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We developed and assessed a method for simultaneous •extraction of plant available nitrogen, phosphorus, sulfur and potassium using anion and cation exchange membranes (ACEM). The technique was found to be highly suitable for routine soil testing due to its simplicity, rapidness and accuracy. The study compared the amount of nutrients extracted by ACEM with conventional chemical-based extractants for P and K (0.5M NaHC03) and N and S (0.001M CaCh) for 135 soil samples representing a wide range of soil types in Western Canada. The nutrient availability predicted by ACEM was significantly correlated with the conventional methods. The correlation was not affected by the two•different shaking times tested (one hour and 15 minutes), suggesting that extraction times as short as 15 minutes could be used in ACEM extraction. To evaluate the relative ability of ACEM and the conventional tests to predict actual nutrient availability to plants, canola plants were grown on soils in the growth chamber and actual plant uptake was compared to test-predicted nutrient availability. Phosphorus and potassium uptake by canola plants was more closely correlated with ACEM extractable P and K (r 2= 0.84*** and 0.54***) than with 0.5M N aHC03 P and K (r 2= 0. 70*** and 0.37***). Also, nitrogen and sulfur uptake by canola plants was significantly correlated with ACEM extractable-N~ and-S04 (r2 = 0.60*** and 0.70***) and with CaCh extractable-N03 and-S04 (r2 = 0.57*** and 0.61 ***). Availab~lity of all four macronutrients can be assessed in a single ACEM extraction. The higher correlation with plant uptake suggests that ACEM is a better index of macronutrient availability than conventional methods. The ACEM soil test could be readily adopted in routine soil analysis because of low cost and simplicity as well as its consistency over a wide range of soil types.

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The purpose of this study was to characterize the quantity, distribution, and variance of water-soluble organic C (WSOC) in a soil under trembling aspen (Populustremuloides Michx.) in the southern boreal forest of Canada. WSOC was determined monthly from May to October 1994 in the forest floor horizons (L, F, H) and mineral soil (Ae) of an aspen stand in Prince Albert National Park, Saskatchewan. The concentration of WSOC varied considerably with profile depth, but varied little among the slope positions and aspects. The L horizon had the highest WSOC concentration (425–8690 mg•kg−1 ovendried soil), followed by the F, H, and Ae horizons. The concentration of WSOC in the Ae horizon was significantly related to the concentration in forest floor horizons above. Water-soluble organic C in the Ae horizon likely was derived from the overlying organic layer by leaching. In a laboratory incubation, the rate of WSOC release (the net result of release and uptake) during incubation decreased continuously over time, but in the field, the rate of WSOC release decreased slightly early in the growing season, but increased later in the season as new litter fall reached the forest floor. This indicates that litter fall is a major factor in the replenishment of WSOC in aspen forest stands.

spatial variations in soil nitrogen and phosphorus supply rates in a boreal aspen forest. Can. J. Soil Sci. 77: 597-612. Soil nitrogen (N) and phosphorus (P) supply is one of the growth limiting factors in many forest ecosystems. Seasonal patterns in soil N and P supply rate were examined during a 2-yr period (1994)(1995) for forest floor (L, F and H) and upper mineral (Ae) horizons in an 80-yr-old aspen forest in Saskatchewan, Canada. Accumulation of plant nutrient ions on ion exchange resins incubated in the field can provide an estimate of nutrient supply rate in soils because ion exchange resins have the potential ability to simulate nutrient flux to plant roots. Nutrient supply rates and the effect of plant uptake on nutrient supply rate was assessed using ion exchange membranes buried inside and outside polyvinyl chloride (PVC) cylinders. The difference between ion flux to the membranes inside (root uptake excluded) versus outside the cylinders was used as an index of plant nutrient uptake. From May to October, nutrient uptake (µg 10 cm -2 2 wk -1 ) by plants ranged from 1.6 to 31.7 (NO 3 --N), from 2.7 to 13.7 (NH 4 + -N) and from 2.6 to 12.7 (P), with maximum N and P uptake in summer. Nutrient uptake by plants also varied among horizons. In general, plant uptake of NO 3 --N, NH 4 + -N and P was highest in the H horizon, followed by the F and Ae horizons, with lowest uptake apparent in the L horizon. The results are consistent with the distribution of plant fine roots: most were found in the H horizon (68%), followed by the Ae and F horizons (15%), and the L (2%) horizon. Autumn litterfall represented a nutrient return of 28-40 kg N ha -1 and 4-7 kg P ha -1 to the forest floor which coincided with an increase in ion supply rates in the forest floor. During the growing season, atmospheric inputs via bulk deposition and throughfall contributed small amounts of N (1.8 kg NH 4 + -N ha -1 and 0.23 kg NO 3 --N ha -1 ) and P (1.38 kg ha -1 inorganic P) to the forest floor. Recycling of nutrients by litterfall and subsequent mineralization and re-assimilation by plant roots in the forest floor is a dynamic and important component of nutrient cycling in boreal aspen forest ecosystems.