Nitrifiers DriveDifferent N2O ProductionPatterns in Tropical River Sediments

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.