Chitosan-functionalized biochar modulates arsenic speciation, distribution, and stress tolerance during rice growth in contaminated paddy systems: role of natural aging

Abstract

This study systematically evaluated the performance and underlying mechanisms of chitosan-functionalized biochar in arsenic (As)-contaminated soil–rice systems, with particular emphasis on the effects of natural aging. Specifically, four biochar-based amendments were prepared: fresh biochar (BC), chitosan-functionalized biochar (CBC), naturally aged biochar (NBC), and naturally aged chitosan-functionalized biochar (NCBC). Chitosan functionalization and natural aging collectively increased the abundance of oxygen/nitrogen-containing functional groups (e.g., C = O and N–H/O–H) and modified biochar surface properties. The CBC decreased soil-available As by 21.1%, whereas BC and NBC increased As bioavailability in soils. Notably, both CBC and NCBC decreased As accumulation in rice roots (48.1% vs. 37.1%) and grains (43.1% vs. 32.8%) compared to the control. Subcellular fractionation showed that both CBC and NCBC enhanced As sequestration in root and leaf cell walls, while reducing its distribution in soluble fractions and organelles. Micro-X-ray fluorescence (μ-XRF) analysis showed strong spatial As–Fe co-localization, indicating that Fe-mediated sequestration was associated with As stabilization, particularly for aged chitosan-functionalized biochar. Additionally, chitosan-functionalized biochars reduced the levels of glutathione and proline while increasing low-molecular-weight organic acids. Structural equation modeling suggested root architecture as an important factor in rice growth, and implied that As-induced root impairment may represent a pathway linking As stress to yield reduction. While natural aging attenuated CBC’s As immobilization performance in soils, the material maintained a significant ability to promote root growth, modulate phytohormones, and control As uptake. These findings suggest that chitosan-functionalized biochar may serve as a promising approach for mitigating As contamination in paddy soils, warranting further field-scale validation.

Graphical Abstract

Keywords

Aged biochar / Heavy metal bioavailability / Modified biochar / Subcellular distribution / Root exudate

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Funding

Key Laboratory for the Synthesis and Application of Organic Functional Molecules, Ministry of Education(2025C02097)

the National Natural Science Foundation of China(42507035)

Key Laboratory of Agricultural Information Service Technology(NO. CATASCXTD202303)

Central Public-interest Scientific Institution Basal Research Fund, Chinese Academy of Fishery Sciences(NO. 1630022024001)

Innovation Research Project for Youth Scholar of School of Environment and Natural Resources, Zhejiang University of Science and Technology(HZQY202401)

Zhejiang Key Laboratory of Soil Remediation and Quality Improvement, Zhejiang A&F University(2026SRQI01)

Shenyang Young and Middle–aged Science and Technology Innovation Talent Support Program(RC240417)

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