Biogeochemical cycles of soil phosphorus in southern Alpine spodosols

We applied the Hedley fractionation to three Spodosols from the southern Al ps to describe the distribution with depth of biological, or organically-bo und forms (bicarb Po and NaOH Po) and of geochemical, or inorganically-boun d forms (resin Pi, bicarb Pi, NaOH Pi, HCl Pi, and residual P) of soil phos phorus. These Spodosols differed considerably in regard to the crystallinit y of free aluminium, the presence of amorphous compounds, and their respect ive distributions with depth. In only one case did the depth function of bi ological and geochemical forms of soil phosphorus clearly reflect the depth trend in amorphous compounds. In two cases, either no trend or no clear tr end was obvious which suggests the presence of controls other than soil che mistry. Organically-bound forms of soil phosphorus are as high as 65% and 4 0% of total P in the topsoil and subsoil, respectively, and in all three so ils, 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 strati fication 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 cyc les in a pedogenetic context. (C) 1999 Elsevier Science B.V. All rights res erved.