Remobilization risk of Cd in aged pig manure biochar under rice rhizosphere influence: Evidence from six-season cultivation

Biochar is widely used to immobilize cadmium (Cd) in contaminated paddy soils; however, the long-term stability of biochar-adsorbed Cd under rice rhizosphere influence remains poorly understood, raising concerns about its potential secondary release. Herein, we investigated the remobilization behavior and speciation evolution of Cd from Cd-loaded pig manure biochar (Cd-PMB) using a six-season rice pot experiment, combined with a sequential simulated aging procedure designed to decouple the contributions of soil solution chemistry, root exudates, and oxidative processes. Two rice cultivars with contrasting Cd accumulation capacities were employed. Total Cd in Cd-PMB decreased by 84.3%–89.2% after six seasons, with >50% of the loss occurring during the first season, driven predominantly by cation exchange and rhizosphere acidification. Speciation analysis revealed that the ion-exchangeable Cd fraction was almost completely released (>95%) irrespective of planting treatment, while physically adsorbed and hydrogen-bonded Cd were preferentially remobilized under rhizosphere influence. Notably, complexed Cd exhibited a dual-phase dynamic, which initially increased by 8.1%–19.3% through secondary complexation onto newly formed carboxyl sites and subsequently declined during prolonged aging as these sites underwent protonation and oxidative degradation, as confirmed by Fourier‐transform infrared and X‐ray photoelectron spectroscopy analyses and further supported by significant Pearson correlations between COOH/ C O 3 2 − and Q com -Cd (r = 0.801, p < 0.05). The simulated aging experiments confirmed that soil solution, root exudates, and oxidative aging synergistically promote Cd release from distinct binding pools. Our findings demonstrate that biochar-adsorbed Cd undergoes substantial, binding form–dependent remobilization during aging, posing a secondary risk that necessitates explicit consideration in long-term safety assessments of biochar-amended soils.