Nitrogen isotope constraint on the zonation of multiple transformations between dissolved and particulate organic nitrogen in the Changjiang plume

Isotopic compositions of organic and inorganic nitrogen reveal processing and source dynamics at septic influenced and undeveloped estuary sites

Historically, dissolved organic nitrogen (DON) has not been characterized in the nitrogen profiles of most estuaries despite its significant contribution to total nitrogen and projected increase in loading. The characterization of dissolved inorganic nitrogen (DIN) and DON processing from groundwater to surface water also remains unconstrained. This study attempts to fill in these knowledge gaps by quantifying the DON pool and potential sources in a semiarid, low inflow estuary (Baffin Bay, Texas) using stable isotope techniques. High NO3- and DON concentrations, and high δ15N-NH4+ (+55.0 ± 56.7 ‰), δ15N-NO3- (+23.9 ± 8.6 ‰) and δ15N-DON (+22.3 ± 6.5 ‰) were observed in groundwaters of a septic-influenced estuarine area, indicating coupled septic contamination and nitrification/denitrification. In contrast, groundwater of an undeveloped area provided evidence of inundation by bay water through high NH4+ concentrations and δ15N-NH4+ (+8.4 ± 3.0 ‰) resembling estuary porewater. NH4+ was the dominant nitrogen species in porewater of both areas and δ15N-NH4+ indicated production via organic nitrogen mineralization and dissimilatory nitrate reduction to ammonium. Surface water had similar nitrogen profiles (DON constituted ∼98 % of dissolved nitrogen pool) and potential source contributions, despite distinct nitrogen processing and profiles found in each water table. This was attributed to low nitrogen removal rates and prolonged mixing associated with long residence time. This study emphasizes the importance of DON in a low-inflow estuary and the isotopic approach to comprehensively examine both inorganic and organic N processing and sources serving as a guide to investigate N cycling in high DON estuaries globally.

Evolution of dissolved organic nitrogen chemistry during transportation to the marginal sea: Insights from nitrogen isotope and molecular composition analyses

Estuaries are hotspots where terrestrially originated dissolved organic matter (DOM) is modified in molecular composition before entering marine environments. However, very few research has considered nitrogen (N) modifications of DOM molecules in estuaries, limiting our understanding of dissolved organic nitrogen (DON) cycling and the associated carbon cycling in estuaries. This study integrated optical, stable isotopes (δ15N and δ13C) and molecular composition (FT-ICR MS) to characterize the transformation of DOM in the Yangtze River Estuary. Both concentration of dissolved organic carbon (DOC) and DON decreased with increasing salinity, while their δ13C and δ15N increased with the increasing salinity. A significant positive correlation was found between δ15N and δ13C during the transportation of DOM to marginal seas, indicating that the behavior of both DOC and DON are primarily controlled by the mixing of freshwater and the seawater in the YRE. During the mixing process, the DON addition was observed using the conservative mixing curves. In the view of molecular composition, DOM molecules became more aromatic as the number of N atoms increased. Spearman correlations reveal that DOM molecules with fewer N atoms exhibited a higher enrichment in protein-like components, while those with more N atoms were more enriched in humic-like components. In addition, the δ15N and δ13C tended to increase as the N content of DOM decreased. Therefore, DON molecules with fewer N atoms were likely to be transformed into those with more N atoms based on the isotopic fractionation theory. This study establishes a linkage between the molecular composition and the δ15N of DOM, and discovers the N transformation pattern within DOM molecules during the transportation to marginal seas.

Multi-isotope tracing nitrate dynamics and sources during thermal stratification in a deep reservoir

Excessive nitrate (NO₃^-) input to reservoirs is a global concern. However, the dynamics and sources of NO₃^- under thermal stratification in deep reservoirs were rarely explored. In this study, multi-stable isotopes (δ¹⁵N/δ¹⁸O-NO₃^-, δ¹⁵N-particulate nitrogen (PN), δ¹⁵N-dissolved total nitrogen (DTN), and δ²H/δ¹⁸O-H₂O) and a Bayesian mixing model were applied to reveal the biogeochemical processes and sources of NO₃^- in a deep reservoir with obvious nitrogen pollution. The results showed that the reservoir was thermally stratified in July while vertically mixed in October. The distribution of δ²H-H₂O suggested that riverine nitrogen migrated to the epilimnion and metalimnion during stratification in the reservoir. In the epilimnion and metalimnion, the significant reduction in NO₃^- concentration was related to the enhancement of assimilation by thermal stratification. Meanwhile, the positive linear correlations between δ¹⁸O-NO₃^- and δ¹⁸O-H₂O suggested that in-reservoir nitrification occurred, with its depth confined above the hypolimnion. In the hypolimnion, denitrification processes were absent due to the aerobic environment. Overall, NO₃^- dynamics were mainly controlled by nitrogen inflow, in-reservoir nitrification, and assimilation during thermal stratification. The results of the Bayesian mixing model showed that manure and sewage, and soil nitrogen were the dominant NO₃^- sources of the reservoir. This study provides new insights and data to help manage and restore deep waters worldwide in tackling a similar situation of nitrogen contamination.