Drivers for ammonia-oxidation along a land-use gradient in grassland soils

In this study, drivers for ammonia-oxidation and the related microbial communities (ammonia-oxidizing bacteria and archaea) were investigated in grassland soils on the local as well as on the regional scale focusing on the role of land-use intensity (LUI). To this end, 150 sites from three distinct regions across Germany were selected, covering the whole range of LUI levels (from natural grasslands up to intensive managed meadows). Furthermore, the role of contrasting soil types was analyzed in one of the regions (high vs low organic matter content) for ammonia-oxidation. We revealed a significant increase in potential nitrification rates and abundance of ammonia-oxidizing microbes at two sites on the local level from extensively to intensively managed sites, which indicates that the response pattern of ammonia-oxidizing microbes in grassland soils is likely triggered to a large extent by LUI. However at a third site, where two different soil types were investigated, no correlation between LUI and potential nitrification rates was observed, and only a site-specific effect was apparent. At this site, on the one hand the specific soil type (Histosol) and the related continuous nutrient mobilization from the former peat matrix, as well as the high groundwater level, which could induce a high abundance of methane- oxidizing microbes in the top soil, may be of greater importance as a driver for potential nitrification rates and abundance of ammonia- oxidizing microbes than LUI. On the other hand, the mineral soils of this site were characterized by extreme water shortage, which may also explain the lack of potential nitrification and the abundance of ammonia-oxidizing bacteria and archaea. Thus any extrapolation of local data to regional predictions must be made with care, as factors other than LUI may be of importance if the nitrification potential of a soil is to be described.