PHOSPHORUS TRANSFORMATIONS AND REDISTRIBUTION DURING PEDOGENESIS OF WESTERN CANADIAN SOILS

Although phosphorus is often assumed to be relatively immobile in soil s, significant redistribution occurs during pedogenesis. We quantified P redistribution as a result of 13,000 yr of pedogenesis in soil prof iles along an environmental gradient of increasing moisture availabili ty and along toposequences in the Brown and Dark Brown Chernozemic, an d Gray Luvisolic soil zones of Saskatchewan, Soil P transformations, t ranslocations, and net gains or losses were measured by P sequential f ractionation, mass balance calculations, and pedogenic indexing. All p rofiles showed a surface accumulation of P and a depletion in the lowe r 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 fro m the depletion zone had leached into the subsoil (50 to 150 cm depth) . Phosphorus fractionation indicated that the subsoil P accumulation c onsisted of secondary inorganic P that was not associated with organic matter or clay movement. Hence, we suggest that deep leaching of P du ring pedogenesis may have occurred as soluble inorganic P, Pedogenic i ndexing showed that some total P had been lost beyond the deepest samp ling, which could partly be accounted for by P movement into groundwat er. No greater P redistribution was found in the subhumid Saskatoon de pression profile than the semiarid Swift Current depression profile. T hus, the expected effects of the environmental gradient between the si tes (a precipitation increase from 350 to 450 mm with a parallel decre ase in potential evapotranspiration from 700 to 450 mm) were overridde n by the effects of local topography on water movement and soil moistu re.