Effects of Passivating Agents on the Availability of Cd and Pb and Microbial Community Function in a Contaminated Acidic Soil

Impact of coconut-fiber biochar on lead translocation, accumulation, and detoxification mechanisms in a soil-rice system under elevated lead stress

Biochar, an environmentally friendly material, was found to passivate lead (Pb) in contaminated soil effectively. This study utilized spectroscopic investigations and partial least squares path modeling (PLS-PM) analysis to examine the impact of coconut-fiber biochar (CFB) on the translocation, accumulation, and detoxification mechanisms of Pb in soil-rice systems. The results demonstrated a significant decrease (p < 0.05) in bioavailable Pb concentration in paddy soils with CFB amendment, as well as reduced Pb concentrations in rice roots, shoots, and brown rice. Synchrotron-based micro X-ray fluorescence analyses revealed that CFB application inhibited the migration of Pb to the rhizospheric soil region, leading to reduced Pb uptake by rice roots. Additionally, the CFB treatment decreased Pb concentrations in the cellular protoplasm of both roots and shoots, and enhanced the activity of antioxidant enzymes in rice plants, improving their Pb stress tolerance. PLS-PM analyses quantified the effects of CFB on the accumulation and detoxification pathways of Pb in the soil-rice system. Understanding how biochar influences the immobilization and detoxification of Pb in soil-rice systems could provide valuable insights for strategically using biochar to address hazardous elements in complex agricultural settings.

Combined use of biochar and phosphate rocks on phosphorus and heavy metal availability: A meta-analysis

Biochar (BC) and phosphate rocks (PR) are alternative nutrient sources with multiple benefits for sustainable agriculture. The combination of these soil amendments serves two main purposes: to increase soil phosphorus (P) availability and to remediate heavy metal (HM) contamination. However, a further demonstration of the benefits and risks associated with the combined use of BC and PR (BC + PR) is needed, considering the specific characteristics of raw materials, soil types, experimental conditions, and climatic contexts. This meta-analysis is based on data from 28 selected studies, including 581 paired combinations evaluating effects on extraction and fractionation of cadmium (Cd) and lead (Pb), and 290 paired combinations for soil labile and non-labile P. The results reveal that BC, PR, and BC + PR significantly increase soil labile and non-labile P, with BC + PR showing a 150% greater increase compared to BC alone. In tropical regions, substantial increases in P levels were observed with BC, PR, and BC + PR exhibiting increments of 317, 798, and 288%, respectively. In contrast, temperate climate conditions showed lower increases, with BC, PR, and BC + PR indicating 54, 123, and 88% rises in soil P levels. Moreover, BC, PR, and BC + PR effectively reduce the bioavailability of Cd and Pb in soil, with BC + PR demonstrating the highest efficacy in immobilizing Cd. The synergistic effect of BC + PR highlights their potential for Cd remediation. BC + PR effectively reduces the exchangeable fraction of Cd and Pb in soil, leading to their immobilization in more stable forms, such as the residual fraction. This study provides valuable insights into the remediation potential and P management benefits of BC and PR, highlighting their importance for sustainable agriculture and soil remediation practices.

Remediation and its biological responses to Cd(II)-Cr(VI)-Pb(II) multi-contaminated soil by supported nano zero-valent iron composites

Multi-metal contaminated soil has received extensive attention. The biochar and bentonite-supported nano zero-valent iron (nZVI) (BC-BE-nZVI) composite was synthesized in this study by the liquid-phase reduction method. Subsequently, the BC-BE-nZVI composite was applied to immobilize cadmium (Cd), chromium (Cr), and lead (Pb) in simulated contaminated soil. The simultaneous immobilization efficiencies of Cd, Cr(VI), Cr_total, and Pb were achieved at 70.95 %, 100 %, 86.21 %, and 100 %, respectively. In addition, mobility and bioavailabilities of Cd, Cr, and Pb were significantly decreased and the risk of iron toxicity was reduced. Stabilized metal species in the contaminated soil (e.g., Cd(OH)₂, Cd-Fe-(OH)₂, Cr_xFe_1-xOOH, Cr_xFe_1-x(OH)₃, PbO, PbCrO₄, and Pb(OH)₂) were formed after the BC-BE-nZVI treatment. Thus, the immobilization mechanisms of Cd, Cr, and Pb, including adsorption, reduction, co-precipitation, and complexation co-exist with the metals. More importantly, bacterial richness, bacterial diversity, soil enzyme activities (dehydrogenase, urease, and fluorescein diacetate hydrolase), and microbial activity were enhanced by applying the BC-BE-nZVI composite, thus increasing the soil metabolic function. Over all, this work applied a promising procedure for remediating multi- metal contaminated soil by using the BC-BE-nZVI composite.

Supplying silicon alters microbial community and reduces soil cadmium bioavailability to promote health wheat growth and yield

Soil amendments of black bone (BB), biochar (BC), silicon fertilizer (SI), and leaf fertilizer (LF) play vital roles in decreasing cadmium (Cd) availability, thereby supporting healthy plant growth and food security in agroecosystems. However, the effect of their additions on soil microbial community and the resulting soil Cd bioavailability, plant Cd uptake and health growth are still unknown. Therefore, in this study, BB, BC, SI, and LF were selected to evaluate Cd amelioration in wheat grown in Cd-contaminated soils. The results showed that relative to the control, all amendments significantly decreased both soil Cd bioavailability and its uptake in plant tissues, promoting healthy wheat growth and yield. This induced-decrease effect in seeds was the most obvious, wherein the effect was the highest in SI (52.54%), followed by LF (43.31%), and lowest in BC (35.24%) and BB (31.98%). Moreover, the induced decrease in soil Cd bioavailability was the highest in SI (29.56%), followed by BC (28.85%), lowest in LF (17.55%), and BB (15.30%). The significant effect in SI likely resulted from a significant increase in both the soil bioavailable Si and microbial community (Acidobacteria and Thaumarchaeota), which significantly decreased soil Cd bioavailability towards plant roots. In particular, a co-occurrence network analysis indicated that soil microbes played a substantial role in wheat yield under Si amendment. Therefore, supplying Si alters the soil microbial community, positively and significantly interacting with soil bioavailable Si and decreasing Cd bioavailability in soils, thereby sustaining healthy crop development and food quality.

The regulating effects and mechanism of biochar and maifanite on copper and cadmium in a polluted soil-Lolium perenne L. system

Arable land polluted by copper (Cu) and cadmium (Cd) is a widespread problem. The use of biochar and/or clay mineral as a soil amendment can effectively solidify heavy metals in the soil. We applied biochar (BC), iron modified biochar (Fe-BC), maifanite (MF, a kind of clay minerals), a combination of BC with MF (BC:MF), and Fe-BC with MF (Fe-BC:MF) at a 2 wt % dose as soil amendments to study their ability to prevent Cu and Cd from accumulating in ryegrass (Lolium perenne L.). We found that after 90 days of cultivation, the Cd and Cu content both significantly decreased in ryegrass shoots from 2.06 and 209.3 mg kg⁻¹ (control) to 1.44–2.01 and 51.50–70.92 mg kg⁻¹, respectively, across treatments (p < 0.05). Similarly, the bioconcentration factor (BCF) for Cd/Cu was significantly smaller (P < 0.05) in all amendments versus control soil. This trend differed among the shoot, BCF, and transportation factor (TF). Combining BC:MF or Fe-BC:MF did not significantly improve the Cd/Cu stabilization in the soil compared to the corresponding single amendment (p > 0.05). Our adsorption balance experiment showed that BC, Fe-BC, and MF physically and chemically adsorbed Cd and Cu by complexation with functional groups (mesoporous nanomaterials) whose porosity measurements ranged from 0.68 to 78.57 m² g⁻¹. Furthermore, the amorphous crystalline iron oxide binding Cd and Cu was the key to immobilizing these metals in the soil. The amendments applied in our study show promise for enhancing immobilization of Cu and Cd in contaminated paddy soils.

Effects of an additive (hydroxyapatite-bentonite-biochar) on Cd and Pb stabilization and microbial community composition in contaminated vegetable soil

A two-year pot experiment was conducted with a pimiento-celery cabbage (Capsicum annuum L.-Brassica pekinensis) rotation in acidic soil contaminated with Cd and Pb, which was amended with 0.0, 1.0, 2.5, 5.0 and 10.0% (w/w) premixtures of hydroxyapatite, bentonite and biochar combinations (HTB, in a ratio of 1 : 2 : 2). The results showed that the application of HTB at 2.5-10.0% significantly increased soil pH and organic carbon by an average of 10.38-17.60% and 35.60-55.34% during the two years, respectively. Compared to the control treatment, 1.0-10.0% HTB decreased the available Cd and Pb concentrations by 40.92-77.53% and 41.60-82.79% on average, respectively. In addition, the diversity and richness of the soil bacterial community improved after the two-year application of HTB. The relative abundances of Acidobacteria, Bacteroidetes and Chloroflexi increased under the HTB treatments, while those of Proteobacteria and Actinobacteria decreased. Redundancy analysis (RDA) and regression analysis indicated that soil pH and Cd and Pb availability were important factors shaping the soil bacterial community. The Cd and Pb concentrations in the edible parts of pimiento and celery cabbage decreased as the HTB application rate increased and met the Food Quality Standard in each season when the HTB application rate was 5.0% or higher. Higher rates of HTB (5.0% and 10.0%) not only ensured the quality of vegetables, but also significantly promoted pimiento and celery cabbage growth. Overall, these results indicated that the application of HTB, especially at a rate of 5.0%, could be an effective way to immobilize Cd and Pb, improve soil quality and ensure vegetables produced in acidic contaminated soil are safe for human consumption.

Effect of Maize Straw Biochar on Bacterial Communities in Agricultural Soil

Biochar has become a popular soil amendment. However, its effect on soil microbial community is still unclear. In the present study, maize straw biochar was pyrolysed at 300°C, 450°C and 600°C, respectively, and then was added to agricultural soil at the ratio of 0.5%, 1% and 2%. Bacterial dynamics was analyzed in the pot experiments using denaturing gradient gel electrophoresis. The results indicated that the pyrolysis temperature has great impact on the elemental composition, pH and porous structures of biochar. Moreover, pyrolysis temperature was primary factor to drive the variation of bacterial community structure in biochar amended soil. In addition, the results suggested that biochar amendments on agricultural soil would decrease the bacterial diversity, and selectively promote growth of functional bacteria to become the dominant community, which could increase the bacterial community organization and improve the stability of bacteria to counteract effects of perturbation.