This study investigated the remediation effects of iron-modified biochar (FeBC-1 and FeBC-2) on arsenic (As) and cadmium (Cd) co-contaminated paddy soil and elucidated the underlying mechanisms from the perspectives of rhizosphere microbial ecology and plant As and Cd accumulation. A pot experiment with rice was conducted, comprising a control (CK) and iron-modified biochar treatments (FeBC-1 and FeBC-2). Parameters such as As and Cd speciation in rhizosphere soil, bacterial community composition, and the abundance of As-related functional genes were analyzed. The results demonstrated that iron-modified biochar reduced As and Cd accumulation in rice grains by promoting the formation of iron plaques on root surfaces. Meanwhile, the iron-modified biochar significantly enhanced the alpha diversity of bacterial communities and altered their composition. Quantitative analysis of functional genes revealed that the abundance of the As oxidase gene (aioA) increased from 3.54 × 10⁵ in the CK treatment to 7.20 × 10⁵ in FeBC-1 and 7.14 × 10⁵ in FeBC-2, and the abundance of the As efflux gene (arsA) decreased in the biochar-treated groups. These results indicate reduced As bioavailability in the rhizosphere and enhanced transformation of As(III) to As(V).

Carbon-based nanomaterials have emerged as promising tools for addressing major challenges in sustainable agriculture, including declining soil fertility, climate change, resource inefficiency, and increasing environmental pollution. Among these materials, carbon nanotubes (CNTs) and carbon quantum dots (CQDs) have attracted considerable attention because of their unique physicochemical properties and diverse interactions with plants and soil systems. This review provides a comprehensive comparative analysis of CNTs and CQDs, emphasizing how their structural characteristics govern environmental fate, plant uptake, physiological responses, and stress mitigation mechanisms. CNTs primarily function as one-dimensional nanostructures that improve soil properties, facilitate nutrient delivery, and immobilize environmental contaminants, whereas CQDs, owing to their ultrasmall size, excellent water dispersibility, and intrinsic fluorescence, actively regulate plant metabolism, photosynthesis, nutrient acquisition, and antioxidant defense. Their distinct transport pathways, rhizosphere interactions, and subcellular localization are critically evaluated alongside recent advances in synthesis, surface functionalization, and physicochemical modification. The review further summarizes current evidence regarding their roles in enhancing tolerance to heavy metal toxicity, salinity, and drought stress through modulation of reactive oxygen species scavenging, osmotic regulation, ion homeostasis, and stress-responsive signaling pathways. Potential phytotoxicity, environmental persistence, ecological risks, and green synthesis strategies are also discussed to provide a balanced assessment of their agricultural applications. Emerging opportunities for synergistic CNT–CQD composite systems are discussed, together with major knowledge gaps and future research priorities, including machine learning-assisted nanomaterial design, multi-omics characterization, long-term field validation, and life cycle-based risk assessment. This review establishes a conditional, context-dependent structure–behavior–function conceptual framework that provides theoretical guidance for the rational design and safe implementation of carbon nanomaterials in next-generation sustainable agriculture.

Three biodegradable organic acids, citric acid (CA), malic acid (MA), and oxalic acid (OA), were evaluated for their ability to remove cadmium (Cd), lead (Pb), and copper (Cu) from contaminated soils. The effects of organic acid concentration, solution pH, and treatment time on metal removal were systematically investigated. Response surface methodology (RSM) was used to optimize these parameters. Sequential extraction was performed to track changes in heavy metal speciation. Under single-factor conditions (75 mmol/L CA, pH 5.0, 60 min), the removal efficiencies were 12.81% for Cd, 10.36% for Pb, and 14.94% for Cu, respectively. Under the optimized conditions (70 mmol/L, pH 5.0, 100 min), the removal efficiencies were further enhanced. The organic acids preferentially targeted bioavailable fractions (water-soluble, exchangeable, and carbonate-bound), which lowered ecological risk. Although CA was less efficient than chemical chelators such as EDTA, it caused much less nutrient loss. Organic acids, especially CA, provide an environmentally friendly alternative for heavy metal extraction with minimal side effects on soil fertility. They represent a promising low-impact option under the tested laboratory conditions. Nevertheless, the absolute removal values in a single washing step remained below 20% for all three metals, indicating that while the method is sustainable and eco-friendly, it is not suited for heavily contaminated soils as a standalone treatment.