Nitrous oxide (N2O) is one of the six greenhouse gases stipulated in the Kyoto Protocol. Its greenhouse potential over the past century was 298 times that of CO2, and the concentration of atmospheric N2O has been continuously and rapidly increasing during the past hundred years. Shallow lakes are an important source of atmospheric N2O. In order to explore the temporal and spatial changes and potential driving factors of N2O emissions from eutrophic water, we conducted field observations in February (winter) and August (summer) in Lake Taihu. We used the coefficient of diffusion-headspace bottle method to trace the variability in the N2O concentration[c(N2O)] and efflux[F(N2O)] from surface water bodies and explored the potential driving factors of N2O emissions. The optical measurements of dissolved organic matter (DOM) are an effective approach for tracing the source and composition of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON). The migration and transformation processes of DOM also release a large amount of inorganic nitrogen, which changes the redox potential of the water column and thereby affects N2O emissions. Our results showed that the variability in c(N2O) and F(N2O) in the surface waters of Lake Taihu were strongly affected by water temperature and nutrient levels. The average c(N2O) of the surface waters was (19.7±2.7) nmol·L-1, corresponding to a mean F(N2O) of (41.1±1.8) μmol·(m2·d)-1, and the means of both c(N2O) and F(N2O) were higher in summer than those in winter (t-test, P<0.01). The input and accumulation of DOM could increase the production and emission potential of N2O in water bodies, as supported by both c(N2O) and F(N2O) significantly increasing with increasing level of terrestrial humic-like C1. The integration ratio of peak C to peak T IC:IT of DOM and the spectral slope S275-295 results indicated that there were high inputs of terrestrial DOM in the northwestern inflowing river mouths, concurring with the high production and emission of N2O found there. This suggested that the accumulation and degradation of terrestrial DOM potentially fueled the emission of N2O. Our results showed that water temperature, DOM composition, and nutrient level were all important factors affecting N2O emission from Lake Taihu. Long-term continuous observation can be applied to better evaluate the impact of various environmental factors on the production and emission of N2O in water bodies and to help with providing scientific emission reduction plans.
Keywords: Lake Taihu; dissolved organic matter (DOM); flux; nitrous oxide(N2O); parallel factor analysis (PARAFAC).