A long-term overview of nitrogen in tropical rainwater and wet deposition fluxes in southern Brazil

Abstract

This work reports inorganic and organic nitrogen concentrations in rainwater and estimates wet deposition fluxes in an agro-industrial region of São Paulo State (Brazil) impacted by biomass burning and vehicular public policies, spanning the period from 2005 to 2020. The overall volume weighted mean (VWM) concentration of nitrate in rainwater was 13.1 ± 0.5 μmol L−1 (n = 696). Despite decreasing vehicular emissions of NOx over the study period, there was no clear trend of nitrate concentrations in rainwater, possibly due to biomass burning emission sources. The overall ammonium VWM concentration was 24.3 ± 1.1 (n = 696). In contrast to nitrate, there was a 94% increase of ammonium in 16 years, attributed to increased emissions from vehicles and fertilizer use, together with variable biomass burning emissions. Dissolved organic nitrogen (DON) concentrations in individual samples ranged from 0.2 to 153 μmol N L−1, corresponding to between 1 and 73% of the total dissolved nitrogen, with VWM of 18.9 ± 0.9 μmol N L−1 (n = 377). Amino acids corresponded, on average, to 15% of the DON. For the majority of the years, the VWM concentrations of nitrate, ammonium, and DON were higher for the pool of samples collected during the dry season, compared to the wet season. These results highlighted the importance of biomass burning sources of reactive nitrogen in drier months, when wildfires are common. The wet deposition flux of total dissolved nitrogen was ∼12 kg N ha−1 yr−1, with ammonium corresponding to 43%, followed by DON (33%) and nitrate (23%). These results demonstrated the importance of including DON when modeling the biogeochemical cycle of nitrogen, especially in a world where wildfires may become more frequent and intense.