• • Magnetic fields (SMF) enhance total nitrogen removal by 10–46% while reducing Shannon diversity by 4.5–22.4% across various bioreactor configurations, indicating a consistent performance–diversity decoupling that challenges conventional ecological paradigms.
• • In a fungal biofilter treating trichloroethylene, a 20 mT SMF increased removal efficiency by 13.5% while Shannon diversity dropped by 38.7%, demonstrating that moderate field strengths can maximize functional gains with minimal diversity loss.
• • For azo dye decolorization, SMF at 24.6–95 mT boosted decolorization efficiency by 1.3–1.4 times, with bacterial diversity decreasing only 3.9–6.4%, while fungal diversity plummeted by over 95% at higher intensities, revealing species-specific susceptibility.
• • In anammox systems, a 65 mT SMF shortened startup time by 16.7 days with negligible diversity loss (0.6%), underscoring the potential for targeted enrichment of slow-growing autotrophs without destabilizing the community.