaminated agricultural soils in southern China; (2) explore how soil properties, especially soil CEC, affect the growth and Cd accumulation of grain amaranth. The results of this study could be helpful to inform effective use of grain amaranth for phytoextracting Cd from soils that were Cd-contaminated with different CEC levels.
Section snippets
Soil properties
Six experimental soils were collected from the surface layer (0–20 cm) of the farmland in 4 districts or counties (Wengyuan, Qujiang, Lechang, Renhua) of Shaoguan, Guangdong province, China (Fig. S1). Each of the soils was air-dried, crushed, thoroughly mixed, and pass through a 0.5 cm sieve for pot experiment, and a 2 mm and a 0.15 mm sieve, respectively, for soil property analysis.
The primary properties and Cd contamination levels of the six experimental soils were presented in Table S1.
Plant growth
Grain amaranth grown in Soil 3 showed the highest biomass production, followed by those on Soil 1 and 6 (Fig. 1a). For medium Cd-contaminated soils, the biomass of grain amaranth grown in Soil 2 was lower than those on Soil 3 by 57% (p < 0.05). For high Cd-contaminated soils, grain amaranth grown in Soil 4 and 5 only yielded 7.09 and 5.67 g pot−1 respectively, significantly lower than those on Soil 6 and other soils (p < 0.05).
Similar to biomass production, grain amaranth grown in Soil 1, 3,
Discordance between plant Cd accumulation and soil Cd level
It is generally agreed that the primary factor affecting plant Cd accumulation was the level of soil Cd contamination. Many studies showed that Cd accumulation in plants was positively related to soil Cd level when the plant was grown in the soil artificially polluted along a concentration gradient (Nie et al., 2016; Wu et al., 2018). However, in the present study, plant Cd accumulation was discordant with soil Cd level. For example, Soil 3 and 6 had higher available Cd than Soil 2 (0.35 and
Conclusion
The growth and Cd accumulation of grain amaranth were affected by both the level of soil Cd contamination and soil CEC. For the high-biomass Cd-accumulating plants like grain amaranth, ensuring sufficient plant biomass production is required for efficient phytoremediation. Our finding suggested that low CEC of the soils is a substantial limiting factor affecting the phytoremediation efficiency of grain amaranth. The impaired plant photosynthetic capacity and Cd detoxification ability caused by
Credit author statement
Xiaoying Cui: Investigation, Formal analysis, Writing - original draft. Peng Mao: Formal analysis, Writing - review & editing. Shuo Sun: Investigation. Rong Huang: Writing - review & editing. Yingxu Fan: Investigation. Yongxing Li: Investigation. Yingwen Li: Investigation. Ping Zhuang: Writing - review & editing. Zhian Li: Methodology, Writing - review & editing, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The research is financially supported by R&D program of Bureau of Science and Information Technology of Guangzhou Municipality (201903010022), Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) (GML2019ZD0408), R & D program of Guangdong Provincial Department of Science and Technology (2018B030324003), National Natural Science Foundation of China (31670513).
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These authors contributed equally to this work.