Biochar aging-driven cadmium release in paddy soils: Mechanistic insights into adsorption capacity loss and biochar...

Biochar application in Cd-contaminated paddy soils relies heavily on its inherent capacity for direct Cd adsorption. However, the long-term stability is critically challenged by Cd re-mobilization during biochar aging. Thus, we conducted a six-season pot experiment to investigate the temporal Cd release patterns under rice cultivation conditions from four types of Cd-loaded biochar: sewage sludge biochar (Cd-SSBC), palm fiber biochar (Cd-PFBC), pig manure biochar (Cd-PMBC), and rice straw biochar (Cd-RSBC). Our findings revealed three key phases of Cd release dynamics: (1) initial rapid Cd release (first growing season) exhibited strong material specificity, following the order: Cd-SSBC (61.59 %) > Cd-PFBC (48.10 %) > Cd-PMBC (27.82 %) > Cd-RSBC (28.90 %), (2) mid-term (second to the fourth growing season) slow release phase where pH decline (ΔpH = -0.55 to 0.81) showed significant positive correlation with Cd release (r = 0.75, p < 0.001), and (3) long-term equilibration phase after six seasons rice with cumulative Cd release reaching 85.56 %-93.92 %. Results also demonstrated distinct stability profiles among Cd-binding forms at initial aging phase: hydrogen bonding (Q_hyd-Cd) and complexed (Q_com-Cd) species exhibited greater persistence compared to physically adsorbed (Q_phy-Cd) and exchangeable (Q_exc-Cd) fractions at initial aging phases. Notably, inter-biochar variability in total Cd release rates decreased from 33.8 % (S1) to 8.4 % (S6), while Cd form consistency increased across all biochar types. This study provides critical evidence for biochar aging-induced pH decline as the primary driver of Cd re-mobilization, and material-specific Cd release patterns. It also suggests that RSBC/PMBC may exhibit superior stability for extended applications in Cd-contaminated agroecosystems.