SOILWATER CHEMISTRY IN A HOLM-OAK (QUERCUS-ILEX) FOREST - INFERENCES ON BIOGEOCHEMICAL PROCESSES FOR A MONTANE MEDITERRANEAN AREA

Soil solution and free-flowing soilwater were sampled at various depth s for 3 years in a plot of holm oak (Quercus ilex L.) in the Montseny mountains (NE Spain). The soil solution retained at -65M kPa in the mi neral soil at depths of 20 and 40 cm had a different chemistry from th at of throughflow under the humic layer (H-layer throughflow) and, to a lesser extent, from deep subsurface flow. The dominant mobile anion in the soil solution and the deep flow was SO4(2-), whereas in the H-l ayer, SO4(2-) was overridden by alkalinity. H-Layer throughflow chemis try was extremely variable, in consequence of the large number of biog eochemical processes affecting it (e.g. rainfall chemistry, dry deposi tion, leaching from canopy and litter, decomposition), and of the quan tity of water available for transport of solutes. The chemistry of the soil solution was more predictable, as it was governed mainly by nutr ient uptake, cation exchange reactions and the seasonal wetting and dr ying cycles. The chemistry of the deep subsurface flow was often inter mediate between that of the soil solution and that of the H-layer thro ughflow; this suggested a mixture of displaced pre-event soil solution and of H-layer throughflow circulating through preferential flow path s. With humid antecedent conditions, the chemistry of the deep subsurf ace flow approached that of the soil solution. The cation concentratio n relationships in the soil solution were strongly linear. In a homoge neous soil, cation exchange theory predicts this to be expected only f or cations of the same charge, whereas cations of different charges sh ould follow power relationships. Our results, however, are consistent with a theoretical approach involving cation exchange reactions in a h ighly heterogeneous environment. Indeed, our study illustrates the het erogeneous nature of the soils, as the power relationship has been obs cured completely. Soilwater chemistries were markedly different from t hose of streamwater, particularly under dry conditions. During baseflo w, the stream is fed by groundwater, and the soils are probably discon nected from the stream. During humid periods, the chemistry of the str eamwater tends toward that of the deep subsurface flow. Chemical hydro graph separation indicates that, on average, stormflow water comprises an approximately one to one mixture of groundwater and deep subsurfac e flow.