New approach strategy for heavy metals immobilization and microbiome structure long-term industrially contaminated soils

Phosphate-modified biochar attenuates cadmium availability in contaminated soil and reduces its transfer to tomato fruits

Cadmium (Cd) is a toxic heavy metal for plant growth and development. Biochar has been proposed as an effective approach to increase immobilized cadmium fractions in soil and reduce its availability and accumulation by plants. In the present study, the enrichment of biochar with water-soluble polyphosphate (PLB) was tested to assess its ability to modify cadmium immobilization capacities in soils compared with standard biochar (SB) and orthophosphate enriched biochar (OLB) under Cd stress. In a pot experiment, the impact of these three biochar forms (SB, PLB, and OLB) on soil properties, soil Cd fractions, photosynthesis, tomato plant growth, yield, fruit quality, and nutrient uptake was assessed under two levels of Cd (0, and 5 mg kg-1). The obtained results showed that the enrichment of biochar with both forms of phosphorus fertilizers (orthophosphate and polyphosphate) significantly and positively impacted P and K contents in the final enriched biochar compared to SB. Results demonstrate that applying PLB under cadmium stress significantly increased phosphorus availability in soil (+32 %) and reduced Cd exchangeable fraction compared to the SB and OLB treatments. These findings suggest that, under cadmium stress, the slow-release properties as well as the organometallic chelation and precipitation capacities of the PLB can help in improving phosphorus, calcium, and iron uptake by plants and reducing Cd uptake and translocation to the shoot tissues, which resulted in significant enhancement of plant photosynthesis efficiency (PIabs: +144 %), shoot dry biomass (+117 %), and fruit yield (308 %) and quality, compared to standard biochar. Therefore, the enrichment of biochar with polyphosphate fertilizers could be proposed as an efficient strategy to enhance plant growth, physiology, and nutrition and mitigate cadmium stress and toxicity in contaminated soils.

Influence of freeze-thaw cycles on change of arsenic speciation and toxic effects to bacteria in paddy soil

Global warming increases the freeze-thaw (FT) cycles, however, the impact of increased FT cycles on the environmental behavior of arsenic (As) in soils and the toxic effect of As to microorganisms are still unknown. Herein, the influence of FT cycles on As forms, available As, and microbial community structure in paddy soils was investigated. After 60 FT cycles, the content of exchangeable As and residual state As decreased by 1.77% and 14.18%, respectively, while the carbonate-bound As, iron-manganese oxide-bound As, and organic-bound As increased by 4.53%, 6.5%, and 5.35%, respectively. The available As in soil and As(III) in soil water increased by 6.53 mg/kg and 38.9 μg/L, respectively. High throughput sequencing data indicated that FT cycles reduced Alpha diversity and significantly changed Beta diversity of soil microorganisms. FT cycles considerably enhanced the relative abundance of Sphingomonas and Lysobacter. Kyoto Encyclopedia of Genes and Genomes function predictions revealed that FT cycles significantly activated cellular gene segments involved in soil bacterial immunological disorders, cell motility, parasite infectious diseases, and neurological diseases. This study would serve as a reference for future study on environmental behavior and toxic effects of heavy metals in farm soils of seasonal FT aeras.

Influence of freeze-thaw process on As migration and microorganisms in aggregates of paddy soil

A natural phenomenon known as the seasonal freeze-thaw (FT) cycle happens in cold temperature zone such as high latitude and high altitude regions where the soil frequently freezes and thaws in response to temperature variations. Global warming would increase the number of FT cycles in FT regions. However, the influence of FT process on arsenic (As) migration in paddy soil is seldom investigated. Herein, indoor simulation experiment was conducted to investigate the influence of FT process (60 cycles) on As migration from surface to deep soil and microorganisms in paddy soil column. Compared to non FT treatment groups, the concentrations of As in microaggregates of 8-10 cm depth and 18-20 cm depth in soil column of FT treatment group increased by 3.69 mg/kg and 4.16 mg/kg, respectively; the concentrations of As in macroaggregates of 8-10 cm depth and 18-20 cm depth in soil column of FT treatment group increased by 3.34 mg/kg and 3.94 mg/kg, respectively, indicating that FT process accelerated the As migration from surface to deep soil. FT process affected the microbial community structure by changing the physicochemical properties of the soil, which decreased the diversity and uniformity of bacterial community distribution in the soil. The relative abundance of two As-resistant bacteria, e.g., Sphingomonas and Lysobacter, increased by 8.2% and 11.35% after 60 cycles, respectively; moreover, total As in the soil was significantly (p < 0.05) negatively correlated with the alpha index of the soil microorganisms. This study would provide basic data for future study on determining environmental behavior and risk of metals in farm soils in seasonal FT aeras.

Biological roles of soil microbial consortium on promoting safe crop production in heavy metal(loid) contaminated soil: A systematic review

Heavy metal(loid) (HM) pollution of agricultural soils is a growing global environmental concern that affects planetary health. Numerous studies have shown that soil microbial consortia can inhibit the accumulation of HMs in crops. However, our current understanding of the effects and mechanisms of inhibition is fragmented. In this review, we summarise extant studies and knowledge to provide a comprehensive view of HM toxicity on crop growth and development at the biological, cellular and the molecular levels. In a meta-analysis, we find that microbial consortia can improve crop resistance and reduce HM uptake, which in turn promotes healthy crop growth, demonstrating that microbial consortia are more effective than single microorganisms. We then review three main mechanisms by which microbial consortia reduce the toxicity of HMs to crops and inhibit HMs accumulation in crops: 1) reducing the bioavailability of HMs in soil (e.g. biosorption, bioaccumulation and biotransformation); 2) improving crop resistance to HMs (e.g. facilitating the absorption of nutrients); and 3) synergistic effects between microorganisms. Finally, we discuss the prospects of microbial consortium applications in simultaneous crop safety production and soil remediation, indicating that they play a key role in sustainable agricultural development, and conclude by identifying research challenges and future directions for the microbial consortium to promote safe crop production.

Current and emerging technologies for the remediation of difficult-to-measure radionuclides at nuclear sites

Difficult-to-measure radionuclides (DTMRs), defined by an absence of high energy gamma emissions during decay, are problematic in groundwaters at nuclear sites. DTMRs are common contaminants at many nuclear facilities, with (often) long half-lives and high radiotoxicities within the human body. Effective remediation is, therefore, essential if nuclear site end-state targets are to be met. However, due to a lack of techniques for in situ DTMR detection, technologies designed to remediate these nuclides are underdeveloped and tend to be environmentally invasive. With a growing agenda for sustainable remediation and reduction in nuclear decommissioning costs, there is renewed international focus on the development of less invasive technologies for DTMR clean-up. Here, we review recent developments for remediation of selected problem DTMRs (129I, 99Tc, 90Sr and 3H), with a focus on industrial and site-scale applications. We find that pump and treat (P&T) is the most used technique despite efficacy issues for 129I and 3H. Permeable reactive barriers (PRBs) are a less invasive alternative but have only been demonstrated for removal of 99Tc and 90Sr at scale. Phytoremediation shows promise for site-scale removal of 3H but is unsuitable for 129I and 99Tc due to biotoxicity and bioavailability hazards, respectively. No single technique can remediate all DTMRs of focus. Likewise, there has been no successful site-applied technology with high removal efficiencies for iodine species typically present in groundwaters (iodide/I-, iodate/IO3- and organoiodine). Further work is needed to adapt and improve current techniques to field scales, as well as further research into targeted application of emerging technologies.