Transformation of soil organic phosphorus along the Hailuogou post-glacial chronosequence, southeastern edge of the Tibetan Plateau

Organic phosphorus (P) accumulates in soil during pedogenesis, yet information on the composition and transformation of organic P during the early stages of soil development remains scarce. We studied the top 5 cm of mineral soil immediately beneath the organic horizon from six sites (0, 35, 45, 57, 85 and 125 years) along the Hailuogou glacier foreland chronosequence, on the southeastern edge of the Tibetan Plateau. Phosphorus compounds in the soils were determined by NaOH-EDTA extraction and solution phosphorus-31 nuclear magnetic resonance (P-31 NMR) spectroscopy. Extractable P was dominated by phosphomonoesters (up to 51.5%) and orthophosphate (37.9-44.6%) throughout the chronosequence. The phosphomonoesters were mainly hydrolysis products of RNA and phospholipids and followed a unimodal pattern with soil age, with maximum concentration at the 57-year-old site. myo-Inositol hexakisphosphate was not detected, although scyllo-inositol hexakisphosphate accounted for 4.7-9.3% of the extracted P and D-chiro- and neo-inositol hexakisphosphates occurred small amounts in a few soils (1.1-3.3% of the extracted P). DNA accounted for 4.0-8.3% of extracted P and increased continuously along the chronosequence, associated with increased inputs of plant and microbial residues and stronger sorption to soil surfaces during the rapid decline in soil pH. Pyrophosphate, an inorganic polyphosphate, occurred in small concentrations (up to 5.0%) that fluctuated with soil age. We conclude that organic P compounds accumulate rapidly in the top mineral soil during the early stages of pedogenesis, predominantly as relatively labile compounds from plant and microbial residues.