Resolving the interactive effects of warming- and eutrophication-induced hypoxia in nitrogen transformations at a subtropical estuary

As the combined result of warming and organic matter input, coastal hypoxia has been affecting the biogeochemical nitrogen cycling. Specifically, the coordinated regulatory mechanisms in nitrification and nitrous oxide (N2O) production rates during the multifaceted factors of hypoxic events induced by warming, stratification, and organic matter input remain critical knowledge gaps to be filled. In this study, samples from the subtropical Pearl River Estuary showed increasing nitrification rates by two orders of magnitude from winter at ∼ 18°C to summer at ∼ 28°C. Winter water temperatures below 20°C appeared unfavorable for nitrification and N2O production; still, manipulated temperature increase, deoxygenation, and organic matter input for the water sample collected in winter within 24 hours could effectively stimulate N2O production. When incubated at temperatures ≥ 25°C, laboratory-controlled experiments confirmed that hypoxic DO (∼ 60 μmol·L−1) could enhance nitrification and associated N2O production compared to in situ, oxygenated conditions (DO > 140 μmol·L−1) for all sampled sites. Deoxygenation alone may inhibit partial denitrification across the estuary, challenging the existing paradigm. The elevated organic matter input had a more pronounced increase in N2O production via nitrate denitrification at the upper and middle estuary than at the oligotrophic lower estuary. Taken together, future coastal warming may alleviate the temperature limitation in winter, thereby allowing hypoxia and organic matter effects to potentiate N2O production; intense organic matter degradation causes hypoxia events that may increase both nitrification rates and N2O production via ammonium oxidation and nitrate denitrification at the upper and middle estuary.