Significant diurnal variations in nitrous oxide (NO) emissions from two contrasting habitats in a large eutrophic lake (Lake Taihu, China).

Algae and macrophytes in lake ecosystems regulate nitrous oxide (NO) emissions from eutrophic lakes. However, knowledge of diurnal NO emission patterns from different habitats remains limited. To understand the diurnal patterns and driving mechanisms of NO emissions from contrasting habitats, continuous in situ observations (72 h) of NO fluxes from an algae-dominated zone (ADZ) and reed-dominated zone (RDZ) in Lake Taihu were conducted using the Floating Chamber method. The results showed average NO emission fluxes of 0.15 ± 0.06 and 0.02 ± 0.04 μmol m h in the ADZ and RDZ in autumn, respectively. The significantly higher (p < 0.05) NO fluxes in the ADZ were mainly attributed to differences in nitrogen (N) levels. The results also showed significant diurnal differences (p < 0.05) in the NO emission fluxes within the ADZ and RDZ, and daytime fluxes were significantly higher (p < 0.05) than nighttime fluxes. The statistical results indicated that NO emissions from the ADZ were mainly driven by diurnal variations in N loading and the dissolved oxygen (DO) concentration, and those from the RDZ were more influenced by DO, redox potential, and pH. Finally, we determined the proper time for routine monitoring of NO flux in the two habitats. Our results highlight the importance of considering diverse habitats and diurnal variations when estimating NO budgets at a whole-lake scale.