Nitrification along a grassland gradient: Inhibition found in matgrass swards

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.