Fertilization and intercropping reduce Pb accumulation in plants by influencing rhizosphere soil phosphorus forms in soil-plant systems

Abstract

Fertilization and planting practices in crop systems have become important ways to reduce the uptake of heavy metals in polluted soils. However, the relative effectiveness of different management modes and the underlying mechanisms are not fully understood. In this study, we conducted experiments to assess how fertilization and planting modes affect the bioavailability of lead (Pb) in soil and its accumulation in plants. The results revealed that planting and fertilization modes significantly affected Pb speciation in rhizosphere soil and Pb accumulation characteristics in plants (p < 0.01). Specifically, fertilization with nitrogen–phosphorus–potassium (N-P-K) had stronger effects than did N-K fertilization in terms of a reduction in soil available Pb (APb) and an increase in both chemically stable Pb (CHS-Pb) and the immobilization efficiency of Pb (R). A significantly greater concentration of soil CHS-Pb was detected, at 39.29 mg/kg and 28.43 mg/kg, in the intercropping system under N-P-K and N-K fertilization (p < 0.05), respectively, compared with the control. In addition, the Pb concentration in plants and the enrichment coefficient under the N-P-K fertilization mode were significantly lower than those under the other fertilization modes (p < 0.05). Notably, P fertilization was found to be more effective than intercropping in reducing Pb accumulation in plants. With decreasing soil pH, Pb²⁺ was adsorbed or immobilized through exchange with cations in the soil, and Pb²⁺ further formed precipitates with Fe-P, H₂PO₄^- and Ca²⁺. Therefore, the changes in Pb speciation and the increases in Pb immobilization efficiency induced by changes in pH and P fractions within rhizosphere soil were the primary mechanisms governing the reduction in Pb accumulation in plants.