Combined application of calcium fertilizer improves the immobilization of biochar-ferromanganese materials on cd/as co-polluted paddy soils under acidifying conditions.

Soil acidification is an increasingly serious issue in southern China. A decrease in soil pH tends to interfere with the immobilization of heavy metals by amendments. Here, we investigated the effect of soil acidification on the remediation of Cd/As co-contaminated soil by biochar-ferromanganese material (BFM) and sought effective measures to alleviate these adverse effects by applying different calcium fertilizers. The results showed that BFM effectively promotes the transformation of Cd and As in soil to a stable state, reducing the available Cd and As content by 42.7%-64.3% and 8.1%-13.6%, respectively. However, soil acidification negatively impacted the ability of BFM to immobilize Cd and As, especially Cd. Specifically, the proportion of unstable Cd in soil treated with different levels of BFM increased by 16.1%-49.1% under acidifying conditions. Importantly, when compared to BFM alone, the combined application of BFM and calcium fertilizer promoted the transformation of Cd and As to more stable forms in the soil under acidifying conditions, particularly for Cd, with its percentage in the residual state increasing from 63.0%-64.6% to 75.1%-87.0%. Therefore, the use of BFM in conjunction with calcium fertilizer may represent a viable strategy for managing Cd/As co-contaminated soils, with optimal results achieved using 1% BFM alongside 50 kg/acre calcium magnesium phosphate fertilizer.