DISTRIBUTION AND MOBILITY OF PHOSPHORUS IN GRASSLAND AND FOREST SOILS OF SASKATCHEWAN

Abstract

Soil phosphorus transformations and losses were measured in three profiles representative of an environmental gradient covering a grassland to forest transition: a Calcareous Brown, an Orthic Black and an Orthic Gray Luvisol. A sequential extraction procedure was used to measure the different forms of both inorganic (Pi) and organic (Po) phosphorus present in soil horizons. The mobilities of phosphate anions and of some selected (Po) compounds (glucose 6-phosphate, choline phosphate and adenosine triphosphate) were estimated through isotopic dilution kinetic experiments, and indicated that all the tested Po compounds had a higher mobility than phosphate anions in B horizons; also, Po was predominant over Pi in the water extracts from these soils. A mass-balance approach (Pedogenic Index) which incorporates depth-bulk density parameters in measuring the composition of soil horizons was used to calculate losses of Pt and apatite-P (HCl-Pi). Both grassland soils behaved similarly; they lost 500–550 kg P ha−1 (20%) of their original Pt and 30% of the initial HCl-Pi fraction was either transformed to various other forms of P or lost from the solum. Losses of Pt (3020 kg P ha−1: 41 %)and of HCl-Pi (70%) were much higher in the Gray Luvisol. Since losses of Pt could not be due to leaching of phosphate anions through the profile, the importance of Po in P leaching should be considered along with surface runoff in determining the fate of P in these soils. Key words: Pedogenic index, P mass balance, P fractionation, organic P leaching, P mobility