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Seasonal and spatial variations in soil nitrogen and phosphorus supply rates in a boreal aspen forest

Huang, W.Z. and J.J. Schoenau . 1997. Can. J. Soil Sci. 77: 597-612

Abstract

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 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 (mg 10cm-2 2wk-1) by plants ranged from 1.6 to 31.7 (NO3--N), from 2.7 to 13.7 (NH4 -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 NO3--N, NH4 -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 through fall contributed small amounts of N (1.8 kg NH4 -N ha-1 and 0.23 kg NO3--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.

Key Words

Forest floor, ion exchange membranes, nutrient supply