Changes in nitrification and bacterial community structure upon cross-inoculation of Scots pine forest soils with different initial nitrification rates

Nitrification occurs slowly in many acid Scots pine forest soils. We examined if bacterial community structure and interactions between members of the bacterial community in these forest soils prohibit growth of ammonia-oxidising microorganisms and their nitrifying activity. Native and gamma-irradiated Scots pine forest soils known to have low net nitrification rates were augmented with fresh soils or soil slurries from nitrifying Scots pine forest soil, and vice versa. Augmentation of native non-nitrifying soils with nitrifying soils induced net nitrification, although no significant changes in bacterial community structure, as measured by 16S rRNA gene-based denaturing gradient gel electrophoresis (DGGE), were observed. In sterilised soils, the inoculum, i.e. native nitrifying soil or non-nitrifying soil, determined the occurrence of net nitrification and bacterial community structure, and not the origin of the sterilised soils. Our results demonstrate that low net nitrification rates in acid Scots pine forest soils cannot be (solely) explained by unfavourable abiotic soil conditions, but that still uncaptured biotic factors contribute to suppression of nitrification.