Nitrogen availability plays an important role in determining plant community composition and forest productivity, and likely influences the composition of the microbial communities. The objective of the study was to examine the variation of ammonia-oxidizing and the denitrifying bacterial communities by comparing: (i) three model forest ecosystems common in Fennoscandia, encompassing the natural variation of N supply and plant productivity, and with contrasting microbial communities, with (ii) a four-year old large scale tree girdling (terminates below ground inputs of C by cutting off the bark including the phloem) experiment known to have experienced a significant decline in microbial biomass C  and with (iii) a N fertilization experiment with up to 34 years of annual N loading at four levels of ammonium nitrate additions, of which the highest was terminated 14 years ago. Terminal restriction fragment length polymorphism (T-RFLP) analysis of the genes encoding ammonium mono-oxygenase (amoA) and nitrous oxide reductase (nosZ) was used to characterize the communities of ammonia-oxidizing and denitrifying bacteria, respectively. Amplification of amoA and nosZ was clearly more successful in the forest types with higher N availability, which likely relates to differences in population sizes of ammonia-oxidizers and denitrifiers. We expect to observe variations in community composition of ammonia-oxidizing and denitrifying bacteria in response to changes both in soil N availability and to altered tree belowground C input in the two experiments