Human activities are drastically altering the structure and function of ecosystems.  Many studies have shown that multiple co-occurring global changes (such as increased CO2, precipitation, temperature, nitrogen deposition and wildfire frequency) can significantly affect above-ground plant communities.  Recent studies have indicated that these effects extend to microbial communities and the processes they mediate.  Particularly, soil ammonia-oxidizing Bacteria (AOB), which mediate nitrification, the process often assumed to be the rate-limiting step in the global nitrogen cycle, exhibit significant shifts in community structure in response to increased nitrogen deposition.  Less is known about the response of ammonia-oxidizing Archaea (AOA) to global changes.  This study focuses on the effects of global changes, particularly nitrogen deposition and wildfire, on AOA and AOB in the Jasper Ridge Global Change Experiment (JRGCE) over a 3-year period.  We examine how these variables affect the two ammonia-oxidizing communities in both structure, using terminal restriction fragment length polymorphism (TRFLP) profiling, and abundance, using amoA DNA quantitative-PCR.  These measures are then compared to enzyme and nutrient variables to determine the relationship between functional community and environmental activity.

Results/Conclusions

Our results show that AOB community structure and abundance responds strongly to increases in nitrogen deposition.  Furthermore, AOB community structure and abundance are related to nitrification enzyme rates and [NH3].  Interestingly, AOA, which are at least an order of magnitude more abundant than AOB in Jasper Ridge soils, do not exhibit changes in community structure or in abundance in response to increased nitrogen deposition.  AOA abundance is not related to nitrification rate or to [NH3], which may indicate that despite their prevalence in the environment, their involvement in ammonia-oxidation is limited.  Also, AOA and AOB abundances are related, suggesting that competition between these two groups is not strong.  These results suggest that the Bacterial community is more active in nitrification than the Archaeal community in this soil environment, and calls into question the role of the highly abundant AOA community in below-ground processes.