Forms and profile distribution of soil phosphorus in alpine Inceptisols and Spodosols (Pyrenees, France)

Soil phosphorus in different depths or horizons of two Inceptisols and four Spodosols was fractionated using the Hedley, Stewart and Chauhan method. The soils were sampled at altitudes between 2000 and 2400 m in the Eastern Pyrenees (France). The total phosphorus content varied considerably between soils and between horizons in the soils. The total phosphorus content ranged from 700 to 1100 μg g−1 in the Inceptisols, and from 216 to 955 μg g−1 in the Spodosol horizons. At all depths in Inceptisols, the NaOH-Po form (range 330–665 μg g−1) represented over 45% of the sum of the phosphorus fractions. In the Spodosols, the concentrations were generally lower (between 72 and 125 μg g−1 in the eluvial horizons and between 115 and 290 μg g−1 in the illuvial horizons). Moreover, there was a correlation between organic phosphorus extracted by NaOH and extractable aluminum. The partial correlation with %C when Al concentration was considered as constant was highly significant. Eluviation processes occurred with migration of soluble aluminum leading to eluvial and illuvial horizons with marked differences in their phosphorus contents.