| Summary: | The objective of this research was to document the forms, amounts and cycling patterns of carbon (C), nitrogen (N), phosphorus (P) and sulfur (S) in soil profiles under trembling aspen (<i>Populus tremuloides</i> Michx.) stands located in Prince Albert National Park, Saskatchewan, Canada. Nutrient cycling was assessed by examining the forms, amounts and distribution of nutrients in components of soil and plant litter and changes in supply rates of nutrients seasonally over a two year period. A novel technique, involving in situ burial of ion exchange resin membranes (IEM) was used in the study to follow changes in supply rates of nutrient in the field and laboratory. The technique was also developed to assess differences in soil N, P and K supply rates in situ over small depth increments in the forest floor horizons (ie. the L, F and H horizons). The total mass of organic C stored in the forest floor and mineral soil to a depth of 1-m ranged from 95.4 to 103.4 Mg ha$\sp{-1},$ with a mean of 99.2 Mg ha$\sp{-1}.$ Organic C and N in the forest floor accounted for the greatest proportion of the total storage (47.3% of C and 34.2% of N), followed by the B horizon (22.4% of C and 32.7% of N), the A horizon (17.3% of C and 18.3% of N) and the C horizon (13.0% of C and 14.8% of N). Unlike C and N, more than 96% of the total P was found in the mineral soil and only 3.5% in the forest floor. Much of the P stored in the mineral horizons is contained in non-labile primary mineral forms. The greatest proportion (36.5%) of organic S was found in the C horizon, with 26.6% in the forest floor. Water-soluble organic C (WSOC) was determined monthly from May to October 1994 in the forest floor and Ae horizons. The distribution of WSOC within the soil profiles was related to the distribution of organic C. The concentration of WSOC varied significantly with profile depth, but varied little among the slope positions and aspects. The L horizon had the highest WSOC concentration (425-8690 mg kg<sup>-1</sup> oven dry soil), followed by the F, H and the Ae horizon. The seasonal and spatial patterns in nutrient supply rate showed that the supply of readily available N, P and S in forest soils, especially in the forest floor, varied greatly over the season and by horizon. (Abstract shortened by UMI.)
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