Environmental controls on the diversity of ammonia-oxidizing microorganisms in temperate forest soils

Background/Question/Methods

     Nitrification, the oxidation of ammonia to nitrate, is an essential ecosystem process, which links mineralization and denitrification, thereby acting as a bottle-neck between nitrogen regeneration and the major pathway for nitrogen removal in many ecosystems.  Nitrification is a two-step process consisting of the oxidation of ammonium to nitrite followed by the oxidation of nitrite to nitrate.  In soils, the oxidation of ammonium to nitrite is performed by two distinct groups of microorganisms: ammonia-oxidizing bacteria (AOB) and archaea (AOA).
     Here we present the results of an observational study, which investigates environmental controls on AOA and AOB diversity in temperate forest soils.  We sampled soils from Coweeta Hydrologic Laboratory, a Long Term Ecological Research site in western North Carolina, and assessed both edaphic conditions, and ammonia-oxidizer diversity by high-throughput sequencing of ammonia monooxygenase genes present in soil DNA extracts.  We predicted that ammonia availability would control ammonia-oxidizer diversity in forest soils, since ammonia provides the substrate for chemoautotrophic growth in these organisms, and resource availability has been shown to drive richness in other systems.  We also conducted in situnet-nitrification incubations at each site to investigate how ammonia-oxidizer diversity is related ecosystem function.

Results/Conclusions

     By sampling across experimental watersheds with a variety of historical land-use treatments, we found large gradients in ammonium availability, net nitrification rates, and other edaphic conditions, such as soil pH, within a geographically constrained area.  We found gradients in both AOA and AOB diversity in these soils as well.  In line with our predictions, we found that soil ammonium concentration regulates AOA diversity in the soils we sampled.  AOB diversity was not driven by ammonium concentration, however.  Instead, AOB diversity showed a positive relationship with soil pH in these soils.  Though soil pH could affect the availability of substrate to these organisms, through the ionization of ammonium, we do not believe that this effect was responsible for the pattern we observed.  We also found a positive relationship between net nitrification and ammonia-oxidizer richness, indicating that the diversity of soil microorganisms may play a role in regulating this important ecosystem function.  This finding could have implications for hillslope nutrient cycling, and watershed nitrogen export.