Atmospheric deposition and ionic concentration in soils under pine and deciduous forests in the river Sor catchment (Galicia, NW Spain)

The chemistry of rainwater, throughfall, soil solution and runoff was studied in a Pinus sylvestris forest and in a mixed forest with Quercus robur and Betula alba from November 1991 to October 1993 in the river Sor watershed, NW Spain. Rainwater had an average pH of around 4.8 and the dominant ions were Cl, SO4 and Na. Throughfall was remarkably enriched in SO4, base cations (mainly K) and H, while concentrations of NO3 were similar to those observed in rainwater; pine forest throughfall was notable for the high content of SO4, H and Al. Soil solutions from Ah (0–10 cm) horizons had similar ionic concentrations in both forests. Soil solutions were also characterized by NH4+ levels tending to decrease with depth. This, accompanied by a parallel increase in NO3 in both soils and a strong H+ neutralization in the pine forest soil are remarkable features in soil solutions. Stream waters showed lower levels of SO4, NO3 and K compared to throughfall, while Ca and Mg levels were similar; levels of NH4 and Al in runoff waters were negligible. The differences between soil solutions and stream waters implies that important changes in water chemistry occurred in the subsoils.