Regional Sources and Seasonal Variability of Rainwater Dissolved Organic and Inorganic Nitrogen at a Mid‐Atlantic, USA Coastal Site

Abstract

AbstractChanges in anthropogenic activities have altered the speciation and concentration of inorganic reactive nitrogen (Nr) delivered to coastal and oceanic waters with precipitation. Less is known about rainwater dissolved organic nitrogen (DON) despite its quantitative importance (>20% of Nr) and potential contributions to primary and secondary production. We document decreases in rainwater nitrogen and carbon amounts between 1994 and 2019 in Delaware, USA with the major reduction observed for nitrate (64%) reflecting emissions technology improvements. [DON] in 2019 was 55% that of 1994, though only 2 years of data are available precluding any assessment of trends. Season, airmass back trajectory (AMBT), rainfall amount, and meteorology influenced Nr amounts in 2018–2019 rain. [DON], which peaked in Summer, had different seasonal patterns than inorganic Nr and dissolved organic carbon, suggesting a biological source. Marine AMBT events showed the lowest Nr abundances. AMBTs from the southwest had the highest concentrations of Nr and DOC partially due to low rainfall amounts. Characterization of the oxidized fraction of DON revealed abundant highly unsaturated aliphatic and peptide‐like formulas suggesting a combination of secondary organic, biomass burning, and biological sources. The large changes in Nr and DOC loads emphasize the dynamic nature of atmosphere to land/water fluxes due to the influence of anthropogenic processes with potential implications for coastal and oceanic water quality and ecology. Models of atmospheric deposition to watersheds and the ocean should be frequently reevaluated with current data to accurately assess inputs from changing atmospheric sources.