Coupled regulation mechanisms of carbon sequestration, microbial communities, and manganese stabilization in manganese-contaminated soil by phosphorus-modified biochar

Biochar is widely recognized for enhancing soil carbon sequestration and immobilizing heavy metals such as cadmium, zinc, and lead. However, research on its application in manganese-contaminated soils remains limited. In this study, Arundo donax L. collected from Mn-contaminated soil was impregnated with KH 2 PO 4 and subsequently pyrolyzed to produce phosphorus-modified biochar (PKBC). A controlled soil incubation experiment was conducted with PKBC applied at 1 %, 2 %, and 4 % to evaluate its effects on soil carbon accumulation, CO 2 emissions, bacterial community structure, and Mn speciation. Increasing PKBC rates significantly enhanced soil available phosphorus (AP), available potassium (AK), and the activities of sucrase (SUR) and urease (URE). Meanwhile, it promoted the accumulation of soil organic carbon (SOC) and its stable fractions, while cumulative CO 2 emissions decreased. Random forest analysis indicated readily oxidizable carbon (ROC) and cation exchange capacity (CEC) as the main drivers of CO 2 emissions. PKBC reshaped the bacterial community structure and shifted assembly mechanism from r- to K-selection, while the assembly process remained largely stochastic. Mantel test indicated that the M2, M3, and M5 modules in the bacterial co-occurrence network responded strongly to changes in soil physicochemical properties. In addition, PKBC appeared to promote the transformation of Mn forms into abiotically effective forms. They may provide a theoretical basis for the application of PKBC to enhance carbon sequestration and ecological restoration in manganese-contaminated soils. • Phosphorus-modified biochar improved properties and stable organic carbon storage. • Bacterial communities under phosphorus-modified biochar had stochastic assembly. • Phosphorus-modified biochar shifted bacterial communities from r- to K-strategists. • Phosphorus-modified biochar enhanced microbial Mn transformation and immobilization.