SEASONALITY OF NUTRIENT AVAILABILITY IN SOILS OF SUB-ARCTIC MOUNTAIN BIRCH WOODLANDS, SWEDISH LAPLAND

The seasonal course of the availability of nitrogen (ammonium and nitr ate) and phosphate in soils of some mountain birch woods in subarctic Sweden was measured in situ by using ion exchange membranes. One meado w system and two heath systems (differing in moisture availability) we re selected for the study, and ion exchange membranes were buried in s oils for between 4 and 37 wk during 1 yr. Annual nitrogen availability increased in the order dry heath < moist heath < meadow system, while phosphate availability was lowest in the meadow system. There were cl ear seasonal patterns of nitrogen and phosphorus availabilities, with a maximum during the, early (nitrogen) and late growing season (phosph orus) and a minimum during the winter season. Nitrate was detected in the meadow and in the moist heath system. The relative availabilities of nitrogen and phosphorus varied considerably among the study sites. Early-season nitrogen availability corresponded well with atmospheric nitrogen input by snow deposition during the winter season. Soil hydro logy was concluded to exert stronger influence on nutrient availabilit y than soil temperature in these woodlands. Nitrogen uptake rates in e xperimentally grown seedlings of mountain birch usually exceeded adsor ption rates to membranes, whilst seedling phosphorus uptake was estima ted to be similar to membrane phosphate adsorption.