Influence of aged silicon-rich biochars (Si-chars) on leaf Cd accumulation in pakchoi (Brassica rapa subsp. chinensis): a pyrolysis temperature-dependent response

Abstract Silicon-rich biochars (Si-chars) have demonstrated effectiveness in heavy metal remediation. However, the evolution of their functionality during environmental aging remains poorly understood. Here, we investigated the effects of artificial aging on rice husk-derived Si-chars pyrolysis at 300 ℃, 500 ℃, and 700 ℃, further evaluating their efficacy in mitigating cadmium (Cd) toxicity in soil-pakchoi systems. Aging induced a temperature-dependent response, which reduced the performance of high-temperature Si-chars but acted as an activation process for low-temperature variants. Notably, aged 300 ℃ Si-char exhibited the strongest suppression of Cd accumulation in pakchoi leaves, reducing concentrations by 27.1% and 15.6% compared to the control and non-aged 300 ℃ Si-char, respectively. This effect was attributed to the aging-induced release of bioavailable Si (ASi) and high-molecular-weight dissolved organic matter (DOM), both of which could interact with Cd to reduce its bioavailability in the amended soils. Meanwhile, ASi and DOM promoted the activities and enrichment of Cd-immobilizing bacteria. Furthermore, Si-char amendment enhanced Si deposition in pakchoi leaves, triggering a molecular defense network characterized by the down-regulation of Cd transporter genes and the up-regulation of stress-responsive pathways. Our findings establish a novel framework in which feedstock Si concentration and initial pyrolysis temperature jointly drive the functional evolution of biochar in the environment. Graphical Abstract