Espécies nitrogenadas em água de chuva de Ribeirão Preto (SP)

Abstract

Nitrogen species have been intensively added to the environment since the development of Haber-Bosch process for N2 transformation to NH3, significantly altering the biogeochemical cycle of nitrogen. Once in the atmosphere, reactive nitrogen is deposited back to the Earth's surface by wet and dry deposition processes. The importance of taking into account the organic forms of nitrogen to estimate atmospheric nitrogen deposition by rain is already known. However, few studies evaluated this fraction due to the difficulties and uncertainties imposed by the available analytical methods. This work presents the development of a new, simple and low-cost method for the determination of organic nitrogen (N-org) in rainwater using photo-Fenton process and a homemade photo-reactor. By adding Fenton solution (50 μmol L-1 Fe 2+ and 2 mmol L-1 H2O2) and keeping the reaction under UV radiation for 90 min (85 °C), it was possible to obtain an average of 106 ± 8% nitrogen recovery for solutions containing 50 μmol N L-1 of the model molecules: urea, serine, glycine and histidine. In the case of arginine, 90 min of radiation was sufficient to degrade solutions containing 10 μmol N L-1. A commercial reactor showed to be more efficient in degradation of the tested compounds (30 min). However, with a time of 75 min, the homemade reactor achieved the same results. With 0.2 g L-1 TiO2 and 120 min of UV radiation, it was also possible to obtain satisfactory results. Nevertheless, this method had high blank values, and filtration of irradiated samples before the analysis was necessary, which increased time and cost to the analytical procedure. The developed method using photo-Fenton was applied to determine N-org in rainwater samples collected in Ribeirão Preto city (SP) from 2013 to 2017. N-org concentrations ranged from 3.5 to 195 μmol N L-1 with a volume-weighted mean concentration (VWM) of 17.7 ± 1.0 μmol N L-1 (n = 236). This value was higher than those reported for rainwater from different parts of the world, and this fact can be attributed to the high biomass burning in the study region. Dissolved free amino acids (AA) mean concentrations represented 15 ± 12% (n = 144) of the organic nitrogen fraction in rain, while urea concentrations were close to or below the limit of quantification of the method (0.5 μmol N L-1). Considering all temporal series, initiated in 2005 at the same sampling site, the VWM concentration calculated for NH4 + ions was 22.2 ± 1.1 μmol L-1 (n = 460) and for NO3was 13.3 ± 0.6 μmol L-1 (n = 466), while NO2mean concentration was irrelevant. Significantly higher concentrations (t-test, P = 0.05) of NH4 + , NO3-, N-org and AA were obtained during the harvest period (dry season) in relation to the non-harvest one (rainy season), for all evaluated years. Although manual harvesting was drastically reduced, the fact that the same seasonal trend has been maintained since 2005 demonstrates that the practice of biomass burning is still intense in the region. In rural area, using fire for land management is still common...