Co-application of biochar and nitrogen fertilizer promotes rice performance, decreases cadmium availability, and shapes rhizosphere bacterial community in paddy soil

Biochar decreases cadmium uptake in indica and japonica rice (Oryza sativa L.): Roles of soil properties, iron plaque, cadmium transporter genes and rhizobacteria

Biochar is an effective and economical strategy for in situ soil cadmium (Cd) remediation. It is essential to comprehensively investigate how biochar mitigates Cd uptake of the main rice subspecies. A pot experiment was established via adding corn stalk biochar into Cd-contaminated soil growing indica Yangdao 6 (YD) and japonica Nangeng 9108 (9108). 9108 had lower shoot biomass (-17.9%) but higher root biomass (+14.4%) and shoot Cd concentration (+29.4%) than YD. Biochar decreased soil available Cd by 25.2% and shoot Cd concentration by 13.6% through the liming and passivation effects. Biochar also favored Cd mitigation by recruiting Fe reducer, Cd remover and plant growth-promoting rhizobacteria (e.g. Bacteroides, Deferrisomatota, Bacillus and Allorhizobium). Besides, biochar reduced Cd uptake by stimulating iron plaques formation for 9108. Moreover, biochar did not reduce Cd uptake by inhibiting Cd transporter genes' expressions and it increased OsHMA2 expression in YD. In conclusion, biochar had great capacity in mitigating Cd pollution and rice subspecies responded differently to biochar in iron plaque formation and Cd transporter genes. The research established a comprehensive understanding of the mechanisms underlying Cd mitigation by biochar and helped to breed low Cd-accumulated rice cultivars to safeguard rice production.

Enhanced immobilization of cadmium and lead in contaminated soil using calcium alginate-modified HAP biochar: Improvements in soil health and microbial diversity

A novel adsorbent, calcium alginate-modified HAP (Hydroxyapatite)-wood ear mushroom sticks biochar (CA-HAPMB), was synthesized to enhance the immobilization of Cd and Pb in soil. Over 150 days, applying CA-HAPMB at concentrations of 0%-3% in contaminated soils from Chenzhou City in Hunan Province (CZ) and Shenyang City in Liaoning Province (SY) resulted in decreased effective concentrations of Cd and Pb. Specifically, in CZ soil, Cd and Pb decreased by 30.9%-69.3% and 31.9%-78.6%, respectively, while in SY soil, they decreased by 27.5%-53.7% and 26.4%-62.3%, respectively. Characterization results, obtained after separating CA-HAPMB from the soil, indicate that complexation, co-precipitation, and ion exchange play crucial roles in the efficient immobilization of Cd and Pb by CA-HAPMB. Additionally, adjusting the amount of CA-HAPMB added allows modulation of soil pH, leading to increased soil organic matter and nutrient content. Following treatment with CA-HAPMB for immobilizing Cd and Pb, soil bacteria abundance and diversity increased, further promoting heavy-metal immobilization.

How does the biochar-supported sulfidized nanoscale zero-valent iron affect the soil environment and microorganisms while remediating cadmium contaminated paddy soil?

In previous studies, iron-based nanomaterials, especially biochar (BC)-supported sulfidized nanoscale zero-valent iron (S-nZVI/BC), have been widely used for the remediation of soil contaminants. However, its potential risks to the soil ecological environment are still unknown. This study aims to explore the effects of 3% added S-nZVI/BC on soil environment and microorganisms during the remediation of Cd contaminated yellow-brown soil of paddy field. The results showed that after 49 d of incubation, S-nZVI/BC significantly reduced physiologically based extraction test (PBET) extractable Cd concentration (P < 0.05), and increased the immobilization efficiency of Cd by 16.51% and 17.43% compared with S-nZVI and nZVI/BC alone, respectively. Meanwhile, the application of S-nZVI/BC significantly increased soil urease and sucrase activities by 0.153 and 0.446 times, respectively (P < 0.05), improving the soil environmental quality and promoting the soil nitrogen cycle and carbon cycle. The results from the analysis of the 16S rRNA genes indicated that S-nZVI/BC treatment had a minimal effect on the bacterial community and did not appreciably alter the species of the original dominant bacterial phylum. Importantly, compared to other iron-based nanomaterials, incorporating S-nZVI/BC significantly increased the soil organic carbon (OC) content and decreased the excessive release of iron (P < 0.05). This study also found a significant negative correlation between OC content and Fe(II) content (P < 0.05). It might originate from the reducing effect of Fe-reducing bacteria, which consumed OC to promote the reduction of Fe(III). Accompanying this process, the redistribution of Cd and Fe mineral phases in the soil as well as the generation of secondary Fe(II) minerals facilitated Cd immobilization. Overall, S-nZVI/BC could effectively reduce the bioavailability of Cd, increase soil nutrients and enzyme activities, with less toxic impacts on the soil microorganisms.

Elevated CO2 and nitrogen addition enhance the symbiosis and functions of rhizosphere microorganisms under cadmium exposure

Soil microbes are fundamental to ecosystem health and productivity. How soil microbial communities are influenced by elevated atmospheric carbon dioxide (eCO2) concentration and nitrogen (N) deposition under heavy metal pollution remains uncertain, despite global exposure of terrestrial ecosystems to eCO2, high N deposition and heavy metal stress. Here, we conducted a four year's open-top chamber experiment to assess the effects of soil cadmium (Cd) treatment (10 kg hm-2 year-1) alone and combined treatments of Cd with eCO2 concentration (700 ppm) and/or N addition (100 kg hm-2 year-1) on tree growth and rhizosphere microbial community. Relative to Cd treatment alone, eCO2 concentration in Cd contaminated soil increased the complexity of microbial networks, including the number links, average degree and positive/negative ratios. The combined effect of eCO2 and N addition in Cd contaminated soil not only increased the complexity of microbial networks, but also enhanced the abundance of microbial urealysis related UreC and nitrifying related amoA1 and amoA2, and the richness of arbuscular mycorrhiza fungi (AMF), thereby improving the symbiotic functions between microorganisms and plants. Results from correlation analysis and structural equation model (SEM) further demonstrated that eCO2 concentration and N addition acted on functions and networks differently. Elevated CO2 positively regulated microbial networks and functions through phosphorus (P) and Cd concentration in roots, while N addition affected microbial functions through soil available N and soil organic carbon (SOC) concentration and microbial network through soil Cd concentration. Overall, our findings highlight that eCO2 concentration and N addition make microbial communities towards ecosystem health that may mitigate Cd stress, and provide new insights into the microbiology supporting phytoremediation for Cd contaminated sites in current and future global change scenarios.

Hydrochar reduces oxytetracycline in soil and Chinese cabbage by altering soil properties, shifting microbial community structure and promoting microbial metabolism

The efficient remediation of antibiotic-contaminated soil is critical for agroecosystem and human health. Using the cost-effective and feedstock-independent hydrochar with rich oxygen-containing functional groups as a soil remediation material has become a hot concern nowadays. However, the feasibility and effectiveness of hydrochar amendment in antibiotic-contaminated soil still remain unknown. Therefore, this study investigated the remediation effect and potential mechanisms of different hydrochars from cow manure (H-CM), corn stalk (H-CS) and Myriophyllum aquaticum (H-MA) at two levels (0.5% and 1.0%) in oxytetracycline (OTC)-contaminated soil using a pot experiment. Results showed that compared with CK, OTC content in the soils amended with H-CM and H-MA was decreased by 14.02-15.43% and 9.23-24.98%, respectively, whereas it was increased by 37.03-42.64% in the soils amended with H-CS. Additionally, all hydrochar amendments effectively reduced the OTC uptake in root and shoot of Chinese cabbage by 10.41-57.99% and 31.92-65.99%, respectively. The response of soil microbial community to hydrochar amendment heavily depended on feedstock type rather than hydrochar level. The soil microbial metabolism (e.g., carbohydrate metabolism, amino acid metabolism) was enhanced by hydrochar amendment. The redundancy analysis suggested that TCA cycle was positively related to the abundances of OTC-degrading bacteria (Proteobacteria, Arthrobacter and Sphingomonas) in all hydrochar-amended soils. The hydrochar amendment accelerated the soil OTC removal and reduced plant uptake in soil-Chinese cabbage system by altering soil properties, enhancing OTC-degrading bacteria and promoting microbial metabolism. These findings demonstrated that the cost-effective and sustainable hydrochar was a promising remediation material for antibiotic-contaminated soil.

Crop effect and mechanism of amino acid-modified biomass ash in remediation of cadmium-contaminated soil

Lignocellulosic biomass ash (BA) has certain adsorption and passivation effects on heavy metals, but its function is generally weak. Amino acid salt can facilitate the leaching of heavy metals in soil. Therefore, modification of BA with amino acid salt may realize a higher leaching rate and better passivation of heavy metals in soil. In this study, BA was modified by amino acid hydrolysate obtained from the hydrolysis of chicken feathers by sulfuric acid. The physicochemical properties of BA and modified BA (MBA), their effects on Chinese cabbage (CC) yield and nutritional quality, and passivation effects on soil cadmium (Cd) were compared, and the related mechanisms were investigated. SEM-EDS, XRD, and FTIR demonstrated that BA was a CaCO3-type soil amendment, while MBA was a CaSO4-type soil amendment with the loading of amino acid. Compared with BA, MBA significantly increased the fresh weight, soluble sugar, vitamin C (Vc), and protein contents of CC in both non-Cd contaminated soil and Cd contaminated soil, and obviously decreased the nitrate content and Cd uptake of CC in Cd-contaminated soil. After the application of MBA, cadmium species in potted soil were transformed from higher plant availability, representing by exchangeable and carbonate-bound, into lower plant availability, representing by iron-manganese oxide bound, which was identified as the key reason for the significant reduction of Cd content in CC under MBA application.

Changes in soil Cd contents and microbial communities following Cd-containing straw return

Contamination of soil with cadmium (Cd) threatens food safety and human health. In general, crop straws from contaminated soils could accumulate considerable amounts of Cd. The addition of Cd-containing rice straw can have negative effects on soil environment. In this study, straws varying in Cd concentration were added to soil at a rate of 5% (w/w) to investigate the effects of Cd-containing straw on soil Cd dynamics and soil microbial communities. Results showed that large amounts of Cd, especially bioavailable Cd, were released into soil during the decomposition of Cd-containing straws. The addition of straws with 10, 20 and 40 mg kg^-1 Cd increased total Cd in soils from 0.31 mg kg^-1 to 0.89, 1.39 and 2.09 mg kg^-1, respectively, exceeding the screening value of total Cd < 0.4 mg kg^-1 for paddy soils of pH 5.5-6.5 according to Chinese Soil Environmental Quality Standards. Moreover, the addition of Cd-containing straw decreased alpha-diversity of bacterial and fungal communities compared to the clean straw. Indeed, changes in soil factors including pH, Eh, dissolved organic C and Cd level jointly reconstructed soil microbial communities. The addition of Cd-containing straw increased the relative abundance of bacterial species Acidobacteria and Proteobacteria but decreased that of Firmicutes. Meanwhile, it increased the relative abundance of fungal species Basidiomycota and Fusarium which were considered Cd-tolerant. This study revealed the potential environmental risk and the variation of microbial communities caused by increasing soil Cd bioavailability after direct application of Cd-containing rice straw to the field.