Substrate availability drives spatial patterns in richness of ammonia-oxidizing bacteria and archaea in temperate forest soils

We sought to investigate the drivers of richness of ammonia-oxidizing bacteria (AOB) and archaea (AOA) in temperate forest soils. We sampled soils across four experimental watersheds in the Coweeta Hydrologic Laboratory, North Carolina USA. These watersheds are geographically close, but vary in soil chemistry due to differences in land use history. While we found a positive relationship between soil pH and AOB richness in the soils we sampled, we provide evidence that this relationship is driven by the effects of soil pH on the availability of NH3, which is the substrate that is directly oxidized by AOB. Conversely, AOA richness responded to NH4+, which these organisms may access directly from the environment. Our results provide evidence that substrate availability may be a dominant driver of both AOA and AOB richness at local scales in forest soils.