DRY and BULK atmospheric nitrogen deposition to a West-African humid forest exposed to terrestrial and oceanic sources

Atmospheric deposition can be a significant source of reactive nitrogen (Nr) in terrestrial ecosystems and might play a relevant role for both N cycle processes and NC cycle interactions. However, detailed studies and annual budgets of Nr deposition are scarce in many regions of the globe, particularly dry depositions in tropical areas. In this study we estimated the magnitude and dynamics of Nr dry and bulk deposition to an African humid forest located 30km from the Atlantic coast, in the National Park of Ankasa (Ghana). Atmospheric concentrations of gaseous (NH3, NO2, HNO3) and aerosol (NH4+, NO3-) nitrogen compounds were measured above the forest canopy, at 45m, using active denuder and filter sampling systems (DELTA). The experimental facility also provided high frequency micrometeorological data, sampled at the same height as DELTA, which were used to estimate deposition fluxes of gas and aerosols by applying an ensemble of four inferential models (six for NH3). Additionally, bulk deposition rates and ionic composition were also determined. Estimated Nr input as dry deposition at the site was -4.9±1.7kgNrha-1yr-1, 50% of which in form of aerosol N (NH4++NO3-). Bulk deposition accounted for 23.1±3.6kgNrha-1yr-1. Two main N deposition peaks were identified during the year, the first occurring in the dry season and being more evident for gaseous N compounds, the second occurring during the wet season, when both aerosol N and bulk N deposition rates reached their maximum value. Savannah burning and ocean emissions were hypothesized to be two important sources for Nr deposition at Ankasa, the former having a more significant role in the dry season, the latter dominating in the wet season. The important role of the oceanic source was supported by the continuous significant input of aerosol N at the site, which was consistent with the constant presence of a sea breeze, causing never less than 50% of the dominant wind direction over the whole year