Cost-effective enhanced iron bioavailability in rice grain grown on calcareous soil by sulfur mediation and its effect on heavy metals mineralization

Iron enriched quinoa biochar enhances Nickel phytoremediation potential of Helianthus annuus L. by its immobilization and attenuation of oxidative stress: implications for human health

The present study was performed to assess Ni-immobilization and the phytoremediation potential of sunflower by the application of quinoa stalks biochar (QSB) and its magnetic nanocomposite (MQSB). The QSB and MQSB were characterized with FTIR, SEM, EDX, and XRD to get an insight of their surface properties. Three-week-old seedlings of sunflower were transplanted to soil spiked with Ni (0, 15, 30, 60, 90 mg kg-1), QSB and MQSB (0, 1, and 2%) in the wire house under natural conditions. The results showed that increasing Ni levels inhibited sunflower growth and yield due to the high production of reactive oxygen species (ROS) and lipid peroxidation. Enzyme activities like superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and peroxidase (POX) also increased as Ni levels increased. However, the application of QSB and MQSB reduced Ni uptake, root-shoot, and shoot-seed translocation and decreased the generation of ROS, and lowered the activity of SOD, CAT, APX, and POX, leading to improved growth and yield, especially with MQSB. This was verified through SEM, EDX, XRD, and FTIR. It can be concluded that QSB and MQSB can effectively enhance Ni-tolerance in sunflowers and mitigate oxidative stress and human health risks.

Effect of biochar amendment on metal mobility, phytotoxicity, soil enzymes, and metal-uptakes by wheat (Triticum aestivum) in contaminated soils

The use of low-cost substances such as biochar could be a sustainable approach to reduce the mobility, accumulation, and toxic impact of heavy metals in crop systems. This study investigates the effect of biochar amendment on heavy metal (Cr, Cd, Cu, Pb, Ni, Zn, Mg and Fe) mobility, bioaccumulation factor (BAF), plant (wheat) metal-uptake, plant oxidative stress, and soil enzymatic profile in contaminated industrial soil. Biochar was obtained from slow pyrolysis of Lantana (LBC), and Parthenium (PBC) biomass, and applied at 3% rates in contaminated soils for wheat crop study under a greenhouse experimental setup. Results show in comparison with control setups, low mobility of Cr (14.15-16.35%), Cd (7.17-15.24%), Cu (9.81-12.97%), Pb (7.99-15.23%), Ni (1.52-2.38%), Zn (10.47-14.42%), Mg (48.85-52.89%), and Fe (19.13-19.90%) contents in soils. The heavy metal uptake rates were 63.08% (Cr), 78.07% (Cd), 74.61% (Cu), 78.11% (Pb), 75.73% (Ni), 69.71% (Zn), 28.78% (Mg), and 49.26% (Fe) lower in biochar amendments, compared with the control treatments. Similarly, the biochar amended treatments exhibited low oxidative stress in wheat plants than control setups. In addition, soil enzymes (dehydrogenase, β-glucosidase, alkaline phosphatase, and urease) alleviated in biochar amended soils indicating reduced toxicity of metals in experimental soils. In summary, this study indicates that biochar amendment in contaminated soils not only improves plant growth but also lowers the rates of soil and plant toxicity and metal bioavailability as well.

Effects of soil properties on heavy metal bioavailability and accumulation in crop grains under different farmland use patterns

Mining activities have increased the accumulation of heavy metals in farmland soil and in food crops. To identify the key soil properties influencing heavy metal bioavailability and accumulation in food crops, 81 crop samples and 81 corresponding agricultural soil samples were collected from rape, wheat, and paddy fields. Heavy metal (copper (Cu), zinc (Zn), lead (Pb), cadmium (Cd), iron (Fe), and manganese (Mn)) concentrations in soils and rape, wheat, rice grains were determined using inductively coupled plasma atomic emission spectroscopy, and soil physicochemical properties (pH, organic matter, total nitrogen, total phosphorus, available phosphorus, and available potassium (AK)) were analyzed. Soil extractable metals were extracted using various single extractants (DTPA, EDTA, NH₄OAc, NH₄NO₃, and HCl). The average concentrations of Cu, Zn, Pb, Cd, and Mn in the soil samples all exceeded the local geochemical background value (background values of Cu, Zn, Pb, Cd, and Mn are 43.0, 81.0, 28.5, 0.196, and 616 mg/kg, respectively), and Cd over-standard rate was the highest, at 98%. Furthermore, soil total Cd concentrations (0.1–24.8 mg/kg) of more than 86% of the samples exceeded the soil pollution risk screening value (GB 15618-2018). The sources of Cu, Zn, Pb, Cd, and Mn in soils were mainly associated with mining activities. The key factors influencing heavy metal bioavailability were associated with the types of extractants (complexing agents or neutral salt extractants) and the metals. Cd and Pb concentrations in most wheat and rice grain samples exceeded the maximum allowable Cd and Pb levels in food, respectively, and Cd concentrations in approximately 10% of the rice grain samples exceeded 1.0 mg/kg. Furthermore, rice and wheat grains exhibited higher Cd accumulation capacity than rape grains, and despite the high soil Cd concentrations in the rape fields, the rape grains were safe for consumption. High soil pH and AK restricted Cd and Cu accumulation in wheat grains, respectively. Soil properties seemed to influence heavy metal accumulation in rice grains the most.

Organic and inorganic amendments for the remediation of nickel contaminated soil and its improvement on Brassica napus growth and oxidative defense

In-situ stabilization has been considered an effective way to remediate metal contaminated soil. Thus, pot experiments were undertaken to investigate the effectiveness of multiple stabilization agents such as biochar (BC), mussel shell (MS), zeolite (ZE) and limestone (LS) on the immobilization of Ni, physicochemical features and enzyme activities in polluted soil. Results showed that the sole application of Ni adversely affected the rapeseed growth, photosynthetic pigments, and antioxidative defense. However, the addition of amendments to the contaminated soil significantly reduced Ni bioavailability. The XRD analysis confirmed the formation of Ni related ligands and FTIR showed the presence of hydroxyl, carboxyl and sulfur functional groups, as well as complexation and adsorption of Ni on amendments. Among multiple amendments, biochar significantly enhanced plant biomass attributes and total chlorophyll content. Moreover, addition of amendments also strengthened the antioxidant defense by decreasing Ni induced oxidative stress (H₂O₂ and O₂^.-), increased macronutrient availability, reduced Ni uptake and improved soil health. The qPCR analysis showed that the Ni transporters were significantly suppressed by amendments, which is correlated with the lower accumulation of Ni in rapeseed. The present study showed that immobilizing agents, especially biochar, is an effective amendment to immobilize Ni in soil, which restricts its entry into the food chain.

Improvement in productivity, nutritional quality, and antioxidative defense mechanisms of sunflower (Helianthus annuus L.) and maize (Zea mays L.) in nickel contaminated soil amended with different biochar and zeolite ratios

Nickel (Ni) contaminated soils pose a potential ecological risk to the environment, soil health, and quality of food produced on them. We hypothesized that application of miscanthus biochar (BC) and cationic zeolite (ZE) at various proportions into a Ni contaminated soil can efficiently immobilize Ni and reduce its bioavailability to sunflower (Helianthus annuus L.) and maize (Zea mays L.). An electroplating effluent contaminated soil was amended with BC and ZE, as sole treatments (2% w/w) and their combinations of various ratios (BC, ZE, BC25%ZE75%, BC50%ZE50% and BC75%ZE25%) for immobilization of Ni in the soil. Furthermore, the associated effects of these treatments on residual and DTPA-extractable Ni from the soil; concentrations of Ni in shoots, roots, and grain; growth, physiology, biochemistry and the antioxidant defence mechanisms of sunflower and maize were investigated. Results revealed that BC50%ZE50% treatment efficiently reduced DTPA-extractable Ni in the soil, Ni concentrations in shoots, roots, and grain, while improved selective parameters of both plants. Interestingly, the BC75%ZE25% treatment significantly improved the biomass, grain yield, physiology, biochemistry and antioxidant defense machinery, while decreased Ni oxidative stress in both sunflower and maize, compared to rest of the treatments. The results demonstrate that the BC50%ZE50% treatment can efficiently reduce Ni concentrations in the roots, shoots and grain of both sunflower and maize whereas, an improvement in biomass, grain yield, physiological, biochemical, and antioxidant defense machinery of both crops can only be achieved with the application of BC75%ZE25% treatment in a Ni contaminated soil.

Assessment of nickel bioavailability through chemical extractants and red clover (Trifolium pratense L.) in an amended soil: Related changes in various parameters of red clover

Application of immobilizing agents may efficiently reduce the bioavailability of nickel (Ni) in the soil. Here we report the effect of biochar (BC), gravel sludge (GS) and zeolite (ZE) as a sole treatment and their combinations on the bioavailability of Ni after their application into a Ni-polluted soil. The bioavailability of Ni after the application of immobilizing agents was assessed through an indicator plant (red clover) and chemical indicators of bioavailability like soil water extract (SWE), DTPA and Ca(NO₃)₂ extracts. Additionally, the effects of Ni bioavailability and immobilizing agents on the growth, physiological and biochemical attributes of red clover were also observed. Application of ZE significantly reduced Ni concentrations in all chemical extracts compared to rest of the treatments. Similarly, the combined application of BC and ZE (BC+ ZE) significantly reduced Ni concentrations, reactive oxygen species (ROS) whereas, significant enhancement in the growth, physiological and biochemical attributes along with an improvement in antioxidant defence machinery of red clover plant, compared to rest of the treatments, were observed. Furthermore, BC+ ZE treatment significantly reduced bioconcentration factor (BCF) and bioaccumulation factor (BAF) of Ni in red clover, compared to rest of the treatments. The Ni concentrations in red clover leaves individually reflected a good correlation with Ni concentrations in the extracts (SWE at R²=0.79, DTPA extract at R²=0.84 and Ca(NO₃)₂ extracts at R²=0.86). Our results indicate that combined application of ZE and BC can significantly reduce the Ni bioavailability in the soil while in parallel improve the antioxidant defence mechanism in plants.

Potential of miscanthus biochar to improve sandy soil health, in situ nickel immobilization in soil and nutritional quality of spinach

The complex interaction of biochar (BC) with soil health reflecting properties, the feedstock used to prepare BC and application rate of BC in sandy soil is still a question for the researchers. An incubation study was conducted where nine different sorts of BC, each prepared from the different feedstock, were applied at 2% rate to evaluate their relative suitability to improve sandy soil health. Results revealed that BC prepared from miscanthus (MIB) significantly increased soil medium and fine pores, available water content (AWC), electrical conductivity (EC), and cation exchange capacity (CEC) while decreased soil wide pores, pH, bulk density (BD) and particle density (PD) compared to the rest sorts of BC. Later, spinach was grown in pots containing same soil but spiked with 50 ppm nickel (Ni) and amended with 1, 2, 3, 4 and 5% rates of MIB. The results showed a significant increment in spinach biomass, reduction in the concentrations of Ni in spinach tissues and DTPA-extractable Ni with the increasing rate of MIB till 3% and later, no significant changes with 4 and 5% rates thereafter. However, significant improvement in the activities of antioxidant enzymes, chemical and biochemical attributes of spinach were observed at 5% MIB when compared to lower rates. Similarly, post-harvest soil physicochemical and enzymatic parameters were also significantly (P < 0.05) improved with increasing rates of MIB. This study implies that application of MIB at 5% rate can improve the nutritional quality of spinach, sandy soil health and can reduce Ni concentrations in spinach tissues.