Simulated Environmental Aging Increases Cadmium Extractability in Acidic Soil Amended with Cadmium-Containing Rice-Straw Biochar

Biochar is widely used to remediate acidic soils, but the long-term risks from its endogenous heavy metals remain unclear. We hypothesized that simulated environmental aging would increase cadmium (Cd) extractability in biochar-amended acidic clean soils. Two rice-straw biochars derived from clean (CB) and polluted (PB) sites were prepared and Cd extractability were investigated in the two biochars and their amended acidic soils before and after accelerated simulated aging. Environmental aging was simulated by combination of the cycles of wetting–drying, freezing–thawing, acid rain, and fertilizer application. Results showed that aging significantly reduced biochar pH by 1.05–2.02 units and increased oxygen-containing functional groups, while decreased aromaticity and graphitization. These changes may have contributed to endogenous Cd release, as indicated by increase in water-soluble and CaCl2-extractable Cd by 19.9–150.7% and 26.1–109.9%, respectively. Biochar application at 5% rate raised soil pH by 1.12 units but increased total soil Cd by 81.3–120.3%. The 1–8 years of aging increased water-soluble (17.19–281.6 µg·kg−1) and CaCl2-extractable (37.07–279.78 µg·kg−1) Cd by 2.53–56.8-fold and 1.76–18.9-fold in amended soils, but only decreased the acid-soluble Cd by 13.7–24.0% for CB-treated soils. Collectively, the accelerated simulated aging equivalent to 8 years weakened pH amelioration, but increased Cd extractability. Our findings suggest that biochar application at such a high rate offers short-term pH benefits in acidic clean soils, but amplifies soil Cd extractability, highlighting the need for long-term field risk assessment.