Oxygen isotopes of phosphate and soil phosphorus cycling across a 6500 year chronosequence under lowland temperate rainforest

Abstract

Phosphorus (P) availability declines during ecosystem development due in part to chemical transformations of P in the soil. Here we report changes in soil P pools and the oxygen isotopic signature of inorganic phosphate (d18Op) in these pools over a 6500-year soil coastal dune chronosequence in a temperate humid environment. Total P declined from 384 to 129 mg P kg- 1 during the first few hundred years of pedogenesis, due mainly to the depletion of primary mineral P in the HCl-extractable pool. The d18Op of HCl-extractable inorganic P initially reflected the signature of the parent material, but shifted over time towards (but not reaching) isotopic equilibrium. In contrast, d18Op signatures of inorganic P extracted in water and NaHCO3 (approximately 9 and 39 mg P kg- 1, respectively) were variable but consistent with isotopic equilibrium with soil water. In the NaOH-extractable P pool, which doubled from 63 to 128 mg P kg- 1 in the early stages of pedogenesis and then gradually declined, the d18Op of the extracted inorganic P changed from equilibrium values early in the chronosequence to more depleted signatures in older soils, indicating greater rates of hydrolysis of labile organic P compounds such as DNA and increase involvement in P cycling as overall P availability declines through the sequence. In summary, this application of d18Op to a long-term soil chronosequence provides novel insight into P dynamics, indicating the importance of efficient recycling through tight uptake and mineralization in maintaining a stable bioavailable P pool during long-term ecosystem development.