Atmospheric deposition in oligotrophic Quercus pyrenaica forests:: implications for forest nutrition

The aim of this study was to analyze the contribution of atmospheric input of elements to tree nutrition in an oligotrophic forest ecosystem of Quercus pyrenaica, located in a low polluted area. The work was conducted over 3 years in four permanent plots, in the Sierra de Gata mountains (CW Spain). To calculate total (wet and dry) atmospheric deposition, we measured element inputs via bulk precipitation (BP), throughfall (TF), stemflow (SF), and dry deposition (DD) (filter gauge method). Available soil and leaf nutrient contents were also analyzed. Aboveground tree nutrient demand was estimated from annual tree growth and nutrient concentrations in trunks, branches, and leaves. Atmospheric depositions were low, especially for N (5.9 kg N ha(-1) per year) and S (7 kg S ha(-1) per year), as expected in an unpolluted area. BP dominated atmospheric deposition, but DD represented an important input of nutrients, especially for N, S, P, K, Mg, and Ca (2.2, 2.2, 0.6, 3.6, 1.4, and 3.4 kg ha(-1) per year, respectively). Low soil contents of N, P, and exchangeable cations were related to low canopy leaching rates, or even to canopy uptakes, showing a certain degree of self-regulation. In fact, the differences in canopy exchange (CE) found between the plots were strongly correlated to leaf contents (r > 0.9, for Mn, Mg, K and P). Comparison of annual aboveground accumulation of nutrients with total annual atmospheric deposition showed that the deposition amount was much higher than the annual demand of the trees for Na, S, and Zn. For P, K, Mn, and Mg annual atmospheric deposition matched the annual tree demand for aboveground growth. For N and Ca, atmospheric depositions reached between 58 and 31%, respectively, of the annual demands. This contribution by atmospheric depositions would explain why these forest ecosystems have persisted in poor soils subjected to permanent and long-lasting losses of nutrients through biomass removal (i.e. leaves, timber, and grazing). (C) 2002 Elsevier Science B.V. All rights reserved.