Enhanced Se enrichment and Cd reduction in rice (Oryza sativa L.) by Zr-Mn bimetallic modified biochar in Se-rich and Cd-contaminated soil

The development of effective amendments for the safe cultivation of rice ( Oryza sativa L.) in selenium-cadmium co-contaminated paddy soils poses a significant challenge. This study synthesized a novel Zr-Mn bimetallic modified peanut shell biochar (ZM@PSB) and evaluate its effects on soil health and the accumulation of selenium (Se) and cadmium (Cd) in rice. Characterization revealed that the modification produced a hierarchical porous structure with uniformly dispersed amorphous Zr-Mn (oxy)hydroxides and a wealth of oxygen-containing functional groups, which contribute to its remediation efficacy. The application of ZM@PSB as a soil amendment improved physicochemical properties in a dose-dependent manner, enhanced the availability of carbon, nitrogen, and phosphorus nutrients, increased microbial biomass, and stimulated enzyme activities. This shift transitioned key regulatory factors from a water-dominated regime to a synergistic pH-EC-Eh environment. ZM@PSB immobilized Cd primarily through complexation and precipitation, resulting in a reduction of bioavailable Cd by 24.37–44.61%. Simultaneously, it mobilized residual Se via manganese (Mn) redox cycling and microbial mineralization. In rice, ZM@PSB promoted the formation of root iron plaques, which served as a barrier to Cd translocation and facilitated Se uptake. Additionally, it enhanced the antioxidant system to alleviate Cd-induced oxidative stress. Consequently, the application of high-dose ZM@PSB increased grain Se concentration by 35.88%, with the proportion of organic selenium species (SeCys and SeMet) in rice grains up to 90.55%, while reducing grain Cd by 32.18%. This reduction was achieved through regulated bioaccumulation and translocation. This study demonstrates that ZM@PSB effectively optimizes the soil-plant system, providing a promising strategy for the synergistic production of Se-enriched and Cd-safe rice in contaminated paddies.