Lei Zheng

Since the 20th century, global riverine nitrous oxide (N2O) emissions have increased 4-fold; however, the N2O emissions of tropical rivers are still unclear. Here, we employed a series of techniques (closed chamber, biological inhibitor, 15N–18O double tracer, metagenomic sequencing, and reverse transcription qPCR) to analyze in situ N2O flux, potential N2O production rate, and N2O production mechanism of China’s tropical rivers. In the 82 sediment samples from the top 10 Hainan rivers, high levels of in situ N2O flux and potential N2O production rate were detected in all samples, indicating that Hainan rivers are significant hotspots of N2O emissions. The higher values were observed in estuary samples (avg: 4.48 ± 0.25 mg m–2 d–1, 39.17 ± 3.28 ng N g–1 d–1) compared to nonestuary samples (avg: 1.98 ± 0.16 mg m–2 d–1, 21.29 ± 4.68 ng N g–1 d–1). Nitrifier denitrification (ND) dominates the N2O production, and its contribution to estuary samples (avg. 49.31–78.90%) is higher than that for nonestuary samples (avg. 32.27–66.19%). We found that complete ammonia-oxidizing bacteria (comammox) Nitrospira nitrificans and ammonia-oxidizing bacteria (AOB) Nitrosomonas marina cooperate to produce N2O via the ND pathway in estuary samples, and AOB Nitrosomonas europaea produces N2O via the ND pathway in nonestuary samples. Salinity, NH4+, pH, and total organic matter (TOM) affect N2O production via three key species. Our findings advance the mechanistic understanding of tropical rivers in the tropical N-cycle and global climate change. Ammonium fertilizer management and estuary ecological restoration should be prioritized in tropical river basins.

Denitrifying phosphorus removal (DPR) systems in wastewater treatment often face challenges from nitrite accumulation, which acts as both an electron acceptor and a metabolic inhibitor. The stability of these systems depends on the microbial community’s composition and adaptability, which can be disrupted under nitrite stress. Here, we distinguish generalist taxa with broad niches from specialist taxa with narrow but critical functions to understand how nitrite stress drives ecological transitions that impact nutrient removal. We analyzed DPR microbiomes under gradient nitrite stress (10, 20, and 25 mg/L) over 120 operational cycles, using metagenomics. At low nitrite (10 mg/L), phosphorus (TP: 92.67 ± 5.72%) removal remained stable, but high nitrite (25 mg/L) impaired phosphorus removal (67.21 ± 13.86%) after 60 cycles. Increasing the nitrite loading markedly suppressed DPR-associated carbon and phosphorus transformations. Under high nitrite stress (25 mg/L), phosphorus release and uptake declined by 3.32-fold and 6.12-fold, respectively. Glycogen consumption and accumulation reduced by 2.18-fold and 2.38-fold, respectively. PHB consumption exhibited a 1.44-fold decrease. Nitrite stress reduced community diversity but increased the niche breadth. Ecological niche analysis identified 16 generalists and 164 specialists, with high nitrite increasing specialist abundance by 30.77% while generalists declined by 25.77%. Evolutionary modeling suggested a shift from generalists to specialists under nitrite stress (tg → s = 11.28). Notably, Thauera aminoaromatica, carrying complete nitrogen and phosphorus pathways, increased significantly under high nitrite stress. These results reveal how nitrite stress drives microbial community transitions and associated functional shifts in DPR systems, providing mechanistic insights into guide strategies for maintaining nutrient removal performance.