Suspended sediment (SPS) triggers nitrogen retention by altering microbial network stability and electron transport behavior during the aerobic-anoxic transition

NO₃^--N transformation, the vital biological process, determines nitrogen removal and retention in aquatic environment. Suspended sediment (SPS) ubiquitous in freshwater ecosystems can accelerate the transitions from aerobic to anoxic states, inevitably impacting NO₃^--N transformation. To elaborate on the microbial mechanism by which SPS content affected NO₃^--N transformation, we explored nitrogen removal and retention, microbial communities, co-occurrence networks, and electron transfer behavior under different SPS content during the aerobic-anoxic transition. We found that higher SPS concentration obviously increased NO₃^--N transformation rates but slightly affected TN removal, as the optimal SPS concentration boosting dissimilatory nitrate reduction to ammonium (DNRA) helped retain nitrogen during the transition. Microbial analysis suggested that the up-regulated SPS content slightly affected dominant bacteria abundance while progressively enhancing the essentiality of deterministic selection in microbial assembly and making microbial network more stable. Further investigations indicated that SPS content indirectly affected nitrogen retention via altering microbial network stability and electron transport system activity (ETSA) rather than bacterial abundance. Notably, elevating ETSA caused by SPS content directly promoted the potential for NO₃^--N being transformed through DNRA, enhancing nitrogen accumulation during the aerobic-anoxic transition. These results would provide supporting theories for the ecological restoration of micro-polluted water with higher SPS content.