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

Cadmium (Cd) and arsenic (As) co-contamination in paddy soils poses a major challenge for rice safety. Humic substances have the potential to regulate the mobility of these metal(loid)s, yet the distinct effects of different humic substances on Cd/As concentrations, distribution in rice and dietary health risks remain unclear. Hence, a pot experiment was conducted using a historically mining-impacted Cd/As co-contaminated paddy soil to compare the effects of two commercial amendments, fulvic acid (FA) and humic acid (HA), under carbon-equivalent application rates on Cd/As extractability and concentration-based distribution across the soil–root surface-grain continuum and the associated potential dietary risk. Each treatment consisted of three replicate pots, with each pot serving as the experimental unit (39 pots in total). The results showed that adding FA and HA decreased CaCl2-Cd by 24.64–67.63%, while increasing the KH2PO4-As by 15.83–44.55%. This reduction in Cd extractability was accompanied by an increase in Fe/Mn oxide-bound Cd. Moreover, the increase in extractable As was associated with increases in soluble and Al-bound As, and a decrease in Fe-bound As. These changes were associated with increased organic carbon, lower Eh, and changes in Fe oxide fractions. The FA and HA additions also decreased grain Cd concentrations to 0.05–0.30 mg kg−1, corresponding to reductions of 30.95–87.68%. However, grain As concentrations showed amendment- and dose-dependent responses, ranging from 0.48 to 0.62 mg kg−1 and decreasing by 10.50–12.47% at higher FA rates but increasing by 12.09–14.21% under some HA treatments. These contrasting grain As responses were associated with changes in plaque-associated As and concentration-based transfer factors, suggesting altered As distribution among rice tissues. Correlation and Random Forest analyses indicated that DOC, metals in iron plaque, and pH were the variables most strongly associated with grain Cd/As variation in this study. Because inorganic As was not determined, total grain As was used as a conservative surrogate for inorganic As in the screening-level health risk assessment. Under this assumption, HQ-As was the dominant contributor to HI, and all HI values remained greater than 1, despite reductions in HQ-Cd. Overall, both amendments reduced grain Cd concentration, but their combined Cd/As responses varied across application rates. Higher FA rates reduced both grain Cd and grain As, whereas intermediate HA rates increased grain As despite reductions in grain Cd. These findings demonstrate amendment- and dose-dependent Cd/As trade-offs across the soil–rice continuum and highlight that increased soil As extractability does not necessarily translate into increased grain As concentration.