Keystone taxa of phoD-harboring bacteria mediate alkaline phosphatase activity during biochar remediation of Cd-contaminated soil

Potassium permanganate-hematite-modified biochar enhances cadmium and zinc passivation and nutrient availability and promotes soil microbial activity in heavy metal-contaminated soil

Modified biochar has garnered considerable attention for its versatile applications in remediating soils contaminated with heavy metals. However, most existing studies have primarily focused on the stabilisation of heavy metals, with limited research exploring the broader environmental effects following the application of modified biochar. In this study, we developed a potassium permanganate (KMnO4)-hematite-modified biochar (MnFeB) as a passivating agent for heavy metals, specifically targeting cadmium (Cd) and zinc (Zn)-contaminated soils. We examined the effects of MnFeB on the biotoxicity of Cd and Zn, soil properties, enzyme activities, heavy metal resistance genes (czcA, czcC, and czcD), and the soil microbial community in contaminated soils. Treatment with MnFeB markedly reduced the soil diethylenetriaminepentaacetic acid (DTPA)-extractable Zn and Cd contents by 18.79% and 43.65%, respectively. Furthermore, soil organic carbon (SOC), cation exchange capacity (CEC), and the availability of nitrogen, phosphorus, and potassium were found to be increased. MnFeB application also enhanced the activities of catalase, urease, and alkaline phosphatase while reducing the expression of czcA by 23.63%. Moreover, changes in the composition and diversity of soil bacterial and fungal communities were observed. These findings highlight the effects of environmental changes induced by MnFeB application on Cd/Zn-contaminated soil and offer theoretical support for employing passivation strategies in the remediation of heavy metal-contaminated soils.

Variations of microbiota and metabolites in rhizosphere soil of Carmona microphylla at the co-contaminated site with polycyclic aromatic hydrocarbons and heavy metals

Co-contamination with organic/inorganic compounds is common in industrial area and poses a great risk to local soil ecological environment. In this study, an operating ink factory site co-contaminated with polycyclic aromatic hydrocarbons (PAHs, mild to moderate pollution level) and heavy metals (HMs, heavy pollution level) was selected and screened for native vegetation, Carmona microphylla. High-throughput sequencing and metabolomics were mainly used to investigate the responses of soil bacteria and metabolites to the composite pollution and rhizosphere effect. As the results showed, among three pollution levels, a medium level of pollution was favorable to increase the richness and diversity of soil bacterial community, while high level of pollution greatly decreased special OTUs number. In addition, HMs were the most significant factors driving bacterial community structure, especially for Cd. The influence of medium molecular weight PAHs with 4 rings (MMW-PAHs) on dominant bacteria was greater than low molecular weight PAHs with 2-3 rings (LMW-PAHs) and high molecular weight PAHs with 5-6 rings (HMW-PAHs). Soil bacterial function was affected mainly by pollution level, but not rhizosphere effect, in which high pollution level changed α diversity and structure and composition of C- and N-cycling bacteria. Rhizosphere promoted network complexity by increasing the connection densities among bacterial communities, metabolites, soil properties and the involved number of metabolites. Compared to HMs, PAHs played a more important role in shaping bacterial community through affecting metabolites in non-rhizosphere soil, which was different from rhizosphere soil with a more significant effect of HMs than PAHs. Some key bacterial taxa have established resistance to HMs in rhizosphere soils, whereas they were sensitive to compound contamination in non-rhizosphere soils. Some key bacterial taxa are resistant to HMs in rhizosphere soils, whereas they are susceptible to complex contamination in non-rhizosphere soils, which could be a consequence of the rhizosphere by regulating soil metabolism. It also provides a valuable reference for how co-contaminants and rhizosphere effect shape together soil bacterial community through the changes of soil metabolites.

Assembly of root-associated bacterial community and soil health in cadmium-contaminated soil affected by nano/bulk-biochar compost associations

Biochar (BC) has been proven effective in promoting the production of safety food in cadmium (Cd)-polluted soil and the impact can be further enhanced through interaction with compost (CM). However, there existed unclear impacts of biochar with varying particle sizes in conjunction with compost on microbiome composition, rhizosphere functions, and soil health. Hence, in this study, two bulk-biochar derived from wood chips and pig manure were fabricated into nano-biochar using a ball-milling method. Subsequently, in a field experiment, the root-associated bacterial community and microbial functions of lettuce were evaluated in respond to Cd-contaminated soil remediated with nano/bulk-BCCM. The results showed that compared to bulk-BCCM, nano-BCCM significantly reduced the Cd concentration in the edible part of lettuce and the available Cd in the soil. Both nano-BCCM and bulk-BCCM strongly influenced the composition of bacterial communities in the four root-associated niches, and enhanced rhizosphere functions involved in nitrogen, phosphorus, and carbon cycling, as well as the relative abundance and biodiversity of keystone modules in rhizosphere soil. Furthermore, soil quality index analysis indicated that nano-BCCM exhibited greater potential than bulk-BCCM in maintaining soil health. The data revealed that nano-BCCM could regulate the Cd concentration in lettuce shoot by promoting microbial biodiversity of keystone modules in soil-root continuum and rhizosphere bacterial functions. These findings suggest that nano-biochar compost associations can be a superior strategy for enhancing microbial functions, maintaining soil health, and ensuring crop production safety in the Cd-contaminated soil compared to the mix of bulk-biochar and compost.

Enhancement of nitrogen cycling and functional microbial flora by artificial inoculation of biological soil crusts in sandy soils of highway slopes

Biological soil crusts (BSCs) are common in arid and semi-arid ecosystems and enhance soil stability and fertility. Highway slopes severely deplete the soil ecological structure and soil nutrients, hindering plant survival. The construction of highway slope BSCs under human intervention is critical to ensure the long-term stable operation of the slope ecosystem. This study investigated the variation rules and interaction mechanisms between soil nutrients and microbial communities in the subsoil BSCs on highway slopes. Bacterial 16S rRNA high-throughput sequencing was employed to investigate the dynamic compositional changes in the microbial community and perform critical metabolic predictive analyses of functional bacteria. This study revealed that the total soil nitrogen increased significantly from 0.557 to 0.864 g/kg after artificial inoculation with desert Phormidium tenue and Scytonema javanicum. Actinobacteria (44-48%) and Proteobacteria (28-31%) were the dominant phyla in all samples. The abundance of Cyanobacteria, Cytophagaceae, and Chitinophagaceae increased significantly after inoculation. PICRUST analysis showed that the main metabolic pathways of soil microorganisms on highway slopes included cofactor and vitamin, nucleotide, and amino acid metabolisms. These findings suggest that the artificial inoculation with Phormidium tenue and Scytonema javanicum could alter soil microbial distribution to promote soil development on highway slopes toward nutrient accumulation.