Characterization of chemical phosphorus forms in volcanic soils using 31P-NMR spectroscopy

Phosphorus (P) is rapidly immobilized in Chilean volcanic soils and its availability to plant is greatly reduced. Chemical forms of P accumulated in soils would eventually determine its availability of plants and its potential to cause pollution in water bodies down stream. To characterize different chemical P species, we carried out chemical fractionation and P-31-Nuclear Magnetic Resonance (NMR) spectroscopy analyses for 10 volcanic ash-derived soils in central southern Chile. Phosphorus ill the alkaline extracts of soils at 0-15 cm and 15-30cm depth was characterized by P-31-NMR. The P-31-NMR results indicate that inorganic-P and monoester-P are the most important P forms in all soils. The diester-P and pyrophosphate were also found in some soils. The patterns of P distribution in the 0-15 and 15-30cm were essentially the same; the signal intensities were usually stronger in the upper samples. The amounts of organic- and inorganic-P determined by P-31-NMR method correlate with those by the chemical fractionation procedure. Furthermore, the organic P of soils, calculated from NMR signals, correlates with the organic matter content of the soils (r = 0.82, p < 0.01). Those results suggest that the observed P-31-NMR signals represent the P forms in soils.