The abundance and composition of ammonia oxidizers and denitrifiers were studied at two boreal forest sites in northern Sweden. One site had a natural gradient of nitrogen availability, which corresponded with different plant communities; the other had a gradient of nitrogen availability imposed as part of a nitrogen fertilization trial. Samples of the mor layer were sampled from replicated plots of different nitrogen availability. The abundances of ammonia oxidizers and denitrifiers were measured by quantitative PCR and their composition was determined by T-RFLP analysis. In general, both abundance and composition of the ammonia oxidizers and denitrifier communities varied systematically with measures of soil nitrogen availability.