Factors Influencing Nitrogen Speciation in Coastal Rainwater

Rainwater was collected from 129 rain events between February 2002 and August 2003 and analyzed for ammonium (NH₄⁺), nitrate (NO₃⁻), organic nitrogen (ON) and free amino acids (AA). Inorganic nitrogen (NO₃⁻ + NH₄⁺) was the dominant form of N representing 85% of total nitrogen based on volume-weighted averages. The remainder of the N occurred as organic nitrogen species of which free amino acids contribute approximately 17%. A significant, and in some cases the majority (> 75%), of the remaining ON could be accounted for by macromolecular uncharacterized humic like substances. This has important ramifications with respect to the long range transport of atmospheric ON because humic materials are recalcitrant and therefore may travel long distances from their source. There was a distinct seasonality to the N speciation data with maximum concentrations of NH₄⁺, ON and AA occurring in the spring. Air-mass back trajectory analysis indicates there is a strong anthropogenic component to the NO₃⁻, NH₄⁺ and AA signal but not ON. There was a strong positive correlation between amino acid concentrations and ammonium which suggests they have similar sources and sinks in rainwater. Finally, large episodic additions of NH₄⁺ and AA during tropical events could significantly impact short term bioavailable N budgets in estuaries impacted by these storms. Approximately three times as much NH₄⁺ and AA were deposited during Hurricane Isabel (317 μ moles ⋅ m⁻² and 84 μ moles ⋅ m⁻² respectively) compared to the mean impact of average summertime rain events at this location.