Responses of sediment nitrogen forms and bacterial communities to different aquatic nitrogen conditions in three submerged macrophyte-type ecological treatment systems

Ecological treatment system (ETS) has been recognized as a promising technology for mitigating agricultural non-point pollution, whereas it remains to be seen how nitrogen (N) forms and bacterial communities in ETS sediments respond to different aquatic N conditions. Therefore, a four-month microcosm experiment was conducted to investigate the effects of three aquatic N conditions (2 mg/L NH4+-N, 2 mg/L NO3  -N and 1 mg/L NH4+N + 1 mg/L NO3  -N) on sediment N forms and bacterial communities in three ETSs vegetated by Potamogeton malaianus, Vallisneria natans and artificial aquatic plant, respectively. Through analysis of four transferable N fractions, the valence states of N in ion-exchange and weak acid extractable fractions were found to be mainly determined by aquatic N conditions, while significant N accumulation was observed only in strong oxidant extractable and strong alkali extractable fractions. Sediment N profiles were primarily influenced by time and plant types, with N condition having secondary effect, while sediment bacterial community structures experienced a significant shift over time and were slightly influenced by plant types. Sediment functional genes related to N fixation, nitrification, assimilable nitrate reduction, dissimilatory nitrite reduction (DNRA) and denitrification were substantially enriched in month 4, and the bacterial co-occurrence network exhibited less complexity but more stability under NO3  condition compared to others. Furthermore, certain sediment N fractions were found to have strong relationships with specific sediment bacteria, such as nitrifiers, denitrifiers and DNRA bacteria. Our findings highlight the significant influence of aquatic N condition in submerged macrophyte-type ETSs on sediment N forms and bacterial communities.