Characterization of phosphorus in a spruce-fir Spodosol by phosphorus-31 nuclear magnetic resonance spectroscopy

Factors affecting the distribution and biogeochemical cycling of soil P are only partially understood. Many previous studies have focused on P distribution within surface mineral soils and cycling of inorganic P. Pew data are available on organic forms of P in forest Boor and mineral horizons of forest soils. Therefore, we used P-31 nuclear magnetic resonance (P-31-NMR) to characterize the P compounds in NaOH-ethylenediaminetetraacetic acid (EDTA) extracts of three Spodosol profiles. The NaOH-EDTA extracted most of the P in the three organic horizons (87, 83, and 69% of total P in Oi, Oe, and Oa horizons, respectively) and 39 to 67% in the Bhs horizon. Major P species detected in all horizons were inorganic orthophosphate and orthophosphate monoesters and diesters that ranged from 11 to 53%, 12 to 52%, and 13 to 45% of total P extracted, respectively. As is typical in Spodosol chemistry, considerable variation was found for organic P distribution in the organic horizons of the three profiles. However, on average, distribution patterns of P forms with depth were observed, including a decrease in the proportion of inorganic orthophosphate with greater decomposition in the forest floor (Oi > Oe > Ga), followed by an increase in the Bhs horizons. Pyrophosphate (5-19% of total P) and polyphosphates (5-13% of total P) were found mainly in the forest Boor horizons, except for one profile where pyrophosphate had evidently accumulated in the Bhs horizon. Traces (approximate to 2%) of phosphonates occurred only in the Oa horizon. Results suggest that biological activity, extractable Fe concentration, structural characteristics of P compounds, and pH are probably responsible for the distribution of P species hn these Spodosols.