1. Phosphorus in aerosol particles in the Amazon Basin
Paulo Artaxo 1, Pascal Guyon 2, Meinrat O. Andreae 2, José Vanderlei Martins 1, Karla M. Longo 1, Willy Maenhaut 3, Rene Van Grieken 4.
1 Instituto de Física, Universidade de São Paulo, Rua do Matão, Travessa R, 187, , São Paulo, S.P., Brazil; 2Max Planck Institute for Chemistry, Mainz, Germany;3 Institute for Nuclear Sciences, University of Gent, Gent, Belgium; 4Department of Chemistry, University of Antwerp - UIA, Antwerp, Belgium.
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Introduction
Moist tropical forests account for a substantial component of terrestrial plant productivity and several lines of evidence suggest that they may be sequestering significant amounts of anthropogenically released carbon at the present time. But there is also some evidence that the productivity of many of these forests is limited by their low phosphorus availability. This has led to some suggestions that moist tropical forests may not be able to increase their growth in response to increases in atmospheric carbon dioxide concentrations to the same degree as would be the case if rates of P availability were higher.
Methods
Aerosol particles and rainwater were colleted in several sites in the Amazon Basin. Aerosols were collected using Stacked Filter Units, that separates particles in the fine (dp<2 μm) and coarse mode (2<dp<10 μm) fraction. Rainwater was collected using wet only rainwater sampler. Aerosol particles were analyzed with the PIXE (Particle Induced X-ray Emission) method. Rainwater samples were analyzed by Ion Chromatography and ICP-MS. Phosphorus was measured in precipitation and in aerosol particles for several sites in the Amazon Basin. The sites were Balbina, Rondônia (pasture), Rondônia (Rebio Jaru forest), Santarem and Alta Floresta. Local support from the LBA staff is acknowledge. One important issue is that even with the lowest possible detection limits, it was not possible to measure phosphorus in the precipitation.
Results
One of the sites were we have a long time series (several years of measurements) is Alta Floresta, on the Mato Grosso state. Figure 1 shows the phosphorus concentration in the fine mode aerosol fraction, while Figure 2 shows the coarse mode phosphorus concentration in Alta Floresta from 1996 to There is a strong seasonality in airborne phosphorus concentrations, with a significant increase during the dry season due to biomass burning emissions. Concentrations increase by a factor of 5 from the wet season to the dry season, for both fine and coarse mode aerosols. Part of this dry season phosphorus is lost to the ecosystem due to long range transport of biomass burning emissions.
Figure 3 shows phosphorus concentration in Balbina (near Manaus) in the coarse mode particles, together with S and K concentrations. During nighttime it was observed a large increase in phosphorus concentration for the coarse mode particles. By some mechanism, the vegetation releases significant amounts of phosphorus during nighttime, mostly in coarse mode particles. These natural biogenic particles are also enriched in potassium, but not in sulphur.
Figure 4 and 5 shows the average trace element concentrations for the three levels of the Jaru tower (Ground level: 5 meters; Canopy level: 25 meters; Upper Level: 52 meters) in Rondônia for the wet season. The data are separated by fine and coarse mode fraction and by day and night for each tower level. There are remarkable trace element gradients in concentrations, such as the ones for S, K, Mn in the fine mode and for P, Cl and Ca in the coarse mode fraction. Zinc is an element always associated with natural biogenic aerosol. Figures 6 and 7 shows the average trace element concentration for about 20 trace elements for each aerosol fraction and separated by day and night sampling. In the fine mode aerosol, during nighttime, a strong increase in concentrations for Fe, Si, K, and other trace elements is noticeable.
Figure 8 shows the fine and coarse mode phosphorus concentration for the three levels of the Jaru Tower, and for the fine and coarse mode aerosol. It is remarkable the very large increase in coarse mode P during nighttime at the lower level in the canopy. This could an internal forest nutrient cycling mechanism.
Conclusions
  It was found that phosphorus presents a very unique behavior in terms of aerosol particles. It is present almost exclusively in the coarse mode particles, and its concentrations increase strongly at nighttime. In measurements from towers, it was observed that within the canopy, phosphorus has maximum concentration near the ground, decreasing quickly with canopy height. At the ground level in Rondônia, P concentrations on the ground level were at ng/m3, whereas in the free atmosphere concentrations were in the range of ng/m3. Virtually all of this phosphorus is in the coarse mode fraction (particles larger than 2 m). Similar patterns concentrations were observed for Balbina and Santarem. This phosphorus release occurs mainly in the nighttime. This represent an internal forest nutrient cycling mechanism. At nighttime and coarse mode fraction, the possibility of the forest to lose this phosphorus is minimal and this essential trace element is kept in the vicinity of the emissions, minimizing the loss of a key forest nutrient.
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Acknowledgements: FAPESP and the LBA infrastructure team