Biogeochemical processes of dissolved organic nitrogen in aquatic environments

Dissolved organic nitrogen (DON) is a dominant form of nitrogen in aquatic environments. Knowing source, transformation and fate of DON is important to understand global carbon and nitrogen cycling. However, our knowledge of DON in aquatic environment is still limited partly due to inherent analytical challenges. In this dissertation, DON dynamics, including urea, peptide, and natural DON, are systematically studied through field sampling and laboratory-based incubation experiments using the state-of-art analytical techniques. The dynamics of urea, a labile DON molecule, and how its cycling relates to blooms of Microcystis in Lake Taihu, China were investigated. The metabolism rate of urea ranges from non-detectable to 1.37 μmol L⁻¹ h⁻¹ for regeneration, and from 0.04 μmol L⁻¹ h⁻¹ to 2.27 μmol L⁻¹ h⁻¹ for potential urea-N removal. Microcystis culture experiments demonstrated that Microcystis only has minor effect on urea dynamics, and bacterial community analysis further suggested that heterotrophic bacteria play a major role in urea metabolism, even though Microcystis is the dominant bloom organism. To investigate the long-term fate of labile DON, incubation experiments were conducted using ¹⁵N-labeled tetra-peptide (Ala-Val-Phe-Ala) as substrate. The fate of ¹⁵N was monitored throughout the incubation with a newly developed holistic analytical method. Over 40% of peptide N was transformed into uncharacterized DON within several months, in comparison with typically less than 10% for C from previous studies. Moving forward into the mystery of the black box of DON and dissolved organic matter (DOM), molecular structure of DOM was investigated using an Ion Mobility Quadrupole Time of Flight Liquid Chromatography Mass Spectrometer (IM Q-TOF LC/MS). Geometric conformation of DOM molecules was introduced into molecular-level analysis via the ion mobility (IM), and an actual measurement of isomers was achieved for the first time. We established a quantitative measurement of isomers in natural DOM from south Texas Rivers and coastal seawater. Our result showed that about 10% of natural DOM molecules had at least one but no more than four isomers. Overall, the new information on the dynamic, the long-term fate, and the molecular-level information of DON provided in the dissertation opens new dimensions of this field in the future.