Nitrification along a grassland gradient: Inhibition found in matgrass swards

Measurements along a hill-slope vegetation gradient in nutrient-poor grasslands from acid grasslands via matgrass swards to calcareous grasslands showed increased ammonium to nitrate ratios in the matgrass swards. These results generated the research question whether there might be a difference in nitrification activity or nitrifying community composition between the different zones in this hill-slope gradient. In each of the vegetation types along the gradient, soil samples were taken in five grassland nature reserves. Potential nitrification rates have been determined as an indication of the size of the active ammonia-oxidising microbial communities. Additionally, the dominant ammonia-oxidising sequences related to the β-Proteobacteria have been determined by a Polymerase Chain Reaction (PCR) based on the 16S rRNA gene in combination with Denaturing Gradient Gel Electrophoresis (DGGE) at one of the nature reserves. Compared to the top and lower zones of the vegetation gradient (i.e. acid grasslands and calcareous grasslands, respectively), potential nitrification rates were clearly repressed in the middle, matgrass swards zone. In contrast to the differences in potential nitrification activities observed in one of the nature reserves (Bemelerberg), no differences in dominant ammonia-oxidising sequences were observed at this location. One sequence belonging to cluster 3 of the Nitrosospira lineage appeared to be dominant among the sequences belonging to the ammonia-oxidising species of the β-Proteobacteria in all vegetation zones. Nitrification was apparently inhibited by the vegetation, whereas no shift in nitrifier populations could be shown. The possible role of repressed nitrification in the decline of this vegetation type is discussed. © 2010 Elsevier Ltd. All rights reserved.