Soil affects throughfall and stemflow under Turkey oak (Quercus cerris L.)

To investigate how soil properties affect throughfall and stemflow, we conducted a study in a forest of central Italy over a full hydrologic year to compare the chemical composition and the water fluxes of the throughfall and stemflow generated by Turkey oaks (Quercus cerris L.). The study was achieved on two adjacent areas that showed the same topography, supported Turkey oaks of the same height (about 20 m) and age (about 60 years), and received uniform precipitation (835 mm year(-1)). However, the two areas differed for soil reaction, one being acidic (area A, mean profile-weighted pH(H2O) = 5.84) and the other sub-alkaline (area B, mean profile weighted pH(H2O) = 7.55). The branching angle and canopy volume of the oaks differed statistically (Wilcoxon signed-rank test at alpha = 0.05) between areas, with the slender trees of area A having more upward thrust branches. As a consequence, the oaks of area A produced more stemflow per unit canopy surface than those of area B, as indicated by the amount of stemflow per unit soil surface (15-cm radius) around the trunk base and by the stemflow funneling ratio per basal area (F-P,F-B). The annual fluxes determined for 17 solutes were higher in throughfall than in rainfall, except for F and HCO3, reflecting the enrichment and acidification of the precipitation water as it flows through the canopy. For the full hydrological year, the enrichment ratios (E-P,E-B and E-P,E-T) indicated that the stemflow of area A was more enriched than that of area B for the following solutes: total N, TOC, total acidity, carboxylic acidity, phenolic acidity, and NH4. Several significant differences in throughfall (electrical conductivity, Ca, Mg, K, NO3, total N, total organic C, organic anions) and in stemflow (pH, electrical conductivity, Ca, Mg, Na, Cl, NO3, HCO3) chemistry were observed between areas over the course of three time series of rainfall events (throughfall series T1, from September to November 2004; throughfall series T2, from December 2004 to February 2005; stemflow series S1, from March to September 2004). The study further demonstrated the existence of strong links between the significant differences in soil properties (pH, exchangeable Ca and K, effective cation exchange capacity, total and organic C content, mineralogy) and the significant differences in throughfall and stemflow chemistry (pH, HCO3, Ca, K, electrical conductivity) recorded between the two areas. The main processes involved in the short-scale spatial differentiation of throughfall and stemflow at the site appeared to be either soil-dominated like pedogenesis, mineral weathering and organic matter transformation, or tree-mediated such as elemental biocycling.