Soil nutrient availability and relationships with aboveground biomass production on postharvested upland white spruce sites in interior Alaska

Soil nutrient availability was assessed on unharvested white spruce (Picea glauca (Moench) Voss) sites, on a recently harvested site and on 14-year-old postharvested sites stratified into four different regeneration types defined by surface soil conditions and colonizing species. These values were compared with field aboveground biomass production that had been estimated in a previous study and with biomass production of bioassay seedlings. All sites were upland and south facing. On this range of sites, laboratory net N mineralization was the soil characteristic that was the most strongly associated with plant growth in the field as well as in the greenhouse bioassay. The significance of this relationship was mainly caused by the presence of sites regenerating to aspen (Populus tremuloides Michx.) which showed high plant biomass production and high soil N availability. Total soil N content, cumulative field soil temperature and soil moisture content were poorly related to N mineralization estimates and to plant biomass production. Soil temperature had an effect on N mineralization and plant growth only when sites where the forest floor had been scraped during the harvesting operations, were included in the computations. Despite a higher soil temperature, these sites showed decreased N mineralization rates and decreased plant biomass production. These results suggest that on south facing postharvested white spruce sites (i) soil temperature does not show enough variability to be an important factor controlling nutrient availability and plant growth unless the soil is severely disturbed, (ii) the rate of N mineralization is controlled by a small pool of rapidly cycling N which is poorly related to forest floor total N concentrations, and (iii) N availability and vegetation production vary with regeneration type.