LAUSR

Background and aimsInorganic nitrogen (N)––ammonium (NH4+-N) and nitrate (NO3--N)––deposition to tropical forests near urban areas frequently exceeds 5-10 kg ha-1 yr-1, surpassing critical loads for many forest types. We hypothesized that throughfall N fluxes to Atlantic Forest and effects on soil N cycling and availability would increase with proximity to a megacity (population ~12 M) on Brazil’s Atlantic Coast.MethodsWe quantified N in rainfall and throughfall, soil nutrient status and N transformations, and relative N availability in three Atlantic Forest sites: an oceanic site ~100 km southwest, a coastal site in, and an inland site ~50 km northeast and downwind from Rio de Janeiro megacity.ResultsAnnual throughfall N flux did not increase with proximity to the megacity but increased along an ocean (7.6 ± 0.3 kg ha-1) to coast (8.7 ± 1.5 kg ha-1) to inland (14 ± 2.5 kg ha-1) gradient. Potential net N mineralization and nitrification rates did not differ among forests. However, NO3:NH4 and resin bag NO3- accumulation in soil were highest at the high-N site.ConclusionsOur results suggest that elevated rainfall and throughfall N increase relative N availability and potential NO3- leaching losses from Atlantic Forest soils downwind urban areas.