Long-Term Nitrogen and Phosphorus Control and Microbial Responses by Tea-Bag-Encapsulated MgO-Modified Biochar at the Sediment–Water Interface

Internal phosphorus release from sediments is an important limiting factor in the remediation of eutrophic water bodies. Tea-bag-encapsulated MgO-modified biochar has advantages in phosphorus adsorption, microbial attachment, and convenient recovery, but its long-term phosphorus control effect and associated microbial responses at the sediment–water interface remain unclear. In this study, a 124-day sediment–water simulation experiment combined with metagenomic sequencing was conducted to evaluate its effects on nitrogen and phosphorus removal, water environmental changes, and microbial phosphorus-cycling functions. The results showed that the removal efficiencies of phosphate, total phosphorus, ammonia nitrogen, and total nitrogen in the treatment group reached 88%, 67%, 90%, and 40%, respectively. After application, the BET specific surface area of the material decreased from 47.85 to 20.18 m2/g, obvious surface deposits appeared, and phosphorus enrichment was detected, indicating that the material continuously participated in interfacial phosphorus fixation. Meanwhile, the relative abundances of Anaerolinea and Nitrospira increased from 5.71% to 10.08% and from 0.72% to 1.06%, respectively. Functional gene analysis showed that the relative abundances of genes related to phosphorus sensing, transport, and polyphosphate metabolism changed, suggesting that the functional potential related to phosphorus acquisition and storage was adjusted. Since this study was based on a small-scale long-term simulation system, the contribution of the material was mainly interpreted through the stage-specific differences between BC and BT, material characterization, and microbial functional responses, while the independent effects of time-varying factors such as temperature and redox conditions still require further controlled experiments. Overall, the sustained phosphorus control effect of tea-bag-encapsulated MgO-modified biochar was mainly associated with interfacial material fixation, microbial community changes, and adjustments in phosphorus cycling functional potential.