Evidence for progressive phosphorus limitation over long-term ecosystem development: Examination of a biogeochemical paradigm

To test predictions of ecosystem theory for changes in P cycling over primary succession, we determined soil phosphorus (P) in labile, primary mineral, organic, and occluded forms along a chronosequence of five wave cut terraces known as the Ecological Staircase. The Ecological Staircase terraces (T1-T5) transition naturally from fertile native coastal forests in California, USA, to diminutive pygmy vegetation over the span of > 500,000 years of pedogenesis. Soil P fractions were quantified to a depth of 40 cm on T1-T5 using a modified Hedley P fractionation procedure. Overall results confirmed the Walker and Syers Model of Phosphorus Transformations During Pedogenesis: total P declined from youngest (194 mg/kg P) to oldest (127 mg/kg P) sites; primary-mineral P decreased sharply from T1 to older sites; and occluded P dominated P pools at the oldest pygmy sites (T3-T5). In addition, foliar P concentrations declined markedly in the pygmy forest, and N/P of vegetation (T1: 6.03, T5: 14.4) and N/P-organic of mineral soils (T1: 6.10, T5: 25.3) increased significantly over time. Results point to P as the primary limiting nutrient in the pygmy forest, exemplifying the terminal steady-state of ecosystem retrogression that underlies the persistence of this unique ecosystem.