Seasonality of Nutrient Availability in Soils of Subarctic Mountain Birch Woodlands, Swedish Lapland

Abstract

The seasonal course of the availability of nitrogen (ammonium and nitrate) and phosphate in soils of some mountain birch woods in subarctic Sweden was measured in situ by using ion exchange membranes. One meadow system and two heath systems (differing in moisture availability) were selected for the study, and ion exchange membranes were buried in soils 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 clear seasonal patterns of nitrogen and phosphorus availabilities, with a maximum during the early (nitrogen) and late growing season (phosphorus) 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 hydrology was concluded to exert stronger influence on nutrient availability than soil temperature in these woodlands. Nitrogen uptake rates in experimentally grown seedlings of mountain birch usually exceeded adsorption rates to membranes, whilst seedling phosphorus uptake was estimated to be similar to membrane phosphate adsorption.