Spatiotemporal stability of an ammonia-oxidizing community in a nitrogen-saturated forest soil

Elevated levels of nitrogen input into various terrestrial environments in recent decades have led to increases in soil nitrate production and leachin g. However, nitrifying potential and nitrifying activity tend to be highly variable over space and time, making broad-scale estimates of nitrate produ ction difficult. This study investigates whether the high spatiotemporal va riation in nitrate production might be explained by differences in the stru cture of ammonia-oxidizing bacterial communities in nitrogen-saturated coni ferous forest soils. The diversity of ammonia-oxidizing bacteria of the bet a -subgroup Proteobacteria was therefore investigated using two different P CR-based approaches. The first targeted the 16S rRNA gene and involved temp oral temperature gradient electrophoresis (TTGE) of specifically amplified PCR products, with subsequent band excision and nucleotide sequence determi nation. The second approach involved the cloning and sequencing of PCR-ampl ified amoA gene fragments. All recovered 16S rDNA sequences were closely re lated to the culture strain Nitrosospira sp. AHB1, which was isolated from an acid soil and is affiliated with Nitrosospira cluster 2, a sequence grou p previously shown to be associated with acid environments. All amoA-like s equences also showed a close affinity with this acid-tolerant Nitrosospira strain, although greater sequence variation could be detected in the amoA a nalysis. The ammonia-oxidizing bacterial community in the nitrogen-saturate d coniferous forest soil was determined to be very stable, showing little v ariation between different organic layers and throughout the year, despite large differences in the total Bacterial community structure as determined by 16S rDNA DGGE community fingerprinting. These results suggest that envir onmental heterogeneity affecting ammonia oxidizer numbers and activity, and not ammonia oxidizer community structure, is chiefly responsible for spati al and temporal variation in nitrate production in these acid forest soils.