Nitrifiers Drive Different N ₂ O Production Patterns in Tropical River Sediments

Since the 20th century, global riverine nitrous oxide (N 2 O) emissions have increased 4-fold; however, the N 2 O emissions of tropical rivers are still unclear. Here, we employed a series of techniques (closed chamber, biological inhibitor, 15 N– 18 O double tracer, metagenomic sequencing, and reverse transcription qPCR) to analyze in situ N 2 O flux, potential N 2 O production rate, and N 2 O production mechanism of China’s tropical rivers. In the 82 sediment samples from the top 10 Hainan rivers, high levels of in situ N 2 O flux and potential N 2 O production rate were detected in all samples, indicating that Hainan rivers are significant hotspots of N 2 O 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 N 2 O 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 N 2 O via the ND pathway in estuary samples, and AOB Nitrosomonas europaea produces N 2 O via the ND pathway in nonestuary samples. Salinity, NH 4 +, pH, and total organic matter (TOM) affect N 2 O 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.