Terrestrial ecosystems are currently exposed to climatic and air quality changes with increased atmospheric CO₂, increased temperature and periodical droughts. At a temperate heath site this was investigated in a unique full factorial in situ experiment (CLIMAITE). The climate change treatments started October 2005 and consisted of increased temperature (T), extended summer drought (D), increased atmospheric CO₂ and all combinations of these treatments (TD, TCO₂, DCO₂ and TDCO₂).

In this thesis, responses in soil inorganic and microbial nutrient concentration were investigated after one year of climate change treatment. Additionally, top soil net mineralization, immobilization and leaf litter decomposition was investigated through the winter half year separately below Calluna and Deschampsia plants, and acquisition of organic nitrogen in plants and soil microorganisms was assessed.

After one year of treatments, warming increased microbial N, C and P and decomposition of leaf litter below Calluna plants. In Deschampsia soil the net nitrification rate decreased significantly in response to drought, by contrast, an increase was observed in Calluna soil. Drought reduced leaf litter decomposition for both species.

In warmed plots an early senescence was observed with effects on green Deschampsia biomass, on Deschampsia root nitrogen concentration and on acquisition of ¹⁵N from glycine.

In this thesis, experiments using the stable isotopes ¹⁵N and ¹³C as tracers of ammonium and amino acid acquisition by plants and soil microorganisms suggest directions of the short term competition at two dwarf shrub heaths, one with sub-arctic climate and one with temperate climate during spring and fall. Soil microorganisms acquired the largest amount of the added nitrogen sources compared to plants at both heath types. At both heaths, plants preferred the inorganic ammonium, yet all nitrogen forms were acquired by both plants and soil microorganisms. At the temperate heath, soil microorganisms acquired the ¹⁵N ¹³C labeled amino acids (glycine, glutamic acid and phenylalanine) as intact compounds, and both dominant plant species showed indications of phenylalanine acquisition as intact compounds. The thesis consists of an introduction collecting the most important findings from the four manuscripts.