Phosphorus transformations and redistribution during pedogenesis of western Canadian soils

Although phosphorus is often assumed to be relatively immobile in soils, significant redistribution occurs during pedogenesis. We quantified P redistribution as a result of 13,000 yr of pedogenesis in soil profiles along an environmental gradient of increasing moisture availability and along toposequences in the Brown and Dark Brown Chernozemic, and Gray Luvisolic soil zones of Saskatchewan, Soil P transformations, translocations, and net gains or losses were measured by P sequential fractionation, mass balance calculations, and pedogenic indexing. All profiles showed a surface accumulation of P and a depletion in the lower A and upper B horizons. Mass balance showed that as much as 130 g m(-2) of P had been moved out of the depletion zone (20 to 50 cm depth) while less than half of that was accounted for by the accumulation of (mostly organic) P in the surface horizon. Over half of the P lost from the depletion zone had leached into the subsoil (50 to 150 cm depth). Phosphorus fractionation indicated that the subsoil P accumulation consisted of secondary inorganic P that was not associated with organic matter or clay movement. Hence, we suggest that deep leaching of P during pedogenesis may have occurred as soluble inorganic P, Pedogenic indexing showed that some total P had been lost beyond the deepest sampling, which could partly be accounted for by P movement into groundwater. No greater P redistribution was found in the subhumid Saskatoon depression profile than the semiarid Swift Current depression profile. Thus, the expected effects of the environmental gradient between the sites (a precipitation increase from 350 to 450 mm with a parallel decrease in potential evapotranspiration from 700 to 450 mm) were overridden by the effects of local topography on water movement and soil moisture.