Pacheco M., Donoso L., Sanhueza E. (mipachec@ivic.ve)

1. Abstract #56 Soluble Organic Nitrogen in Airborne Particles and Rains of La Gran Sabana, Canaima National Park, Venezuela. Pacheco M., Donoso L., Sanhueza E. (mipachec@ivic.ve) Centro de Química. Laboratorio de Química Atmosférica. IVIC. Km 11 Carretera Panamericana. Apdo. postal 21827. Caracas 1020-A Venezuela . • Results and Discussions • 1. Introduction • Nitrogen is one of the major limiting factors that controls the dynamics, biodiversity, and functioning of most ecosystems. Fixed N also affects the growth and dynamics of the herbivore population and their predators. In general, low deposition rates of soluble inorganic nitrogen (SIN) is produced in Venezuelan ecosystems (Sanhueza et al., 1992; Sanhueza and Crutzen, 1998). Recently significant amounts of soluble organic nitrogen (SON) have been found in rainwater and in airborne particles at various sites in the world (e.g., Cornell et al., 2001; Mace et al.,2003), which a major fraction is bioavailable (e.g., Seitzinger and Sander, 1999). We report the content of SON in rains and aerosols collected at La Gran Sabana, a Venezuelan remote site. Results indicate that SON makes a significant contribution to the regional deposition of soluble nitrogen. RAINS In concordance with this work (see Table 1), low levels of NH4+ and NO3- in rain have also been previously measured at the Venezuelan savannah region (Sanhueza et al., 1992). These concentrations are much lower than the ones recorded in rural areas of Europe and North America. SON concentration is in agreement with the report at the review of Cornell et al. (2001). This review presents the SON interquartile range in continental (rural) rains is 0,182-0.476 mg/l-N, with a mean value of 0.332 mg/l-N. Table 1. Nitrogen soluble compounds at La Gran Sabana 2. Field Measurements Sites locations are indicated in the map in Figure 1. In poster #49 the major features of the sites were given. Only wet rain samples were collected at Parupa and total suspended particles were sampled with Hi-Vol at three different places, Luepa, Parupa and Yuruaní. Rain samples were collected immediately after each event and preserved with chloroform, and stored at 4 oC. Samples were not filtered prior to analysis, since insoluble particulate ON in rainwater is not a significant component of nitrogen (Cornell et al. 2003); therefore our SON concentrations may include some minor amounts of insoluble particulate nitrogen. Suspended particles were collected with quartz filter. After collection, filters were stored at 4ºC until analyze. Nitrogen soluble compounds were extracted with deionized water using an ultrasonic method. SON was determined subtracting the inorganic fraction from the total soluble nitrogen (TSN). NH4+ and NO3- were determined directly in rain samples with a selective electrode and ion chromatography, respectively. TSN was analyzed as nitrate, after wet chemical oxidation with persulfate (Solórzano and Sharp, 1980), using the cadmium reduction method. Recoveries of N following persulfate oxidation of standard urea solutions were >80%. Therefore, results reported in this study could underestimate SON concentrations. Graphic 1. Distributions of nitrogen soluble compounds in rains from Parupa Graphics 1 shows the distribution of nitrogen soluble compounds, as consequence of the low levels of the inorganic components a very high contribution of the SON (89%) to the TSN is produced. Airborne Particles Low concentrations of inorganic compounds were also found in the aerosols, Table 1 and SON concentrations were three times higher than inorganic compounds. Concentrations are in the same order of magnitude in the three places measured, which is in agreement with the reported by Sanhueza 2001 et al., about the homogeneity of the Gran Sabana atmosphere. SON represents up to 70% of TSN in the aerosol samples. Therefore, compare with other parts of the world, wet deposition of SON plays a major role in the nitrogen budget in Venezuelan “natural” ecosystems. Figure 1. Sample sites • References • Cornell, S.E., Rendell, A. and Jickells, T.D. 1995. Atmospheric inputs of dissolved organic nitrogen to the ocean, Nature,376, 243-246. • Cornell, S.E., Mace, K.A., Coeppicus, S., Duce, R.A., Huebert, T., Jickells, T.D. and Zhuang, L.-Z. 2001. Organic nitrogen in Hawaiian rain and aerosol. J. Geophys. Res., 106D, 7973-7983. • Cornell, S.E., Jickells, T.D., Cape, J.N., Rowland, A.P. and Duce, R.A. 2003. Organic nitrogen deposition on land and coastal environments: a review of methods and data. Atmos. Environ., 37, 2173-2191. • Jacobson, M.C., Hansson, H.C., Noone, K.J. and Charlson, R.J. 2000. Organic atmospheric aerosols: Review and state of the science. Rev. Geophys.38, 267-294. • Mace, K. A., Duce, R. A. and Tindale, N.W.. 2003. Organic Nitrogen in Rain and Aerosol at Cape Grim, Tasmania, Auatralia. J. Geophys. Res., 108(D11): 4338. • Pacheco, M., Donoso, L. and Sanhueza, E. 2004. Soluble Organic Nitrogen in Venezuelan Rains. Tellus B, in press. • Sanhueza, E. and Crutzen, P.J. 1998. Budgets of fixed nitrogen in the Orinoco savannah region: Role of pyrodenitrification. Global Biogeochem. Cycles, 12, 653-666. • Sanhueza, E., Arias, M.C., Donoso, L., Graterol, N., Hermoso, M., Martí, I., Romero, J., Rondón, A. and Santana, M. 1992. Chemical composition of acid rains in the Venezuelan savanna region. Tellus44B, 54‑62. • Seitzinger, S. and Sanders, R.W. 1999. Atmospheric inputs of dissolved organic nitrogen stimulate estuarine bacteria and phytoplankton. Limnol. Oceonogr., 44, 721-730. • Solórzano, L. and Sharp, J.H. 1980. Determination of total dissolved nitrogen in natural waters. Limnol. Oceanogr., 25, 751-754. • Conclusions • In conclusion, it is clear that SON is an important component of tropical rains and aerosol, especially in remote unpolluted sites, where it represents up to 80% and 70% of TSN in rains and aerosols respectively. Therefore, in order to have an adequate understanding of the cycling of atmospheric nitrogen, information of other components of the cycle (i.e., SON emission fluxes to the atmosphere from the soil vegetation reservoir, SON production by chemical reaction in the atmosphere, SON dry deposition) is needed. Acknowledgements These studies on biogeochemical cycles in tropical ecosystems were supported by the Venezuelan National Science Council (FONACIT) through the Grant No G-98001124.