Interactive effects of bacteria-loaded biochar on the physiological responses of Brassica rapa var. chinensis in the Pb and Zn contaminated soil

Abstract

Lead (Pb) and zinc (Zn) are widely recognized as common environmental contaminants, contributing to soil degradation and posing risks to environmental health. Combining functional carbon-based materials with microorganisms has been considered as an effective and environmentally friendly strategy for remediating Pb/Zn-contaminated soil. However, there is still a lack of understanding the connection between heavy metal immobilization and plant responses, which hampers practical applications. Here, a 90-day pot experiment was conducted to investigate the integrated effects of biochar (WS700) and microorganisms including inorganic phosphate-solubilizing bacteria (IPSB) and sulfate reducing bacteria (SRB) on Pb and Zn synchronous immobilization and the physiological responses of Brassica rapa var. chinensis (Brassica). Compared with CK, bacteria-loaded biochar treatment declined the exchangeable Pb and Zn fraction by 94.69%−98.37% and 94.55%−99.52%, while increasing the residual state Pb and Zn by 75.50%−208.58% and 96.71%−110.85%, respectively. Three amendments enhanced Brassica growth by improving total chlorophyll content and superoxide dismutase (SOD) and peroxidase (POD) activities. The bacteria-loaded biochar treatment effectively regulated stomatal conductance and reduced intercellular CO2 concentration. Moreover, compared with CK, three amendments reduced MDA content by 28.84%, 28.30% and 41.60%, respectively, under the high concentration of Pb and Zn. The findings demonstrated the significant role of bacterial-biochar consortia in immobilizing Pb and Zn and mitigating Pb and Zn-induced stress in plants by regulating photosynthetic characteristics and antioxidant enzyme activities.

1. Introduction

Lead (Pb) and zinc (Zn) are prevalent main contaminants in soil, due to intensive cultivation and industrial activities, such as high input of Zn-containing chemical fertilizers and pesticides, wastewater irrigation, uses of Pb-containing paints and gasoline, chemical manufacturing, mining, and smelting [1 −3]. Although Zn is plant essential micronutrient, acting as structural and enzymatic activation, oversupply of it could cause toxicity. Pb and Zn result in soil degradation by altering the normal ecosystem structure and functioning and inducing toxicity in vegetation and microbial community [1, 4, 5]. Pb and Zn could result in oxidative stress, toxicity and surplus accumulation of reactive oxygen species (ROS) in plants, which can damage DNA and RNA, inhibit synthesis and activities of enzyme and protein, and destroy antioxidant enzymes system in plant cells [6]. Green vegetables grown in Pb and Zn contaminated soils are eaten by humans through the food chain, resulting in threats to human health, such as lung cancer, abdominal pain, kidney failure and stomach trouble [7]. Thus, effective, economical and eco-friendly immobilization strategies are urgently required for Pb and Zn contamination.

Biochar has been considered as a promising inoculant carrier for beneficial microorganisms, due to its large surface aera, highly porous structure, and abundant nutrients and surface functional groups [8, 9]. Our previous work presented that inorganic phosphate-solubilizing bacteria (IPSB) immobilized on biochar could successfully transform labile Pb to stable state, reducing Pb toxicity in soil [10]. In addition, immobilized sulfate reducing bacteria (SRB) on biochar had the largest passivate performance, reducing the bioavailable Zn fraction by 22.2% in the Zn concentration of 1500 mg/kg in soil [11]. It was reported that IPSB encapsulated by sodium alginate containing Ca3(PO4)2 could reinforce Pb passivation effects in sediments by ZHANG et al [12], and the removal efficiencies for 100 mg/L initial concentration of Cu2+ and Zn2+ were 98.17% and 99.67%, respectively, in SRB loaded on Cu and Fe particles after 48 h by ZHOU et al [13]. However, how amendments of complex bacteria loaded biochar alleviated Pb and Zn stress on plants still remains not clear. Moreover, little is known about interactive effects of bacteria cons...