Carbon and nitrogen relations among soils, microbes and plants in boreal forests

Across Fennoscandian boreal forests, variations in hill-slope hydrochemistry cause regular patterns in vegetation composition and forest productivity. The turnover of N, the nutrient limiting plant growth, should be a result of interactions between soils, microbes and plants. This thesis represents a first attempt to describe variations in microbial biomass, community structure and activity associated with these patterns. The main study area was a local 90-mlong natural productivity gradient in northern Sweden, site Betsele, representing three coniferous forest types with increasing productivity: dwarf-shrub (DS), short-herb (SH) and tall-herb (TH) types. Destructive tree girdling was conducted in a DS forest type at another site, Åheden, to enable estimation of the biomass of mycelium of ectomycorrhizal (ECM) fungi. The gradient encompassed the range in soil chemistry and plant community composition in Fennoscandian boreal forests and is thus a useful model for soil-microbialplant interactions. There was also, at this local landscape level, associations between certain soil conditions and plants with specific mycorrhizal types, as proposed typical of different biomes on a continental scale. At Åheden, extramatrical ECM mycelium contributed one third of soil microbial biomass and produced, together with ECM roots, half the dissolved organic carbon. Neither microbial biomass nor soil (root plus microbial) respiration rates varied along the forest productivity gradient. This was unexpected because of the large variation in forest productivity. Gross N mineralization rate increased roughly in proportion to this three-fold increase in productivity. Thus, in contrast to the conventional view, higher rates of N mineralization were not caused by higher microbial activity in general. The susceptibility of the microbial activity of mor soils to physical disturbance decreased when soil fertility increased from the DS to the TH forest type; this was paralleled by a drastic decrease in fungal biomass. Variations in the contribution of mycorrhizal fungi to the microbial biomass confound interpretations of relations among soil and microbial C/N ratios and soil N mineralization. I hypothesize that low N supply and plant productivity, and hence low litter C supply to saprotrophs are associated with a high plant C supply to mycorrhizal fungi, while the reverse occurs under high N supply. Thus, effects of N availability on C supply to these functional groups of microbes acts in opposing directions