Archaea are widely distributed in soils, sediments, and aquatic environments, but little is known about their biological roles in these systems. Here, I present a survey of QPCR-based bacterial:archaeal (B:A) ratios in a variety of soil and freshwater samples from environments including tundra, northern hardwood forest and lowland tropical rainforest soils, leaf litter samples and lake biofilms.  

Results/Conclusions
B:A ratios vary dramatically between sites, ranging from less than 1 to over 600. Likewise, within site variation in ratios is high; in some cases the magnitude of difference between B:A ratios is dramatic with no observable shift in the bacterial community composition between the same samples. Soil organic carbon (C), total nitrogen (N), C:N ratios and ammonium pools are all positively correlated with B:A ratios. B:A ratios also correlate to the relative abundance of specific bacterial taxa. For example, Gammaproteobacteria are positively correlated with B:A ratios while Nitrospira are negatively correlated with B:A ratios. It is difficult to examine the relative contributions of changes in bacterial vs. archaeal relative abundance to the observed shifts in B:A ratios because of potential differences in PCR inhibitors between samples. However, the fraction of archaeal 16S rRNA genes to total soil DNA is variable across samples while bacterial relative abundance appears more stable. I present a possible mechanism to explain these observed relationships between nutrients, B:A ratios and specific microbial functional groups. Specifically, I propose that when C:N ratios are high, bacterial heterotrophs (e.g., Gammaproteobacteria) are abundant. When C:N ratios are lower, autotrophic nitrifiers (e.g., Nitrospira and Archaea) are selected for, consuming available ammonium.