Selenite versus selenium nanoparticles in foliar application: enhancing alfalfa (Medicago sativa L.) and Bacillus subtilis PSL-1 mediated remediation of pyrene-contaminated soil

Polycyclic aromatic hydrocarbons (PAHs) pose a significant threat to the ecological environment, and plant–microbe remediation represents a promising strategy for the treatment of contaminated soils. This study aimed to evaluate the effects of foliar application of selenite or selenium nanoparticles (SeNPs) on the remediation efficiency of pyrene by Alfalfa ( Medicago sativa L.) and Bacillus subtilis PSL-1. A pot experiment was conducted with foliar selenium treatments at different concentrations (5–15 mg kg −1 ), and the effects were comparatively analyzed in terms of plant physiological parameters (chlorophyll content, biomass, and antioxidant enzyme activity), soil enzyme activities (dehydrogenase and catalase), soil pyrene speciation, and microbial community structure. The results showed that low-concentration foliar selenium application (5 mg L −1 ) enhanced antioxidant defense, alleviated oxidative stress, and promoted chlorophyll synthesis and growth of Alfalfa, thereby stimulating soil enzyme activity and enriching pyrene-degrading microorganisms. Consequently, the pyrene degradation rate increased from 62.07% to 67.83–71.91%. In contrast, high selenite concentration (15 mg L −1 ) induced phytotoxicity and weakened the stimulation of soil enzyme and microbial activities. Compared with selenite, SeNPs consistently enhanced the remediation efficiency across all tested concentrations, demonstrating a broader safe concentration range. Overall, foliar selenium application effectively promoted plant–microbe remediation of pyrene-contaminated soil, and SeNPs exhibiting a superior promoting effect and higher application potential than selenite.