Effects of Elevated Nitrogen and Fire on the Community Structure of Soil Ammonia-Oxidizing Bacteria in a California Grassland
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Global climate change is a serious and pervasive threat to the biosphere, and has the potential to alter the nutrient cycling and soil microbial communities in many ecological niches. These changes, in addition to having local consequences, may also affect the rate of global warming. We examined the interactive effects of two global climate change factors, nitrogen and fire, on ammonia-oxidizing bacteria (AOB) community structure at the Jasper Ridge Global Change Experiment, a California annual grassland with a Mediterranean climate near Palo Alto, CA. This work builds on a previous study that examined the post-bum effects of a July 2003 fire on AOB community structure and nitrification rates. In the current study, we determined the proportion of the AOB community composed of Cluster 3a bacteria, a clade associated with high nitrification rates. We examined the AOB community structure immediately, six months, and nine months after a fire using T-RFLP. Unlike previous studies, we found that soil ammonium concentrations in the nitrogen and fire treatments were no higher than the controls, possibly due to unusual weather during 2011-2012. Analysis of AOB community structure showed that neither fire or nitrogen addition led to a change in AOB community structure, although elevated nitrogen had a homogenizing effect on soil AOB diversity. Our data indicate that further studies which examine the effects of global change on soil microbial diversity, in the context of natural environmental variation and disturbance, are necessary for an accurate understanding of future global change predictions.