Anaerobic oxidation of ammonium (anammox) is recognized as an important process in the marine nitrogen cycle yet nothing is known about the distribution, diversity and activity of anammox bacteria in the terrestrial realm. In this study, we report on the detection of anammox sequences of Candidatus 'Brocadia', 'Kuenenia', 'Scalindua' and 'Jettenia' in marshes, lakeshores, a contaminated porous aquifer, permafrost soil, agricultural soil and in samples associated with nitrophilic or nitrogen-fixing plants. This suggests a higher diversity of anammox bacteria in terrestrial than in marine ecosystems and could be a consequence of the larger variety of suitable niches in soils. Anammox bacteria were not ubiquitously present but were only detected in certain soil types and at particular depths, thus reflecting specific ecological requirements. As opposed to marine water column habitats where Candidatus 'Scalindua' dominates anammox guilds, 'Kuenenia' and 'Brocadia' appear to be the most common representatives in terrestrial environments.

Summary The diversity and activity of anammox bacteria has been studied in various environments, but knowledge about their abundance and population dynamics is scarce and particularly lacking for soils. A SYBR Green qPCR protocol was developed for the quantification of anammox bacterial 16S rRNA gene copies using newly designed primers. This primer set allows specific detection of all currently known anammox candidate genera including members of a new anammox cluster. Seven wetland soils were investigated and anammox bacterial 16S rRNA gene copy numbers of 4.01 × 104 to 6.74 × 106 per gram of fresh soil were detected. Anammox bacteria were also quantified along a Gleysol profile at three different sampling dates. Anammox 16S rRNA gene abundance increased significantly with soil depth and varied only little between seasons, suggesting that anammox bacteria represent a minor but stable microbial population in this soil.