Ammonium oxidation by bacteria and archaea have functional implications for nitrification across a forested landscape

Ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) control nitrification in terrestrial systems. Soil pH and substrate availability (NH₄⁺) can influence community composition, which may affect the contributions of these organisms to nitrification in forest soils. Using high-throughput sequencing, we identified the amoA of AOA and AOB from northern forest stands that occur across a natural gradient of nitrification, soil pH, and net N mineralization (i.e., NH₄⁺ availability). Specifically, we investigated changes in relative abundance and community composition of AOA and AOB across a soil pH and net N mineralization gradient, and how turnover in community composition is linked to nitrification. We found that soil pH was a stronger driver of AOA and AOB relative abundance than was NH₄⁺ availability. Generally, AOA and AOB turnover were positively associated with soil pH; however, some AOA taxa also displayed a negative association. Interestingly, the relative abundance of only a small number of AOA and AOB taxa was significantly associated with net nitrification rates. Our findings reveal that coexisting taxonomical groups of ammonia-oxidizers in forest soils have diverse responses to environmental factors, which influence how ammonia-oxidizer communities are structured, likely having direct implications for nitrification and the regulation of N cycling in forest systems.