Effects of Humic Substance Amendments on Cadmium and Arsenic Availability, Concentrations, and Potential Health Risk in Rice

Overview

This pot experiment compared fulvic acid (FA) and humic acid (HA) amendments at seven carbon-equivalent application rates (0 to 5 g C per kg soil) in cadmium/arsenic co-contaminated paddy soil from a mining-impacted site in Hunan Province, China. The study measured soil extractability, root surface retention, tissue distribution, and grain concentrations of cadmium and arsenic, plus dietary health risk estimates.

Both FA and HA reduced CaCl2-extractable cadmium (by 24.64 to 67.63%) and grain cadmium (by 30.95 to 87.68%), but increased KH2PO4-extractable arsenic (by 15.83 to 44.55%). Grain arsenic responses were dose-dependent and differed between amendments: higher FA rates reduced grain arsenic (by 10.50 to 12.47%), while intermediate HA rates increased grain arsenic (by 12.09 to 14.21%). These findings demonstrate amendment- and dose-dependent cadmium/arsenic trade-offs.

Key numbers

Grain cadmium (mg/kg): control 0.43; FA treatments reduced to 0.05 to 0.30 mg/kg (reductions of 30.95 to 65.95%); HA treatments reduced to 0.05 to 0.30 mg/kg (reductions of 40.85 to 87.68%) (p. 1 abstract).

CaCl2-extractable cadmium: decreased by 24.64 to 51.05% under FA, by 25.12 to 67.63% under HA (p. 8).

KH2PO4-extractable arsenic: increased by 15.83 to 44.55% under both amendments (p. 1 abstract).

Grain arsenic: FA rates above 3 g C per kg decreased grain arsenic by 10.50 to 12.47%; intermediate HA rates (1 to 3 g C per kg) increased grain arsenic by 12.09 to 14.21% (p. 1 abstract).

Test soil: pH 6.03, total cadmium 2.07 mg/kg, total arsenic 147.06 mg/kg, SOC 19.86 g/kg (p. 3).

Evidence Fitness

EF-3 (modeled or limited evidence). Pot experiment under greenhouse conditions with artificially high soil contamination from a mining area. Grain-level cadmium and arsenic data are reported, but the soil contamination levels (2.07 mg/kg cadmium, 147 mg/kg arsenic) are much higher than typical agricultural soils, so reductions may not scale to lower-contamination settings. Demonstrates the cadmium/arsenic trade-off with humic amendments. Not representative of market rice.

Methods

Pot experiment, greenhouse, cadmium (Cd)/arsenic (As) co-contaminated paddy soil. Rice cultivar not named in abstract. ICP-MS for soil and tissue cadmium and arsenic; CaCl2 extraction for cadmium availability, KH2PO4 extraction for arsenic availability. Correlation and Random Forest analyses for variable importance.

LOD/LOQ: not reported in abstract. CRM/Recovery: not reported in abstract.

Implications

Certification: Demonstrates that humic amendments can reduce grain cadmium but may simultaneously increase grain arsenic, reinforcing the cadmium/arsenic trade-off known in rice production. Not directly applicable to HMT&C threshold-setting as it is a pot experiment with highly contaminated soil.

Courses: Useful for teaching the cadmium/arsenic trade-off in paddy soil amendments and the importance of evaluating both contaminants simultaneously.

Provenance notes

Open access (CC BY 4.0). MDPI journal (Toxics). Deposit: HMI-MIT-rice.

Related evidence

• Rice

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