Riverine dissolved organic nitrogen (DON) is biologically active and plays a crucial role in estuarine and coastal biogeochemical processes (Seitzinger et al., 2002). The ecological environment of Jiaozhou Bay has undergone significant changes in recent years (Yuan et al., 2018), with a notable annual increase in the proportion of DON in total dissolved nitrogen (TDN) (Lu et al., 2016). However, studies on DON concentration and its isotopic signatures in Jiaozhou Bay remain very rare. Therefore, this study employs a combination of field observations, controlled indoor incubation, high throughput sequencing and statistical analyses to comprehensively examine the transformation processes of dissolved organic nitrogen originated from terrestrial sources in Jiaozhou Bay. Advanced analytical techniques, including nitrogen and oxygen isotope tracing and ultra-high resolution mass spectrometry, have been utilized.
During the incubation experiments, both dissolved organic carbon (DOC) and DON degradation best fit the single-G model. The model fitting results indicated that the fastest degradation rate was observed in the 1-10 KDa fraction of DON, while the >10 KDa fraction exhibited higher bioactivity compared to the other two fractions (<1 KDa and 1-10 KDa). The δ¹⁵N of the different molecular weight DON fractions increased slightly upon incubation and remained relatively stable thereafter, suggesting the degradation of riverine DON. The δ¹⁵N/δ¹⁸O ratio of nitrate (δ¹⁵N_NO3/δ¹⁸O_NO3) was highest in the >10 KDa DON fraction incubation, while the control showed little change, indicating that the nitrogen cycling process was more active in the incubation of the >10 KDa DON fraction. Significant ¹⁵N and ¹⁸O fractionation occurred across DON incubations (<1, 1-10, >10 KDa). The ¹⁵N and ¹⁸O fractionation in the >10 KDa DON incubation generally aligned with the 1:1 line, while that of the other two fractions initially followed the 2:1 line and later conformed to the 1:1 line after 54 days. Principal component analysis demonstrated marked differences among the three incubations of different DON molecular weights (<1, 1-10, >10 KDa), compared to the controls.
In summary, we draw the following conclusions: (1) the transformation processes of riverine DON with different molecular weights exhibited significant differences; the biological activity of larger molecular weight DON fractions (>10 KDa) was highest, whereas the degradation rate of 1-10 KDa DON was the greatest; (2) The transformation process of riverine DON in Jiaozhou Bay is not solely a simple degradation process, but may also involve other biological processes, including DON production and regeneration.

riverine dissolved organic nitrogen, Jiaozhou Bay, nitrogen isotope tracing, nitrogen cycling