Contamination of agricultural soils by heavy metals has long been a threat to soil quality, food security and human health (Wang et al., 2019; Suhani et al., 2021). Cadmium (Cd) is considered to one of the most toxic heavy metals (Genchi et al., 2020). Cd-contaminated farmland in China accounts for 20% of the total agricultural area (Rafiq et al., 2014). Various remediation techniques and enhancement strategies have been proposed and tested to address this challenging issue (Boparai et al., 2011; Vasudevan et al., 2011; An et al, 2015; Tang et al., 2016).
Phytoremediation is a cost-effective and environmentally friendly technique to remove heavy metals from soil using plants, especially hyperaccumulator species (Raskin et al., 1997; Mahar et al., 2016). To improve the efficiency of phytoextraction, chemical additives and biological agents have been used for phytoremediation practices of Cd enriched soils (He et al., 2015; Guo et al., 2017). However, application of different fertilizers to enhance cadmium phytoextraction has become an emerging common approach (Cheng et al., 2017; Yang et al., 2019).
The effects of nitrogen fertilizers on Cd uptake by plant species have been studied (He et al., 2015; Yang et al., 2019; Wu et al., 2021). It has been reported either from hydroponic or pot experiments that the supply of nitrogen fertilizers, including NH4+-N and/or NO3-N, was able to increase Cd accumulation (Cd content/plant) in frequently studied Cd-hyperaccumulator plants, such as Sedum alfredii H. (Zhu et al., 2010), Sedum plumbizincicola (Hu et al., 2013), Solanum nigrum (Yang et al., 2019; Yang et al., 2022), Rorippa globosa (Turcz.) Thell (Wei et al., 2015), Bidens pilosa (Dai et al., 2020). The efficiency of Cd extraction from soils depends on its concentration in plant tissues and biomass (Jacobs et al., 2019; Cui et al., 2021). However, for the Cd-hyperaccumulators studied, there was no consistent relationship between Cd concentrations (mg/kg DW) and Cd accumulations in their shoots and roots. This phenomenon may be attributed to the dilution effect resulted from an increased biomass production (Olsen, 1983; Rodríguez-Ortíz et al., 2006).
Nevertheless, some studies have shown a simultaneous increase in terms of both Cd accumulation and Cd concentration in the Cd hyperaccumulator Phytolacca americana (Wang et al., 2019), the halophytic plant Carpobrotus rossii (Cheng et al., 2017), the cash crop tobacco (Rodríguez-Ortíz et al., 2006), the vegetable lettuce (Florijn et al., 1992), the forage grass Italian ryegrass (Ji et al., 2020) and the ornamental plant Tagetes patula (Ye et al., 2019), which were supplied with nitrogen fertilizers.
Organic fertilizers not only serve as a good source of plant nutrient in agricultural practices (Hamid et al., 2019a; Okoroafor et al., 2022), but have also been increasingly used for the purpose of phytoremediation in Cd-enriched soils (Hamid et al., 2020). A great deal of previous research into organic fertilizers immobilizing soil Cd has been reviewed by Hamid et al. (2020). In the case of Cd hyperaccumulator plants, Yang et al. (2019) reported that Solanum nigrum showed a significant decrease in Cd concentrations in roots and shoots when supplemented with animal manure and commercial organic fertilizer. On the other hand, a significant increase in Cd concentration was observed in roots of Sedum alfredii when applied with pig manure compost, but no effect was observed in shoots (Xiao et al., 2017). In another pot experiment, Addai et al. (2023) also showed that poultry litter compost increased Cd and Pb uptake in lettuce (Lactuca sativa L). In general, the effect of organic fertilizers on Cd mobility and bioavailability in soil may depend on the Cd adsorption, pH value and microbial reduction processes in the soil where plants grow (Chen and Cutright, 2002; Tlustoš et al., 2006; Elouear et al., 2016; Yuan et al., 2021).
Phytolacca americana L. (pokeweed) is considered as a promising Cd (hyper)accumulator plant (Yan et al., 2009; Liu et al., 2010; Liu et al 2015). It is a semi-succulent perennial herbaceous plant characterized by high aboveground biomass, rapid growth, favorable adaptability and environmental benignity (Li et al., 2018; Liu 2015; Zhao et al., 2011) and therefore has a great potential for phytoextraction from Cd-contaminated farmland and rare-earth-element enriched mine sites (Liu et al., 2020).
Several pot or hydroponic studies (Yan et al., 2009; Liu et al., 2010; Wang et al., 2019) have demonstrated that pokeweed has a strong capacity to take up Cd from soil, with shoot Cd concentration reaching up to 637 mg·kg− 1 and bio-concentration factor (BCF) and translocation factor (TF) ranging from 4 to 10 and 1 to 3, respectively. Furthermore, the plant grown in the natural habitat, where soil Cd concentration was 1,083 mg·kg− 1, had its leaf Cd concentration of up to 402 mg·kg− 1 (Liu et al., 2010).
Our previous pot experiment (Wang et al., 2019) showed that the application of ammonium sulfate and urea as nitrogen sources can all significantly increase the aboveground Cd concentration of pokeweed. However, litter information on Cd uptake and accumulation by the Cd-stressed plant under farmland conditions is lacking, and it appeared that no field experiment had been conducted to investigate the effect of fertilizer application on Cd uptake from farmland soils.
The objectives of this study were: 1) to determine the Cd uptake by pokeweed in Cd-contaminated farmland; 2) to examine the enhancement of Cd accumulation using commercial nitrogenous and organic fertilizers, and 3) to evaluate the phytoextraction of the plant as a phytoremediation tool in Cd-contaminated farmland.