Dominance of nitrous oxide production by nitrification and denitrification in the shallow Chaohu Lake, Eastern China: Insight from isotopic characteristics of dissolved nitrous oxide.

In recent decades, most lakes in Eastern China have suffered unprecedented nitrogen pollution, making them potential "hotspots" for NO production and emission. Understanding the mechanisms of NO production and quantifying emissions in these lakes is essential for assessing regional and global NO budgets and for mitigating NO emissions. Here, we measure isotopic compositions (δN-NO and δO-NO) and site preference (SP) of dissolved NO in an attempt to differentiate the relative contribution of NO production processes in the shallow, eutrophic Chaohu Lake, Eastern China. Our results show that the bulk isotope ratios for δN-NO, δO-NO, and SP were 5.8 ± 3.9‰, 29.3 ± 13.4‰, and 18.6 ± 3.2‰, respectively. More than 76.8% of the dissolved NO was produced via microbial processes. Findings suggest that dissolved NO is primarily produced via nitrification (between 27.3% and 48.0%) and denitrification (between 31.9% and 49.5%). In addition, isotopic data exhibit significant NO consumption during denitrification. We estimate the average NO emission rate (27.5 ± 26.0 μg N m h), which is higher than that from rivers in the Changjiang River network (CRN). We scaled-up the regional NO emission (from 1.98 Gg N yr to 4.58 Gg N yr) using a NO emission factor (0.51 ± 0.63%) for shallow lakes in the middle and lower region of the CRN. We suggest that beneficial circumstances for promoting complete denitrification may be helpful for reducing NO production and emissions in fresh surface waters.