Phosphorus transformations along a soil/vegetation series of fire-prone, dolomitic, semi-arid shrublands of southern Spain Soil P and Mediterranean shrubland dynamic

Models of P transformations duringpedogenesis and with succession have developed fromstudies in temperate humid regions with neutral toacidic soils. Little is known about P biogeochemistryand P availability in semi-arid Mediterranean-typeshrublands with alkaline soils. We studied Ptransformations in a series of semi-arid, dolomiticshrublands in southeastern Spain, ranging from afrequently-burned, open gorse-scrubland on erodedTypic Xerorthents to a long-unburned, mature garrigueon Entic Haploxerolls. In contrast to the commonpattern of decreasing total P concentrations in thesoil profile with soil development due to leaching,total P increased markedly in this system. This is dueto concentration increases of relatively insolubleelements (P, Al, Fe, Ti) as karstification of parentmaterial (dolomitic marbles with up to 94% Ca-Mgcarbonate) during pedogenesis released bicarbonateand, subsequently, Ca and Mg leached from the profileat a higher rate. The total element to Ti ratiosindicated that the relative ion weathering losssequence, from easily weathered to resistant ions, wasCa>Mg > > > Fe>=Al>Ti, showing that P islost from these shrublands at an intermediate rate. Inone extreme of the series (the open gorse-scrubland),most soil P was Ca bound and organic P concentrations,organic matter content and phosphatase activity werevery low, as predicted by the model of Walker andSyers for the initial stages of soil development.However, this site showed the highest inorganic soilsolution P concentration, low soil P fixation capacityand the lowest foliar N:P ratios. Soils from theintermediate stages of the series showed the highestlabile inorganic and labile organic P concentrations.At the other extreme of the series (the maturegarrigue), a high proportion of soil P was in occludedinorganic and organic forms as predicted by the Walkerand Syers'; model. However, Ca bound P still accountedfor the largest single P fraction. Soils showed veryhigh sorption capacity (and high extractable Fe and Alconcentrations) and released very little P tosolution. Increasing values for NaOH extractable Po,organic matter and phosphatase activity indicate thatcycling of P through organic matter is increasinglyimportant with ecosystem development through theseries.