Characterization of Phosphorus in a Spruce‐Fir Spodosol by Phosphorus‐31 Nuclear Magnetic Resonance Spectroscopy

Abstract

Abstract 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. Few data are available on organic forms of P in forest floor and mineral horizons of forest soils. Therefore, we used 31 P nuclear magnetic resonance ( 31 P‐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 > Oa), 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 floor horizons, except for one profile where pyrophosphate had evidently accumulated in the Bhs horizon. Traces (≈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 in these Spodosols.