FACTORS AFFECTING PHOSPHATE SORPTION ALONG A MEDITERRANEAN, DOLOMITICSOIL AND VEGETATION CHRONOSEQUENCE

Phosphate sorption by calcareous soils has been studied mainly on heav ily fertilized agricultural soils and soils with calcite as the main c arbonate mineral. We examined factors affecting phosphate adsorption i n the soils of a semi-arid, mediterranean, dolomitic, soil and vegetat ion chronosequence in southeastern Spain. The youngest soils are highl y eroded, Sandy Regosols (Typic Xerorthents) under grorse-scrubland ve getation. These have small P sorption capacities, large Mg-Ca carbonat e contents but small amounts of Fe and Al oxides. Small total P (HNO3/ HClO4 digestion) concentrations (30-130 mu g P g(-1)), of which up to 90% is Ca-bound (HCl-extractable), are typical of these young soils. P sorption markedly increased when Ca2+ was added to the solution. The fractionation of previously sorbed P indicates that the fate of most o f this extra-sorbed P is the labile-P fraction sorbed on to (carbonate ) surfaces and the apatite-like fraction (NaHCO3-extractable and HCl-e xtractable fractions). At the other extreme, older more-intensively we athered, sandy-clay-loam rendzinas (Entic Haploxerolls), supporting de nse mature garrigue, have a much greater P adsorption capacity and lar ger clay and Fe and Al oxide concentrations. They have more total P (c a 400 mu g P g(-1)), much of it in occluded form (residual fraction). These soils show no significant differences in P sorption whether or n ot CaCl2 was used as a background electrolyte. Considering the overall variations within the chronosequence, dithionite extractable Fe and A l are the properties best correlated with P sorption. This support the general finding that crystalline Fe-oxides (e.g. goethite and haemati te) appear to be the most important P-sorbing component for soils in t he Mediterranean region, rather than amorphous Fe-oxides (e.g. ferrihy drite) as is reported for more mesic areas. Stepwise multiple regressi on and fractionation data, however, suggest that, provided the soil so lution is rich in Ca2+, carbonate may also be a significant contributi ng factor to P sorption, especially in the youngest of these dolomitic soils.