Biogeochemical cycles of soil phosphorus in southern Alpine spodosols

Abstract

We applied the Hedley fractionation to three Spodosols from the southern Alps to describe the distribution with depth of biological, or organically-bound forms (bicarb Po and NaOH Po) and of geochemical, or inorganically-bound forms (resin Pi, bicarb Pi, NaOH Pi, HCl Pi, and residual P) of soil phosphorus. These Spodosols differed considerably in regard to the crystallinity of free aluminium, the presence of amorphous compounds, and their respective distributions with depth. In only one case did the depth function of biological and geochemical forms of soil phosphorus clearly reflect the depth trend in amorphous compounds. In two cases, either no trend or no clear trend was obvious which suggests the presence of controls other than soil chemistry. Organically-bound forms of soil phosphorus are as high as 65% and 40% of total P in the topsoil and subsoil, respectively, and in all three soils, bicarb Po reaches a maximum of 80% of total available soil phosphorus at a depth of about 30 cm. We could not confirm previous claims of a stratification of biological and geochemical cycles in Spodosols, and we caution against an unwarranted reliance on soil taxonomy, instead of relevant soil chemical data, as a framework for the interpretation of soil phosphorus cycles in a pedogenetic context.