Soil organic phosphorus transformations along a coastal dune chronosequence under New Zealand temperate rain forest

Abstract

The chemical composition of soil phosphorus can vary markedly during pedogenesis, which has implications for phosphorus availability to plant and microbial communities during long-term ecosystem development. We used NaOH–EDTA extraction and solution 31P NMR spectroscopy to examine changes in soil phosphorus composition along the Haast chronosequence, a 6,500 year sequence of coastal dunes under lowland temperate rain forest on the west coast of the South Island of New Zealand. Soils along the chronosequence contained a variety of inorganic (orthophosphate, pyrophosphate, and long-chain polyphosphate) and organic (phosphomonoesters, phosphodiesters, and phosphonates) phosphorus compounds, although long-chain polyphosphates were detected only in the organic horizon and phosphonates were detected only in mineral soil. The concentrations of most compounds increased initially during the first few hundred years of pedogenesis and then declined as soils aged. However, concentrations of phospholipids, DNA, and long-chain polyphosphate all increased markedly in the organic horizon of older sites. The four inositol hexakisphosphate stereoisomers (myo, scyllo, neo, and d-chiro) accounted for a considerable proportion of the phosphomonoesters in mineral soil along the sequence (36–52 % of the organic phosphorus), but were not detected in quantifiable concentrations in the youngest mineral soil and all but one organic horizon. Concentrations of the two most abundant isomers (myo- and scyllo) declined along the chronosequence, but the scyllo isomer increased markedly as a proportion of the soil organic phosphorus as soils aged. Amorphous aluminum and iron oxides (i.e., extractable in acid-ammonium oxalate) increased continually throughout the chronosequence, indicating that the decline in inositol hexakisphosphate is due to low phosphorus availability rather than a decline in stabilization potential. Overall, these results provide further evidence that the chemical composition of organic and inorganic phosphorus pools vary markedly during pedogenesis, which has important implications for our understanding of biologically-available organic phosphorus during ecosystem development.