Are nitrous oxide emissions indirectly fueled by input of terrestrial dissolved organic nitrogen in a large eutrophic Lake Taihu, China?

Lakes actively transform nitrogen (N) and emit disproportionately large amounts of NO relative to their surface area. Studies have investigated the relative importance of denitrification or nitrification on NO emissions; however, the linkage between NO efflux and dissolved organic nitrogen (DON) and carbon (DOC) remains largely unknown. Long-term (2012-2017) seasonal field observations and a series of degradation experiments were used to unravel how DON composition impacts NO emissions from Lake Taihu, China. In the northwestern part of the lake, large riverine inflow and high NO emissions occur in all seasons (24.6 ± 25.2 μmol m d), coincident with high levels of terrestrial DON and DOC here. The degradation of labile DON and DOC likely enhanced ammonification as supported by the correlations between NH-N and DON, DOC, a(350), and terrestrial humic-like C3. The area with large riverine inputs in the northwestern part of the lake was characterized by low DO which may enhance incomplete aerobic nitrification and incomplete denitrification, both leading to NO production. Twenty days laboratory experiments indicated greater NO production in the northwest inflow samples (NO on day 20: 120.9 nmol L and 17.3 nmol L for bio- and photo-degradation samples, respectively) compared with the central lake samples (NO on day 20: 20.3 nmol L and 12.3 nmol L for bio- and photo-degradation samples, respectively), despite both having low Chl-a. Our DON and DOC degradation experiments confirmed the occurrence of ammonification along with consumption of NH-N and thereafter NO-N. Our results collectively suggest that terrestrial DON fueled ammonification, enhanced nitrification and incomplete denitrification, and thereby became an important contributor to the NO efflux from Lake Taihu.