Nitrogen and phosphorus enrichment alter the composition of ammonia-oxidizing bacteria in salt marsh sediments

Ammonia oxidation is a central process in the nitrogen cycle. Particularly in marine and estuarine environments, few experiments have been conducted to tease apart the factors influencing their abundance and composition. To investigate the effect of nitrogen and phosphorus availability on ammonia-oxidizing bacteria (AOB), we conducted a nutrient enrichment experiment in a Maine salt marsh and sampled sediment communities in three seasons over 2 years. We assessed community composition using terminal restriction fragment length polymorphism analysis and sequencing of cloned fragments of the ammonia monooxygenase (amoA) gene. Almost all of the amoA sequences fell within the marine and estuarine-specific Nitrosospira-like clade. Applied separately, nitrogen and phosphorus significantly altered AOB composition; however, together the nutrients had an interactive effect, and composition did not change. In contrast, nutrient enrichment did not alter AOB abundance. Furthermore, the response of AOB composition to nutrient enrichment varied over time. We conclude that closely related taxa within the marine/estuarine-specific Nitrosospira-like clade vary in their preference for nutrient concentrations, and this preference may depend on other temporally variable abiotic factors. Finally, AOB composition was highly variable within and across years even in untreated plots. Further studies are needed to test how these different aspects of compositional variability in AOB communities influence nitrogen cycling.