PHOSPHORUS TRANSFORMATIONS ALONG A SOIL VEGETATION SERIES OF FIRE-PRONE, DOLOMITIC, SEMIARID SHRUBLANDS OF SOUTHERN SPAIN - SOIL P AND MEDITERRANEAN SHRUBLAND DYNAMIC/

Models of P transformations during pedogenesis and with succession hav e developed from studies in temperate humid regions with neutral to ac idic soils. Little is known about P biogeochemistry and P availability in semi-arid Mediterranean-type shrublands with alkaline soils. We st udied P transformations in a series of semi-arid, dolomitic shrublands in southeastern Spain, ranging from a frequently-burned, open gorse-s crubland on eroded Typic Xerorthents to a long-unburned, mature garrig ue on Entic Haploxerolls. In contrast to the common pattern of decreas ing total P concentrations in the soil profile with soil development d ue to leaching, total P increased markedly in this system. This is due to concentration increases of relatively insoluble elements (P, Al, F e, Ti) as karstification of parent material (dolomitic marbles with up to 94% Ca-Mg carbonate) during pedogenesis released bicarbonate and, subsequently, Ca and Mg leached from the profile at a higher rate. The total element to Ti ratios indicated that the relative ion weathering loss sequence, from easily weathered to resistant ions, was Ca>Mg>>>P >Fe>=Al>Ti, showing that P is lost from these shrublands at an interme diate rate. In one extreme of the series (the open gorse-scrubland), m ost soil P was Ca bound and organic P concentrations, organic matter c ontent and phosphatase activity were very low, as predicted by the mod el of Walker and Syers for the initial stages of soil development. How ever, this site showed the highest inorganic soil solution P concentra tion, low soil P fixation capacity and the lowest foliar N:P ratios. S oils from the intermediate stages of the series showed the highest lab ile inorganic and labile organic P concentrations. At the other extrem e of the series (the mature garrigue), a high proportion of soil P was in occluded inorganic and organic forms as predicted by the Walker an d Syers' model. However, Ca bound P still accounted for the largest si ngle P fraction. Soils showed very high sorption capacity (and high ex tractable Fe and Al concentrations) and released very little P to solu tion. Increasing values for NaOH extractable Po, organic matter and ph osphatase activity indicate that cycling of P through organic matter i s increasingly important with ecosystem development through the series .