Dissolved Organic Nitrogen in Runoff/Surface Water from Agricultural Fields.

Dissolved organic nitrogen in runoff/surface water from Agricultural Fields Dissolved organic nitrogen (DON), along with inorganic nitrogen can be a significant part of the reactive N in aquatic ecosystems and can lead to eutrophication and harmful algal blooms. One-year observation was coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) to identify molecular composition of DON in the agriculture storm runoff water from citrus grove and pasture. More than 3000 N-bearing organic formulas were identified by FT-ICR-MS in the storm runoff water. These molecular formulas represented a wide range of biomolecules such as lipids, proteins, amino sugars, lignin, tannins, unsaturated H/Cs, condensed H/Cs and carbohydrates. Most of them were lignin. During the waterway, molecular weight increased and aromaticity index decreased. From the source to the discharge point, 1447 unique N-bearing were identified, and the median molecular weight increased. Molecular weight and aromaticity index also varied with seasons. This work implies that the storm runoff water from agriculture production system has the potential to influence biogeochemical processes in downstream water bodies. Key words: Mass spectrometry, Estuarine chemistry, Spatial variations, Temporal variations, Management