Atmospheric organic and inorganic nitrogen inputs to coastal urban and montane Atlantic Forest sites in southeastern Brazil

• TDN deposition decreased with distance from a coastal urban area to downwind forest.
• In bulk precipitation, the relative contribution of DIN to TDN was higher that of DON.
• Urea was an important component of DON in bulk precipitation.
• Non-urea DON comprised most of the net N flux from the forest canopy.
• DIN in throughfall was 1.5–3 fold higher than the critical load for tropical forest.

Tropical regions are currently experiencing changes in the quantity and form of nitrogen (N) deposition as a result of urban and industrial emissions. We quantified atmospheric N inputs to two coastal urban and two montane (400 m and 1000 m) Atlantic Forest sites downwind of the Metropolitan Region of Rio de Janeiro (MRRJ), Brazil, from August 2008 to August 2009. Concentrations of total dissolved nitrogen (TDN), dissolved inorganic nitrogen (DIN) and urea were measured in bulk precipitation at all sites, as well as in canopy throughfall in the lower montane forest. Dissolved organic nitrogen (DON) was calculated as the difference between TDN and DIN (NH4+ + NO3− + NO2−). Annual volume-weighted mean bulk concentrations of all N species were higher at the coastal urban than montane forest sites, with DON accounting for 32–56% and 26–32%, respectively, of the TDN concentration in bulk precipitation. Bulk deposition of TDN ranged 12.1–17.2 kg N ha− 1 yr− 1 and tended to decrease with increasing distance from the coastal urban region. In the lower montane forest, throughfall TDN flux, 34.3 kg N ha− 1 yr− 1, was over 2-fold higher than bulk TDN deposition, and DON comprised 57% of the total N deposited by throughfall to the forest soil. Urea comprised 27% of DON in throughfall compared to up to 100% in bulk precipitation. Our findings show that DON is an important, yet understudied, component of TDN deposition in tropical forest regions, comprising one-third to greater than one-half of the N deposited in rainfall and throughfall. Further, in this lower montane Atlantic Forest site, throughfall DIN flux was 1.5–3 fold higher than the suggested empirical critical load for humid tropical forests, highlighting the potential for increasing N pollution emitted from the MRRJ to impact N cycling in adjacent ecosystems.