Dissolved organic nitrogen (DON) in wet deposition is often overlooked in monitoring programs and foundational theories of ecosystem biogeochemical cycles. However, where DON is monitored, concentrations have increased over time showing distinct phenology that aligns with the growing season in the northern hemisphere. Concentrations of DON in wet deposition are at magnitudes that raise questions about the role it plays in terrestrial and aquatic biogeochemical cycles, particularly its contribution to an ecosystem's exceedance of critical loads of nitrogen (CL-N). Currently, exceedance calculations do not consider DON wet deposition due to limited monitoring. This study re-analyzed 284 archived wet deposition samples from 17 NADP NTN sites (2017-2018) to estimate DON's contribution to total N deposition. Samples were re-analyzed for NO₃ and NH₄ and validated for stability. DON was then calculated by subtracting NO₃ and NH₄ from total dissolved N (TDN). The relative abundance was weighted by precipitation depth to create an annual DON scalar, ranging from +1.3% to 15.5%, that was applied to TDep wet deposition within the level 3 Ecoregion where the sample was collected.

Comparing TDep Total N and TDep Total N + DON maps against two CL datasets revealed a 0.4-0.6% increase in area exceeding CL-N for sensitive lichen richness and a 7.4-8.8% increase for herbaceous species richness. The CL-N for sensitive lichen richness had increases in area of exceedance in 17 to 18 Class I areas, while the CL-N for herbaceous species richness had an increase in 6 to 8 Class I areas using 2017 and 2018 data, respectively. These findings suggest that while the relative contribution of wet deposition DON to TDep is small, when considered as an additional input of N it can push ecosystems over CL thresholds, posing risks to ecosystem health. Importantly, this analysis likely underestimates the annual contribution of DON to total N deposition due to incomplete spatial and temporal coverage of our DON data. Nonetheless, this study provides a foundational understanding for the degree to which DON loads should be considered in monitoring programs moving forward.

¹ Department of Natural Resources and the Environment, University of New Hampshire, Durham, NH

² National Park Service, Air Resources Division, Denver, CO

* Corresponding Author: desneiges.murray@unh.edu