The Xiamen Symposium on Marine Environmental Sciences (XMAS 2025)

Ammonia oxidation, mediated by microorganisms, converts ammonia into nitrite, a crucial step in organic matter decomposition and a central link in the nitrogen cycle. This process consumes oxygen, releases hydrogen ions, and produces greenhouse gas nitrous oxide (N₂O), making it closely relate to major environmental and climate issues such as coastal hypoxia, acidification, and global warming. Due to the limitations of trace N₂O measurement techniques, current research on the response of ammonia oxidation to warming has primarily focused on ammonia oxidation rates, with little investigation into the thermal response of its by-product, N₂O. This study utilized isotope labeling techniques to explore the temperature response characteristics of both ammonia oxidation rates and N₂O production rates in eutrophic coastal waters. The results show that, below the optimal temperature, both ammonia oxidation rates and N₂O production rates increase with rising temperatures. However, when temperatures exceed the optimal threshold, these rates decline with further increases in temperature. Notably, the temperature sensitivity coefficient (Q10) of N₂O production (2.3 ± 0.6) is greater than that of ammonia oxidation, suggesting that predictive biogeochemical and climate models need to include the differing temperature response characteristics of ammonia oxidation and N₂O production rates.