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Impact of metabolites of the forest vegetation on the chemistry of throughfall
Skřivan, Petr ; Navrátil, Tomáš ; Vach, Marek ; Špičková, Jitka ; Fottová, D.
The chemistry of throughfall is a result of numerous processes that affect the concentrations of its individual components in the original wet precipitation above the tree crowns. The precipitation entering the tree crowns is generally enriched throughout its interactions with the above - ground part of the vegetation. The samples of open place precipitation and throughfall have been collected since 1989 in the region of Kostelec n. Č.lesy. The monitored area is situated predominantly in the Nature State Reserve Voděradské bučiny on the bedrock of the Říčany and Jevany granites. The exact evaluation of the impact of metabolites on the resultant concentration and fluxes of individual monitored elements in throughfall, based on mere comparison with the corresponding values in open place precipitation, is faced with several problems. First, the chemistry of throughfall reflects more the chemical composition of aerosol from lower parts of the atmosphere, as it is more or less effectively swept out by the above ground growth. The vegetation surface also entraps the reactive atmospheric gasses. Finally, the original precipitation solution is thickened through the evapotranspiration and it is enriched by the excluded and leached metabolites. The estimate of the extent of metabolic activity of the forest trees on the chemistry of throughfall was therefore limited on the comparison of a relative magnitude of normalised fluxes of individual elements in a given type of throughfall. The fluxes of individual elements in a beech- and spruce throughfall were normalised on the Na fluxes, with respect to the supposed zero metabolic share of Na in throughfall and to its predominantly natural sources in the atmospheric aerosol. The calculation of the enrichment factor Ef of the individual elements was then executed by means of the relation EfTFB,TFS (Me) = FBP Na / FTFB,TFS Na* FTFB,TFS Me / FBP Me, where EfTFB,TFS (Me) is the enrichment factor od the particular element in a given type of throughfall, FBP Na a FBP Me are the mean annual deposition fluxes of Na and the element Me on an open place, and FTFB,TFS Na a FTFB,TFS Me are the mean annual fluxes of Na and the element Me in throughfall. Values of the enrichment factor for the individual elements are presented in the article in the Tab. 1. Values higher than 1 should generally indicate the presence of metabolic products of the vegetation. The highest Ef values were found, after expectations, for the basic nutrients and significant essential elements, in a row of K > Mg > Mn > Ca. Together with these elements, significant impact of leached metabolites was also found for Rb, Sr and Ba. The enrichment of throughfall with rubidium is extraordinary high, probably even higher than that of its homologue potassium. Surprisngly high values of the Rb deposition fluxes in both tyes of throughfall certify the extensive input of this element through the root uptake of the forest vegetation, which is contingent on very close chemical characteristics of rubidium and potassium (ionic radii, electronegativity, ionic character of bonding of these alkaline metals in all their chemical compounds etc.). We have not found, however, similar pronounced paralell between Ca, Sr and especially Ba, which is even more abundant in the underlying rocks and soils than Sr.
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