Long-neglected contribution of nitrification to N2O emissions in the Yellow River.

Rivers play a significant role in the global nitrous oxide (N 2 O) budget. However, the microbial sources and sinks of N 2 O in river systems are not well understood or quantified, resulting in the prolonged neglect of nitrification. This study investigated the isotopic signatures of N 2 O, thereby quantifying the microbial source of N 2 O production and the degree of N 2 O reduction in the Yellow River. Although denitrification has long been considered to be the dominant pathway of N 2 O production in rivers, our findings indicated that denitrification only accounted for 18.3% (8.2%–43.0%) of the total contribution to N 2 O production in the Yellow River, with 50.2%–80.2% being concurrently reduced. The denitrification contribution to N 2 O production (R² = 0.44, p < 0.01) and N 2 O reduction degree (R² = 0.70, p < 0.01) were positively related to the dissolved organic carbon (DOC) content. Similar to urban rivers and eutrophic lakes, denitrification was the primary process responsible for N 2 O production (43.0%) in certain reaches with high organic content (DOC = 5.29 mg/L). Nevertheless, the denitrification activity was generally constrained by the availability of electron donors (average DOC = 2.51 mg/L) throughout the Yellow River basin. Consequently, nitrification emerged as the primary contributor in the well-oxygenated Yellow River. Additionally, our findings further distinguished the respective contribution of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to N 2 O emissions. Although AOB dominated the N 2 O production in the Yellow River, the AOA specie abundance (AOA/(AOA + AOB)) contributed up to 32.6%, which resulted in 25.6% of the total nitrifier-produced N 2 O, suggesting a significant occurrence of AOA in the oligotrophic Yellow River. Overall, this study provided a non-invasive approach for quantifying the microbial sources and sinks to N 2 O emissions, and demonstrated the substantial role of nitrification in the large oligotrophic rivers. [Display omitted] • Nitrification was the primary contributor to N 2 O in the well-oxygenated Yellow River. • DOC content determined the denitrification activity and N 2 O reduction degree. • Denitrification was generally constrained by electron donors in the Yellow River. • 50.2%–80.2% N 2 O was reduced to N 2 in the Yellow River. The quantification of the microbial sources to N 2 O emissions highlights the important role of nitrification to N 2 O emission in large rivers. [ABSTRACT FROM AUTHOR]