Initial Stages Of Soil Development Research Articles

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 during pedogenesis and with succession have devel- oped from studies in temperate humid regions with neutral to acidic soils. Little is known about P biogeochemistry and P availability in semi-arid Mediterranean-type shrublands with alkaline soils. We studied P transformations in a series of semi-arid, dolomitic shrublands in southeastern Spain, ranging from a frequently-burned, open gorse-scrubland on eroded Typic Xerorthents to a long-unburned, mature garrigue on Entic Haploxerolls. In contrast to the common pattern of decreasing total P concentrations in the soil profile with soil develop- ment due to leaching, total P increased markedly in this system. This is due to concentration increases of relatively insoluble elements (P, Al, Fe, 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 intermediate rate. In one extreme of the series (the open gorse-scrubland), most soil P was Ca bound and organic P concentrations, organic matter content and phosphatase activity were very low, as predicted by the model of Walker and Syers for the initial stages of soil development. However, this site showed the highest inorganic soil solution P concentration, low soil P fixation capacity and the lowest foliar N:P ratios. Soils from the intermediate stages of the series showed the highest labile inorganic and labile organic P concentrations. At the other extreme of the series (the mature garrigue), a high proportion of soil P was in occluded inorganic and organic forms as predicted by the Walker and Syers' model. However, Ca bound P still accounted for the largest single P fraction. Soils showed very high sorption capacity (and high extractable Fe and Al concentrations) and released very little P to solution. Increasing values for NaOH extractable Po, organic matter and phosphatase activity indicate that cycling of P through organic matter is increasingly important with ecosystem development through the series.