Crop straw-derived biochar alleviated cadmium and copper phytotoxicity by reducing bioavailability and accumulation in a field experiment of rice-rape-corn rotation system

Exploring metal(loid)s dynamics and bacterial community shifts in contaminated paddy soil: Impact of MgO-laden biochar under different water conditions

In this study, a soil incubation experiment was conducted to explore the influence MgO-treated corn straw biochar (MCB) on the bioavailability and chemical forms of cadmium (Cd), lead (Pb), and arsenic (As), alongside the impact on the bacterial community within paddy soil subjected to both flooded and non-flooded conditions. Raw corn straw biochar (CB) served as the unmodified biochar control, aiding in the understanding of the biochar's role within the composite. The results showed that even at a minimal concentration of 0.5 %, MCB exhibited higher effectiveness in reducing the bioavailability of Pb and Cd compared to 1 % CB. In non-flooded conditions, 0.5 % MCB reduced the bioavailable Pb and Cd by 99.7 % and 87.4 %, respectively, while NaH2PO4-extracted As displayed a 14.5 % increase. With increasing MCB concentrations (from 0.5 % to 1.5 %), soil pH, DOC, EC, available phosphorus, and bioavailable As increased, while bioavailable Pb and Cd exhibited declining tendencies. Flooding did not notably alter MCB's role in reducing Pb and Cd bioavailability, yet it systematically amplified As release. Heavy metal fractions extracted by acetic acid increased in the MCB groups under flooding conditions, especially for As. The inclusion of 0.5 % MCB did not noticeably affect bacterial diversity, whereas higher doses led to reduced diversity and substantial changes in community composition. Specifically, the groups with MCB showed an increase in the Bacteroidetes and Proteobacteria phyla, accompanied by a decrease in Acidobacteria. These alterations were primarily attributed to the increased pH and EC resulting from MgO hydrolysis. Consequently, for Pb/Cd stabilization and soil bacterial diversity, a low dosage of MgO-treated biochar is recommended. However, caution is advised when employing MgO-treated biochar in soils with elevated arsenic levels, particularly under flooded conditions.

Quantification of the effect of biochar application on heavy metals in paddy systems: Impact, mechanisms and future prospects

Biochar (BC) has shown great potential in remediating heavy metal(loid)s (HMs) contamination in paddy fields. Variation in feedstock sources, pyrolysis temperatures, modification methods, and application rates of BC can result in great changes in its effects on HM bioavailability and bioaccumulation in soil-rice systems and remediation mechanisms. Meanwhile, there is a lack of application guidelines for BC with specific properties and application rates when targeting rice fields contaminated with certain HMs. To elucidate this topic, this review focuses on i) the effects of feedstock type, pyrolysis temperature, and modification method on the properties of BC; ii) the changes in bioavailability and bioaccumulation of HMs in soil-rice systems applying BC with different feedstocks, pyrolysis temperatures, modification methods, and application rates; and iii) exploration of potential remediation mechanisms for applying BC to reduce the mobility and bioaccumulation of HMs in rice field systems. In general, the application of Fe/Mn modified organic waste (OW) derived BC for mid-temperature pyrolysis is still a well-optimized choice for the remediation of HM contamination in rice fields. From the viewpoint of remediation efficiency, the application rate of BC should be appropriately increased to immobilize Cd, Pb, and Cu in rice paddies, while the application rate of BC for immobilizing As should be <2.0 % (w/w). The mechanism of remediation of HM-contaminated rice fields by applying BC is mainly the direct adsorption of HMs by BC in soil pore water and the mediation of soil microenvironmental changes. In addition, the application of Fe/Mn modified BC induced the formation of iron plaque (IP) on the root surface of rice, which reduced the uptake of HM by the plant. Finally, this paper describes the prospects and challenges for the extension of various BCs for the remediation of HM contamination in paddy fields and makes some suggestions for future development.

Chemical and mechanical coating of sulfur on baby corn biochar and their role in soil Pb availability, uptake, and growth of tomato under Pb contamination

In recent ages, industrial revolution and natural weathering processes have been increasing lead (Pb) contamination in agricultural soils, therefore, green remediation technologies are becoming attractive and cost-effective. In the current pot study, 1% and 2% (w/w) application rates of sulfur (S) alone and novel chemo-mechanically S-modified baby corn biochars (CSB and MSB) were applied in a Pb-contaminated (500 mg/kg) soil to evaluate tomato (Lycopersicon esculentum L.) growth, Pb uptake and its soil availability. The results from SEM-EDS and XRD patterns confirmed the S enrichment on the surface of baby-corn biochar. Further, Pb treatment alone imposed a significant reduction in biomass accumulation, photosynthetic pigments, antioxidative mechanism, root traits, and Pb-tolerance index because of increased soil Pb availability and its uptake, translocation and biological accumulation in various tissues of tomato. However, incorporation of lower rate of elemental S (1%) and higher rates of biochars, especially chemically S-modified biochar, CSB (2%) significantly improved dry biomass production, Pb-tolerance index, physiological attributes and antioxidative defense system of tomato plants. These results might be due to a prominent decrease in soil Pb availability by 37.5%, Pb concentration in shoot by 66.7% and root by 58.3%, soil to root transfer by 33.8%, and root to shoot transfer by 20.2% in tomato plants under 2% application rate of CSB, as compared with the Pb treatment without any amendment. Moreover, sulfur treatment induced a significant impact in reduction of soil pH (from 8.97-7.47) as compared to the biochar treatments under Pb-toxicity. The current findings provided an insight that 2% chemically S-modified biochar (CSB) has significant potential to improve the tomato growth by reducing Pb bioavailability in the Pb-contaminated soil, compared to the S alone and MSB amendments.

The Hydrolysis of Pigment-Protein Phycoerythrin by Bromelain Enhances the Color Stability

Phycoerythrin (PE) is a natural protein-pigment complex with a strong pink color, but it is sensitive to thermal and light variations. In this study, PE was extracted from Porphyra haitanensis in a yield of 0.2% (w/w). The phycoerythrin hydrolysates (PEH) (3-10 kDa) were prepared by enzymatic hydrolysis of PE with bromelain (8000 U/g) at 47 °C for 30 min, with a degree of hydrolysis (DH) of 11.57 ± 0.39% and a color degradation rate of 7.98 ± 0.39%. The physicochemical properties of PEH were evaluated. The UV and fluorescence spectra indicated that bromelain changed the microenvironment around phycoerythrobilin (PEB). The infrared spectrum revealed that the bromelain hydrolysis increased the α-helix content of PEH. The scanning electron microscope showed that bromelain destroyed the dense and smooth structure of PE, resulting in irregular porous structures. The radical scavenging activities of DPPH and ABTS of PEH were increased relative to that of PE (p < 0.05). The thermal (50-80 °C)-, UV (0.5-3 h)-, visible light irradiation (2-8 h)-, and metal ion exposing stabilities of PEH were significantly improved (p < 0.05). This study provides a potential scheme for overcoming the sensitivity of PE to thermal and light variations and facilitates PEH as a natural colorant ingredient in food and pigment applications.

Low pe+pH inhibits Cd transfer from paddy soil to rice tissues driven by S addition

Both soil irrigation and sulfur (S) are associated with the precipitation of cadmium (Cd)-sulfide in paddy soil, their interaction affecting on Cd solubility and extractability is still unknown. This study primarily discusses the effect of exogenous S addition on the bioavailability of Cd in paddy soil under unsteady pe + pH conditions. The experiment was treated with three different water strategies: continuous dryness (CD), continuous flooding (CF), and alternating dry-wet cycles for one cycle (DW). These strategies were combined with three different S concentrations. The results indicate that the CF treatment, particularly when combined with S addition, had the most significant effect on reducing pe + pH and Cd bioavailability in the soil. The reduction of pe + pH from 10.2 to 5.5 resulted in a decrease in soil Cd availability by 58.3%, and Cd accumulation in rice grain by 52.8%, compared to the other treatments. While it was more conducive to the formation of iron plaque on the root surface in DW treatment with S addition at rice maturing stage and enhanced the gathering of Fe/S/Cd. Structural equation model (SEM) analysis further confirmed a significant negative correlation (r = -0.916) between the abundance of soil Fer-reducing bacteria (FeRB) and sulfate-reducing bacteria (SRB) like Desulfuromonas, Pseudomonas, Geobacter, and the Cd content in rice grains. This study provides a basic mechanistic understanding of how soil redox status (pe + pH), S addition, and FeRB/SRB interacted with Cd transfer in paddy soil-rice tissues.

Different composites inhibit Cd accumulation in grains under the rice-oilseed rape rotation mode of karst area: A field study

Ensuring the safe production of food and oil crops in soils with elevated cadmium (Cd) content in karst regions is crucial. We tested a field experiment to examine the long-term remediation effects of compound microorganisms (CM), strong anion exchange adsorbent (SAX), processed oyster shell (POS), and composite humic acids (CHA) on Cd contamination in paddy fields under a rice-oilseed rape rotation system. In comparison to the control group (CK), the application of amendments significantly increased soil pH, cation exchange capacity (CEC), and soil organic matter (SOM) content while markedly decreasing the content of available Cd (ACd). During the rice cultivation season, Cd was predominantly concentrated in the roots. Relative to the control (CK), the Cd content in each organ was significantly reduced. The Cd content in brown rice decreased by 19.18-85.45%. The Cd content in brown rice following different treatments exhibited the order of CM > POS > CHA > SAX, which was lower than the Chinese Food Safety Standard (GB 2762-2017) (0.20 mg/kg). Intriguingly, during the oilseed rape cultivation season, we discovered that oilseed rape possesses potential phytoremediation capabilities, with Cd mainly accumulating in roots and stems. Notably, CHA treatment alone significantly decreased the Cd content in oilseed rape grains to 0.156 mg/kg. CHA treatment also maintained soil pH and SOM content, consistently reduced soil ACd content, and stabilized Cd content in RSF within the rice-oilseed rape rotation system. Importantly, CHA treatment not only enhances crop production but also has a low total cost (1255.230 US$/hm2). Our research demonstrated that CHA provides a consistent and stable remediation effect on Cd-contaminated rice fields within the crop rotation system, as evidenced by the analysis of Cd reduction efficiency, crop yield, soil environmental change, and total cost. These findings offer valuable guidance for sustainable soil utilization and safe production of grain and oil crops in the context of high Cd concentrations in karst mountainous regions.

Changes in the Availability and Distribution of Microelement Copper in Cadmium Contaminated Soil and its Accumulation in Rice (Oryza sativa L.) After Biochar Application

This experiment explored the side effect of biochar application on the availability and distribution of the soil microelement copper (Cu) in cadmium (Cd) contaminated soil and its uptake in rice tissues. Biochar was added at 0, 10, 20, 30, and 40 t ha^-1 on topsoil. Results observed that both the concentration of available Cu in the topsoil and subsoil layer decreased by 16.3%-28.9% and 7.5%-59.3%, respectively, after biochar application. The Cu concentration in the < 0.053 mm and 1-2 mm soil aggregates increased as biochar application rate increased. Increasing application rate of biochar reduced the proportion of exchangeable Cu, carbonate bound Cu, and Fe/Mn oxide Cu fraction in the soil profile. However, the fraction of organic bound and residual Cu increased by 5.0%-178.4% and 7.0%-15.6%, respectively. Biochar could immobilize microelement Cu in the soil profile and limit Cu uptake by rice.

Recent advances in biochar amendments for immobilization of heavy metals in an agricultural ecosystem: A systematic review

Over the last several decades, extensive and inefficient use of contemporary technologies has resulted in substantial environmental pollution, predominantly caused by potentially hazardous elements (PTEs), like heavy metals that severely harm living species. To combat the presence of heavy metals (HMs) in the agrarian system, biochar becomes an attractive approach for stabilizing and limiting availability of HMs in soils due to its high surface area, porosity, pH, aromatic structure as well as several functional groups, which mostly rely on the feedstock and pyrolysis temperature. Additionally, agricultural waste-derived biochar is an effective management option to ensure carbon neutrality and circular economy while also addressing social and environmental concerns. Given these diverse parameters, the present systematic evaluation seeks to (i) ascertain the effectiveness of heavy metal immobilization by agro waste-derived biochar; (ii) examine the presence of biochar on soil physico-chemical, and thermal properties, along with microbial diversity; (iii) explore the underlying mechanisms responsible for the reduction in heavy metal concentration; and (iv) possibility of biochar implications to advance circular economy approach. The collection of more than 200 papers catalogues the immobilization efficiency of biochar in agricultural soil and its impacts on soil from multi-angle perspectives. The data gathered suggests that pristine biochar effectively reduced cationic heavy metals (Pb, Cd, Cu, Ni) and Cr mobilization and uptake by plants, whereas modified biochar effectively reduced As in soil and plant systems. However, the exact mechanism underlying is a complex biochar-soil interaction. In addition to successfully immobilizing heavy metals in the soil, the application of biochar improved soil fertility and increased agricultural productivity. However, the lack of knowledge on unfavorable impacts on the agricultural systems, along with discrepancies between the use of biochar and experimental conditions, impeded a thorough understanding on a deeper level.

Remediation of cadmium-contaminated soil by micro-nano nitrogen-doped biochar and its mechanisms

Cadmium-contaminated soils are an urgent problem that needs to be solved in many countries and regions. In this study, a new heavy metal passivator, micro-nano nitrogen-doped biochar (Nm-NBC), was prepared by introducing nitrogen into biochar. Soybean was used as an experimental plant to compare the effects of corn straw biochar (CBC, not modified), ammonium chloride modified corn straw biochar (NBC), and micro-nano nitrogen-doped biochar (Nm-NBC) on the remediation of Cdcontaminated soil. The results showed that the biomass of soybean, pH, organic matter, and total nitrogen content of the Cd-contaminated soil significantly increased, and the available Cd in soil significantly reduced (P < 0.05) when CBC, NBC, and Nm-NBC were added. The effect was as follows: Nm-NBC > NBC > CBC; Nm-NBC had the best result. When 1% Nm-NBC added to the soil, the Cd content in beans reduced by 68.09%. BET, FTIR, XPS, and SEM were used to analyze the characteristics of Nm-NBC and its mechanisms in the remediation of Cd-contaminated soils. The results showed that Nm-NBC had larger specific surface area and abundant functional groups; -COOH and graphitic nitrogen in Nm-NBC can form Cd-O bond and Cd-π with Cd(II) in the soil. Therefore, Nm-NBC prepared by introducing nitrogen into biochar has a promising application in the remediation of Cd-contaminated soil.

Nitrogen modulates early growth and physio-biochemical attributes in fragrant rice grown under cadmium and multiwall carbon nanotubes stresses

Nitrogen (N) modulates plant growth, but its impact on the early growth and physio-biochemical characteristics of rice under cadmium (Cd) and multiwall carbon nanotubes (MWCNTs) toxicity has received little attention. In this study, a hydroponic experiment was conducted on two fragrant rice cultivars, e.g., Xiangyaxiangzhan (XYXZ) and Yuxiangyouzhan (YXYZ), grown under two N levels (N and 1/4 N) and various Cd and MWCNTs treatments (CK: without CdCl₂ or MWCNTs; Cd: 100 μmol L^-1 CdCl₂; MWCNTs: 100 mg L^-1 MWCNTs; and Cd-MWCNTs: 100 μmol L^-1 CdCl₂ + 100 mg L^-1 MWCNTs). Results showed that when compared to CK, the total dry weight of the Cd and MWCNTs treatments did not change under 1/4 N for both varieties, whereas Cd and Cd-MWCNTs treatments resulted in a significant reduction in total dry weight by 18.78-37.85% for XYXZ and the Cd-MWCNTs treatment resulted in a significant reduction in the total dry weight by 20.24% for YXYZ. The changes in total dry weight were linked to changes in the dry weight of the different parts of the plant. Besides, the antioxidant parameters and the enzymes involved in the nitrogen metabolism changed in different varieties and different plant parts under two N levels and various Cd and MWCNTs treatments. In addition, differences in total dry weight changes at the N levels and various Cd and MWCNTs treatments were identified between the two varieties, and the relations between total dry weight and other investigated parameters indicated that the modulation processes varied between varieties. Overall, N modulates the early growth and physio-biochemical attributes in fragrant rice seedlings under Cd, MWCNTs, and their combined toxicity.

Effects of the increases in soil pH and pH buffering capacity induced by crop residue biochars on available Cd contents in acidic paddy soils

To explore the effects of the increases in pH and pH buffering capacity (pHBC) induced by crop residue biochars on the changes in soil available Cd content, six acidic paddy soils developed from different parents were amended with seeded sunflower plate biochar (SSPBC), peanut straw biochar (PSBC) and corn straw biochar (CSBC). The pH, pHBC, and available Cd of the soils were measured after laboratory incubation. The results showed that the incorporation of crop residue biochars led to the increases in soil pH and pHBC, but a decrease in soil available Cd content. The decreasing order of available Cd content was SSPBC > PSBC > CSBC and was consistent with the changes in soil pH induced by the biochars. During submerging and draining, soil pH increased first and then declined, however the content of available Cd decreased first and then increased significantly. Soil pH in the treatments with biochars showed little change during draining, which was different from the control without the biochars added. This was attributed to the enhancing effect of the biochars on soil pHBC. Also, there was a significant negative correlation between the change in available Cd content and soil pHBC during submerging/draining alternation and suggested that higher pHBC corresponded to smaller soil available Cd content. Consequently, the amount of Cd absorbed by rice was reduced, thereby reducing the potential risk of soil Cd to humans. These results can provide useful references for the remediation of Cd-contaminated paddy soils.

The potential of saltmarsh halophytes for phytoremediation of metals and persistent organic pollutants: An Australian perspective

Persistent organic and inorganic pollutants are among the most concerning pollutants in Australian estuaries due to their persistent, ubiquitous, and potentially toxic nature. Traditional methods of soil remediation often fall short of practical implementation due to high monetary investment, environmental disturbance, and potential for re-contamination. Phytoremediation is gaining traction as an alternative, or synergistic mechanism of contaminated soil remediation. Phytoremediation utilises plants and associated rhizospheric microorganisms to stabilise, degrade, transform, or remove xenobiotics from contaminated mediums. Due to their apparent cross-tolerance to salt, metals, and organic contaminants, halophytes have shown promise as phytoremediation species. This review examines the potential of 93 species of Australian saltmarsh halophytes for xenobiotic phytoremediation. Considerations for the practical application of phytoremediation in Australia are discussed, including mechanisms of enhancement, and methods of harvesting and disposal. Knowledge gaps for the implementation of phytoremediation in Australian saline environments are identified, and areas for future research are suggested.

Mechanisms of Stress Alleviation after Lime and Biochar Applications for Brassica napus L. in Cadmium-Contaminated Soil

Lime and biochar amendments are widely used to immobilize cadmium (Cd) in agricultural soils and to ensure food security. However, the effects of these two soil amendments on the mechanisms of Cd stress alleviation in crops are unclear. Therefore, the effects of lime and biochar applications on Cd uptake, transport, subcellular distribution, antioxidant system, N metabolism, and related factors were examined in a soil-Brassica napus L. (B. napus) system. We found that lime application significantly increased the root Cd content by 41.5% but decreased Cd TF and shoot Cd by 81.0% and 74.3%, respectively, whereas biochar amendment decreased root and shoot Cd contents by 67.6% and 34.3%, respectively, but increased Cd TF by 104.1%. Lime treatment immobilized Cd in the cell wall of the root to reduce Cd transport, but biochar treatment increased the soluble fraction of Cd in root cells to improve the migration capacity of Cd. The significant negative relationship between the soil exchangeable Cd and Ca and the positive relationships between Cd and Ca both in shoot and root indicated that the Ca mediated Cd transport from soil to B. napus after lime and biochar applications. Additionally, lime amendment increased Cd proportion in the root cell walls to immobilize Cd, but biochar amendment increased Cd proportion in the soluble fraction to enhance Cd migration. Furthermore, biochar application significantly increased SOD, CAT, and POD by 17.5%, 95.4%, and 26.6%, whereas lime amendment only significantly enhanced CAT by 51.0%. Besides, both of biochar and lime applications increased shoot N content and GDH activity, but only the shoot NO3- content and nitrate reductase under biochar treatment were significantly altered. Overall, these findings suggested that lime is more efficient in reducing the transport of Cd from underground to aboveground and in improving Cd tolerance, whereas biochar tends to improve the antioxidant capacity and facilitate N metabolism. These results will provide significant strategies for selecting appropriate amendments to ensure the crops safety.

Remediation of Cd and Cu contaminated water and soil using novel nanomaterials derived from sugar beet processing- and clay brick factory-solid wastes

Producing nanomaterials from hazardous wastes for water and soil treatment is of great concern. Here, we produced and fully characterized two novel nanomaterials from sugar beet processing (SBR)- and brick factory-residuals (BFR) and assed their ability for Cd and Cu sorption in water and reducing metal availability in a contaminated soil. The SBR removed up to 99% of Cu and 91% of Cd in water, and exhibited a significantly faster and higher sorption capacity (q_max (g kg^-1) = 1111.1 for Cu and 33.3 for Cd) than BFR (q_max (g kg^-1) = 33.3 for Cu and 10.0 for Cd), even at acidic pH. Soil metal availability was significantly reduced by SBR (up to 57% for Cu and 86% for Cd) and BFR (up to 36% for Cu and 68% for Cd) compared to the unamended soil. The higher removal efficacy of SBR over BFR could be attributed to its higher alkalinity (pH = 12.5), carbonate content (82%), and specific surface area, as well as the activity of hydroxyl -OH and Si-O groups. The nano-scale SBR and BFR, the former particularly, are novel, of low cost, and environmental friendly amendments that can be used for the remediation of metal-contaminated water and soil.

Recent Advances in Minimizing Cadmium Accumulation in Wheat

Cadmium (Cd), a toxic heavy metal, affects the yield and quality of crops. Wheat (Triticum aestivum L.) can accumulate high Cd content in the grain, which poses a major worldwide hazard to human health. Advances in our understanding of Cd toxicity for plants and humans, different parameters influencing Cd uptake and accumulation, as well as phytoremediation technologies to relieve Cd pollution in wheat have been made very recently. In particular, the molecular mechanisms of wheat under Cd stress have been increasingly recognized. In this review, we focus on the recently described omics and functional genes uncovering Cd stress, as well as different mitigation strategies to reduce Cd toxicity in wheat.

Comparative study on the potential risk of contaminated-rice straw, its derived biochar and phosphorus modified biochar as an amendment and their implication for environment

Direct application of contaminated-rice straw (CRS) to soil can cause the secondary pollution in agricultural land because of high content of Cd in rice straw. This study employed biochar or modified biochar technique to reduce the potential pollution risk of Cd in CRS. In the pot experiment, the CRS, straw biochar prepared at 300 °C (B300) and 500 °C (B500), and phosphorus modified biochar pyrolyzed at 300 °C (PB300) and 500 °C (PB500) were added at dosage of 5% into three typical paddy soils. The results showed that CRS and its derived biochar could enhance soil pH, EC, Eh, organic carbon, exchangeable base cations (K⁺, Na⁺, Ca²⁺ and Mg²⁺), and available phosphate. The application of CRS, biochar and phosphorus modified biochar significantly increased the contents of total Cd in soils relative to control soil. Compared to CRS, the biochar application (especially the PB500) decreased the contents of 0.01M CaCl₂-extractable Cd. The application of CRS significantly increased the content of exchangeable Cd fraction (F1), whereas biochar increased residual Cd content (F4). The biochar and phosphorous modified biochar significantly decreased the contents of bioavailable Cd in soils compared to CRS application. The increased soil pH and dissolve organic matter were found to be the main factors in reducing the release of Cd in biochar. The possible mechanisms of biochar in reducing bioavailability of Cd were to significantly increase soil pH, enhance the complexation of Cd ions, and promote the transformation of Cd from easily available to stable (residual) forms. It could conclude that conversion of contaminated rice straw into biochar was an efficient way to minimize Cd availability in soil and reduce the pollution risk of Cd in rice straw.