Comparison of AOA and AOB communities from soils with different underlying geologies

Grasslands (~40% of the earths land surface) are high in organic matter and represent a large reservoir for C and N storage. Seasonal ammonification and nitrification rates were measured together with bacterial, archaeal and ammonia-oxidizer communities in U.K. grassland soils, whose geologies differed between clay, greensand and chalk. Across sites, ammonification and nitrification rates were slower in the autumn which correlated with lower soil ammonium concentrations. Turnover times for soil ammonium pools were <1 day, whilst soil nitrate pools were several days. In clays, nitrification accounted for 123% of ammonification, 74% in chalk, and only 54% in greensand. Clays harboured lower abundances of bacteria, archaea and ammonia oxidisers. Moreover microbial communities in the clay soils were clearly distinct from those found in chalk and greensand soils. Ammonia-oxidising archaea (AOA), namely Nitrososphaera comprised 37.4% of the total archaeal communities, with the vast majority of AOA sequences found in chalk and greensand soils. Across sites, AOA were dominant over ammonia oxidising bacteria (AOB) (notably all Nitrosospiras). Candidatus Nitrososphaera gargensis and Candidatus Nitrosocosmicus franklandus were found in high abundance, suggesting these AOA taxa are the likely drivers of nitrification in these low ammonium soils, further supporting the concept of niche requirements for ammonia-oxidisers.