Characterization of DON sources linked with water quality for inland water bodies in the Songnen Plain of Northeast China

Dissolved organic nitrogen (DON) in inland water bodies plays an important role in the global nitrogen and carbon cycles. A total of 14 fresh water and 5 brackish water bodies (electrical conductivity (EC) threshold value = 1200 mu S cm(-1)) were selected to assess the dynamics of DON for inland water bodies in the semiarid Songnen Plain of Northeast China. The spatial characteristics of DON concentrations, chromophoric dissolved organic matter (CDOM) absorption, CDOM fluorescent components (two humic-like C1 and C3, one tryptophan-like C2) and their correlations with water quality for these 19 water bodies were firstly evaluated using excitation-emission matrix (EEM) fluorescence spectroscopy coupled with parallel factor (PARAFAC) with the data collected in May 2021. Results showed that there were significant differences in DON concentrations, CDOM absorption a(254) and CDOM fluorescent components C1-C3 between fresh water and brackish water bodies (t test, p < 0.01), respectively. DON concentrations were moderately correlated with EC (R-2 = 0.766, p < 0.01), trophic state index (TSI) (R-2 = 0.757, p < 0.01) and chemical oxygen demand (COD) (R-2 = 0.740, p < 0.01) for all 19 water bodies, respectively, suggesting that DON can be used as the potential predictors for TSI and COD. Further, there was a significant positive linear relationship between the CDOM fluorescent component C1 and TN (R-2 = 0.654, p < 0.01), DON (R-2 = 0.746, p < 0.01) or COD (R-2 = 0.893, p < 0.01), respectively, indicating that the nitrogen and carbon in CDOM were originated from similar terrestrial humic-like substances. These results demonstrated that CDOM humic-like fluorescent component C1 can be used for the best optical predictors for nutrients providing a possible way to directly monitor DON and associated with water quality for inland water bodies in the semiarid regions of Northeast China, which has potential implication for inland water bodies with similar characteristics.

Automated summary

Dissolved organic nitrogen (DON) plays a crucial role in the global nitrogen and carbon cycles. This study assessed the dynamics of DON in inland water bodies in the semiarid Songnen Plain of Northeast China. The results showed significant differences in DON concentrations and CDOM characteristics between fresh water and brackish water bodies. DON concentrations were moderately correlated with water quality parameters and CDOM fluorescent components. CDOM component C1 was identified as the best optical predictor for nutrients in the water bodies.