Edaphic Factors affect the abundance and activity of ammonia oxidizing microbes in Soils at Coweeta LTER

Ammonia Oxidation is the rate-limiting step of nitrification, a critical nitrogen transformation that controls nitrogen mobility in terrestrial ecosystems.  Ammonia oxidation is performed by two phylogenetically disparate groups of microorganisms, Ammonia Oxidizing Bacteria (AOB) and recently-discovered Ammonia Oxidizing Archaea (AOA).  We use culture-independent molecular methods to investigate environmental controls on the abundance and diversity of AOA and AOB in soils at the Coweeta LTER site.  Additionally, we are investigating the function of these organisms by pairing molecular methods with net nitrification assays, thereby building on a long established research program on nitrogen cycling at Coweeta.  In our investigations we have found a strong positive relationship between soil pH and AOB abundance, while AOA abundance peaked at low pH (pH ≈ 5.3) conditions.  Furthermore, we found that AOA abundance was better predicted by ammonium (NH₄⁺) availability, while AOB abundance was better predicted by ammonia (NH₃) availability, which we estimated from temperature and edaphic conditions at each site.  In all cases, AOB abundance was a better predictor of net nitrification than AOA abundance, though AOA far outnumber AOB in these soils.  Interestingly, we have also found evidence for phosphorous limitation of AOB, which may have ramifications for coupled nutrient cycling at Coweeta.  Ongoing research addresses the relative roles of these two groups in controlling rates of nitrification in Coweeta soils.