ERP004638 PRJEB5248 ena-STUDY-NIOO-23-01-2014-17:13:45:506-56 Nitrifiers and MOB in volcanic grassland soil Ammonium/ ammonia is the sole energy substrate of ammonia-oxidizers, but also an essential nitrogen source for all other microorganisms. Excess soil nitrification stimulated by ammonium can lead to loss of nitrogen via nitrate-leaching and cause N-limitation to microbes and plants. For example, elevated ammonium levels have been shown to either stimulate or inhibit the activity of methane-oxidizing bacteria (MOB) pointing at interactions between ammonia-oxidizers and MOB. Since both functional groups are linked to greenhouse gas emissions it is possible that such interactions will have important consequences for climate change. Here we report on the interactions between nitrifying communities dominated by AOA and Nitrospira-like NOB, and communities of MOB in controlled microcosm experiments with two levels of ammonium and methane availability. We observed strong stimulatory effects of ammonium concentration on the processes of nitrification and methane oxidation as well as on the abundances of actively growing nitrifiers. However, the key players in nitrification and methane oxidation, identified by stable isotope-labelling using 13CO2 and 13CH4, were the same under both ammonium levels, namely type 1.1a AOA, sub-lineage II Nitrospira-like NOB, and Methylomicrobium-/ Methylosarcina-like MOB. AOB were nearly absent, and ammonia oxidation could almost exclusively be attributed to AOA. Interestingly, while gene numbers of AOA increased tenfold during incubation there was very limited evidence of autotrophic growth, suggesting a partly mixotrophic lifestyle. Furthermore, we could document that nitrification and autotrophic growth of AOA and NOB were inhibited by active MOB depending on ammonium levels, resulting in suppression of net nitrification under elevated ammonium conditions. Our results hint to the existence of a previously unaccounted-for competition for nitrogen between nitrifiers and methane oxidisers in soils, thus linking two of the most important biogeochemical cycles in nature. CH4_NH4 Ammonium/ ammonia is the sole energy substrate of ammonia-oxidizers, but also an essential nitrogen source for all other microorganisms. Excess soil nitrification stimulated by ammonium can lead to loss of nitrogen via nitrate-leaching and cause N-limitation to microbes and plants. For example, elevated ammonium levels have been shown to either stimulate or inhibit the activity of methane-oxidizing bacteria (MOB) pointing at interactions between ammonia-oxidizers and MOB. Since both functional groups are linked to greenhouse gas emissions it is possible that such interactions will have important consequences for climate change. Here we report on the interactions between nitrifying communities dominated by AOA and Nitrospira-like NOB, and communities of MOB in controlled microcosm experiments with two levels of ammonium and methane availability. We observed strong stimulatory effects of ammonium concentration on the processes of nitrification and methane oxidation as well as on the abundances of actively growing nitrifiers. However, the key players in nitrification and methane oxidation, identified by stable isotope-labelling using 13CO2 and 13CH4, were the same under both ammonium levels, namely type 1.1a AOA, sub-lineage II Nitrospira-like NOB, and Methylomicrobium-/ Methylosarcina-like MOB. AOB were nearly absent, and ammonia oxidation could almost exclusively be attributed to AOA. Interestingly, while gene numbers of AOA increased tenfold during incubation there was very limited evidence of autotrophic growth, suggesting a partly mixotrophic lifestyle. Furthermore, we could document that nitrification and autotrophic growth of AOA and NOB were inhibited by active MOB depending on ammonium levels, resulting in suppression of net nitrification under elevated ammonium conditions. Our results hint to the existence of a previously unaccounted-for competition for nitrogen between nitrifiers and methane oxidisers in soils, thus linking two of the most important biogeochemical cycles in nature.