The Domains Archaea and Bacteria contain the vast majority of Earth’s biodiversity and biomass, and their members play critical, often exclusive, roles in many biogeochemical cycles and ecosystem services.  Human-induced global change, particularly with respect to increased nitrogen deposition, has the potential to drastically alter how soil nitrifying communities perform their biogeochemical function.  Additionally, multi-factor global change can alter how microbial communities interact with each other and with the associated plant communities. This study, performed in the context of the long-term Jasper Ridge Global Change Experiment (JRGCE) in a California grassland ecosystem, examines how ammonia-oxidizing Archaea and Bacteria (AOA and AOB, respectively) respond to multi-factor global change.  Manipulations at the JRGCE include simultaneous increases in CO₂, warming, precipitation and nitrogen deposition.  Past studies have utilized DNA-fingerprinting methods to assess ammonia-oxidizer response to multi-factor global change.  This study compares how seed bank (DNA-based) versus metabolically active (RNA-based) ammonia-oxidizing communities respond to global change manipulations over several seasons.  We have employed ultra-deep 454-pyrosequencing techniques to examine these communities using the ammonia mono-oxygenase (amoA) functional gene marker.  Effects of global change have been examined at several phylogenetic levels and linked this community information to gross rates of nitrification using ¹⁵N stable isotopic methods.  Ammonia-oxidizer and plant communities have been compared using multivariate statistical methods.

Results/Conclusions

Our results show that both AOB and AOA communities are highly influenced by nitrogen deposition in both their abundance and community structure.  These changes are linked to increased nitrification rate in the elevated nitrogen deposition plots.  Our results further show that the relationship between the AOB and plant communities fundamentally changes under long-term nitrogen deposition manipulation.  This positive feedback loop may enhance the rate of change in ammonia-oxidizer communities, which may further elevate nitrification rates.  This study provides strong evidence that incorporating microbial community and abundance information into global change predictions is crucial for understanding how ecosystem-level nutrient cycling rates may change.