Over one-third of N2O emissions originate from water in large, shallow and eutrophic Taihu Lake, China

In shallow lakes, sediments are often considered hotspots for nitrogen cycling and nitrous oxide (N₂O) emissions due to their abundant nutrients and microorganisms. However, in large, shallow eutrophic lakes, severe sediment resuspension and algal proliferation lead to high concentrations of particles in the water, which may host active nitrogen cycling and significantly influence N₂O emissions. Nevertheless, the contribution of waterborne N₂O emissions and the underlying mechanism remain unclear. To explore it, this study investigated N₂O flux from Taihu Lake water, China, quantified its contribution to total emissions, and elucidated the associated microbial mechanisms using real-time quantitative polymerase chain reaction. The results revealed that N₂O flux from Taihu Lake water significantly contributed to total emissions, accounting for an annual average of 34.1 %, with contributions exceeding 50 % in spring and winter. In contrast, during autumn, the water served as a sink for N₂O. A variety of nitrifying and denitrifying microorganisms were detected in the water, which regulate the source-sink dynamics and intensity of N₂O emissions. From 2006 to 2023, the total N₂O emissions from Taihu Lake water amounted to 5382.7 Tg, exhibiting a declining trend over time. When tracing N₂O emissions and developing mitigation strategies for large, shallow, eutrophic lakes, it is essential to consider the contribution of the water column rather than attributing the sources solely to sediments.