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 (P i ) and organic (P o ) phosphorus present in soil horizons. The mobilities of phosphate anions and of some selected (P o ) compounds (glucose 6-phosphate, choline phosphate and adenosine triphosphate) were estimated through isotopic dilution kinetic experiments, and indicated that all the tested P o compounds had a higher mobility than phosphate anions in B horizons; also, P o was predominant over P i 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 P t and apatite-P (HCl-P i ). Both grassland soils behaved similarly; they lost 500–550 kg P ha −1 (20%) of their original P t and 30% of the initial HCl-P i fraction was either transformed to various other forms of P or lost from the solum. Losses of P t (3020 kg P ha −1 : 41 %)and of HCl-P i (70%) were much higher in the Gray Luvisol. Since losses of P t could not be due to leaching of phosphate anions through the profile, the importance of P o 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