Effect of different amounts of fruit peel-based activator combined with phosphate-solubilizing bacteria on enhancing phytoextraction of Cd from farmland soil by ryegrass

Highlights

• The combination of Orange and GLDA have the highest activation efficiency.

• The activation of cadmium by AP was obvious on the seventh day.

• AP-OG increased soil pH and EC, and changed nutrients.

• Ryegrass biomass and Cd uptake were optimum when AP-OG was 80 mL/kg.

• RDA analysis showed that TN had the greatest impact on the whole process.

Abstract

To improve the uptake of heavy metals by plants and increase the effectiveness of phytoextraction, chelating agents are employed to change the speciation of heavy metals in soil and increase their bioavailability. However, the effect of a single activator is limited. In recent years, compound activators have been applied widely to improve phytoextraction efficiency. In this study, a fruit peel-based activator (OG) was prepared, containing a mixture of orange peel extracts and tetrasodium glutamate diacetate (GLDA) (1.6% v/v) in a ratio of 1:1 (v/v). The pot experiment was used to investigate the effects of different amounts of OG combined with phosphate-solubilizing bacteria (Acinetobacter pitti, AP) on the extraction of Cd from farmland soil by ryegrass (Lolium perenne L). The results indicated that the addition of OG and AP increased the pH and EC of the soil and improved the content of nutrient elements in the soil. The optimal combination of the application rates of OG and AP improved the growth of ryegrass and enhanced the phytoextraction of Cd. Redundancy analysis (RDA) showed that total soil nitrogen had the greatest influence on phytoextraction, with a contribution rate of 85.3%, followed by pH, with a contribution rate of 7.7%. Total nitrogen, pH, available phosphorus, alkaline nitrogen, and total organic matter were correlated positively with plant Cd, soil Cd decrease ratio, and the bioaccumulation factor but negatively with total Cd and available Cd. Based on the findings of this study, it is feasible to apply the fruit peel-based activator (amended with GLDA) and phosphate-solubilizing bacteria to enhance phytoextraction of Cd, which will provide a valuable reference for the treatment of heavy metal-contaminated soils and the reutilization of fruit peel waste. When applying the compound activator, it is recommended to consider the influence of the additional amount of compound activator on the extraction efficiency.

Introduction

In recent years, the pollution of farmland soil by Cd has seriously affected the quality and yield of rice, as well as ecological safety (Li et al., 2014; Luo et al., 2017). Farmland with excessive Cd cannot be used for growing rice or other food, which will lead to enormous economic losses. Reducing heavy metal contamination in farmland soil is essential for this step. The main methods to remediate Cd pollution in farmland include excavation, backfilling, optimized water management, and phytoextraction to reduce the uptake of Cd by crop (Guo et al., 2021; Wielgusz et al., 2022; Yuan et al., 2019). Among these methods, phytoextraction technology has several advantages, such as low cost, being benign for the environment, and being sustainable, and it has been studied widely (He et al., 2020; Mahar et al., 2016). In the present study, we used phytoextraction technology to lower the concentration of Cd in farmland soil during its fallow period, which can minimize financial losses and control soil pollution (Mayerová et al., 2017).

Plant species that hyperaccumulate metals are the basis for phytoextraction. However, hyperaccumulator biomass is generally relatively low due to the relatively harsh growing conditions (Yaashikaa et al., 2022). Accordingly, it is urgent to select plants with a strong accumulation capacity of heavy metals and large biomass (Cobbett, 2000). Perennial ryegrass grows fast and has been used in the phytoextraction of Cd from contaminated soil (Zhang et al., 2019a; Zhang et al., 2019c). However, the relatively low metal-enrichment factor limited its remediation efficiency (Diarra et al., 2021). Therefore, chelating agents were added to change the form of heavy metals and stimulate the efficiency of ryegrass, taking them up from the soil. At present, the most common chelating agents are ethylene diamine tetraacetate (EDTA), tetrasodium glutamate diacetate (GLDA), and nitrilotriacetic acid (NTA) (Chauhan et al., 2015; Guo et al., 2018; Ferraro et al., 2016; Tananonchai et al., 2019). However, the addition of EDTA causes secondary soil pollution, which carries potential risks (Zhang et al., 2019b). Wang et al. (2018) found that GLDA had similar effects compared with EDTA, and it is an environmentally benign chelating agent. After remediation, soil enzyme activity significantly increased (by 5%–94%), accompanied by an increase in total microbial biomass.

In addition, low molecular weight organic acids (LMWOA) are also favored by many researchers because they can change the form of heavy metals and improve their bioavailability (Dinh et al., 2017; Han et al., 2021; Ma et al., 2020). Ma et al. (2020) showed that organic acids could improve soil fertility, promote the cycling of soil nutrients, boost plant growth, and increase the availability of Cd to a certain extent. Some researchers combined LMWOA with GLDA to prepare a composite activator and found a significant change in soil properties and the enhanced growth of pakchoi (Guo et al., 2018). Large amounts of fruit peel waste are produced every year in the world, and this fruit peel waste contains lots of organic acids, sugars, cellulose and other substances (Ersus and Cam, 2007; Hernández et al., 2009). Although many studies have been devoted to the recycling of peel waste, most of that researches ignored the potential of organic acids in fruit peel waste (Fu et al., 2017). Therefore, it is logical to combine GLDA with fruit peel waste to prepare a kind of fruit peel-based activator that can not only stimulate the phytoextraction of Cd but also achieve the reutilization of the fruit peel waste.

During the phytoextraction process, the role of microorganisms in soil cannot be ignored. Numerous studies have found that phosphate-solubilizing bacteria (PSB) typically increase the availability of soil phosphorus and have a similar effect on heavy metals in the soil as well (Jeong et al., 2013; Xiao et al., 2021). Moreover, PSB can release indoleacetic acid and ACC deaminase in soil, which is beneficial for plant growth and Cd extraction (Ma et al., 2019). Based on the positive roles of chelating agents and PSB, the combined application of OG and phosphate-solubilizing bacteria (Acinetobacter pitti, AP) is a promising way to enhance phytoextraction efficiency. However, there is little research on the interaction between chelating agents and PSB in phytoextraction.

In the present study, a fruit peel-based activator (OG) was prepared by combining suitable fruit peel waste and the biodegradable chelating agent GLDA. Pot experiments were conducted to explore the efficiency of ryegrass in extracting Cd after adding different amounts of OG and AP. The content of Cd in the soil and the ryegrass, as well as the physical and chemical properties of the soil were analyzed. Fresh weight, the length of the overground part, and the chlorophyll content of ryegrass were determined. This study contributed to the theory and practice of remediating Cd-contaminated soil.