Understanding phosphorus fluxes in soils in terms of the availability of phosphorus to plants has important implications for ecosystem productivity and resilience. While phosphorus is a major plant nutrient, biological and geochemical sinks generally maintain plant-available phosphorus as a very small fraction of the total soil phosphorus pool. In this study nine different organic and inorganic soil phosphorus fractions were measured using the Hedley sequential extraction procedure as modified by Tiessen and Moir (1993). The soils examined in this study were collected from the following five sites; a chronosequence of landslide scars ranging from 1 to 55+ years plus a late-succession vegetation association which represents a potential mature forest stage for aggrading landslide scars in the Luquillo Experimental Forest (LEF), Puerto Rico; abandoned pasture and mature tabonuco forest plots in the Luquillo Experimental Forest (LEF), Puerto Rico; Calciustols from the Guánica Forest Biosphere Reserve, a subtropical dry forest ( sensu Holdridge 1967, Ewel and Whitmore 1973) in southwestern Puerto Rico; soils were also collected from an area of primary forest, which has escaped disturbance from forest harvesting, land clearing and the deposition of anthropogenic chemicals in an old, unglaciated landscape at 600-705 m in the Cordillera de Piuchué, Chile (42° 30 ' S, 74°W); and mineral soils from a sandy Oxisol in successional secondary and primary forests ranging in age from 5 to 100+ years, in the Bragantina region, Para, Brazil.
It appeared that soil phosphorus was unlikely to limit aboveground growth in the study forests examined in this dissertation, if the consensus that resin Pi, bicarbonate Pi, and bicarbonate Po are readily available to plants in the short term is correct. Alternatively, if the productivity of any of the mature forests used in this study is shown to be phosphorus limited (by fertilization studies, for instance), then the interpretations of the availability of the Hedley phosphorus fractions would require revision. The results clearly suggest that soil organic matter (SOM) is an important source of labile or easily mineralized phosphorus and that changes in SOM are likely accompanied by changes in labile/plant available P.
