Soil nitrification is a key process in regulating the relative availability of the various inorganic N forms to plants. In the current study, we investigated the effect of different plant species on numbers of ammonia-oxidizing microbial cells by measuring the potential nitrification activity (PAA). Soil from matgrass sward and from calcareous grassland was collected in the field and four characteristic plant species of each vegetation type were cultivated from seed. These plant species grew for 4 months in the two soil types. After those 4 months, PAA were significantly higher in calcareous soil compared to the matgrass sward soil and the presence of matgrass sward species had significantly decreased PAA in this soil. In soils from matgrass stands, PAA were much lower, and no effect of the different plant species could be detected. Plant biomass of the calcareous grassland species was overall positively correlated with PAA, whereas for matgrass plant species a negative trend was found. We conclude that matgrass sward plant species had a clear repressing effect on the potential ammonia-oxidizing activity in calcareous grassland soil within this 4-month growth experiment. The observed repression of PAA is in accordance with earlier field observations of PAA in the different vegetation zones, where repressed PAA and significant higher ammonium to nitrate ratios were observed in the matgrass sward vegetation compared to the other vegetation zones. Major findings of this study indicate that plant species can influence the nitrification potential of their habitats, i.e. certain species can repress nitrification potential and others can increase nitrification potential of their habitats.
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