H+ budget in oligotrophic Quercus pyrenaica forests:: atmospheric and management-induced soil acidification?

The aim of the present work was to quantify the importance of the atmospheric deposition and the internal processes in soil acidification in oligotrophic Quercus pyrenaica forests, and determine the mechanism(s) involved in the neutralization of the acidity through the canopy and the soil. Four permanent oak forest plots were monitored in the Sierra de Gata mountains (Central-Western Spain): pH and mineral elements were determined in bulk precipitation, dry deposition, throughfall, stemflow, surface runoff, and in the soil, the water flowing through the humic (Ah) and mineral ( Bw) horizons. Soil acidification has been estimated from the balance of the elements (atmospheric input minus output by surface runoff and deep drainage) and the net accumulation of nutrients by biomass. The results showed that rainwater pH was 5.5, and dry deposition of H+ was lower than bulk deposition (20 vs. 31 eq H+ ha(-1) y(-1)). Throughfall was slightly less acid (pH 5.6) than the incident rain; however, stemflow was more acid than rain (pH 5.2). As a result, only 22 eq H+ ha(-1) y(-1) reached the soil surface, and the rest was taken up by the canopy (29 eq H+ ha(-)1 y(-1)), which could only explain between 4 and 8% of the cation leaching. The pH of water was increased slightly when percolating through the soil (from pH 5.6 to 5.8). Atmospheric deposition of free H+ and N-compounds had a minor contribution to the soil acidification (3 and 12%, respectively). The rest, 77% of H+, were produced by cation uptake, and 8% by mobilization of anion from mineral weathering and by precipitation or adsorption of cations. Almost all the acidity generated by the external and internal sources, was buffered by the soil (2.40 keq ha(-1) y(-1) on average), and losses from the soil were less than 1% of total H+. Neutralization occurred mainly by release of base cations from exchange surfaces and weathered minerals (71%); aluminium mobilization contributed to only 8%. If the forest reaches its degenerative phase, the lost acid-neutralizing capacity (ANC) could be completely restored by release of cations during biomass mineralization. However, if biomass is exported from these forests, further acidification of these already oligotrophic oak coppices will result in the future.