Nitrification is the process by which ammonia is broken down into nitrite and then nitrate by way of aerobic oxidation. This process was long thought to be a two-step reaction carried out by chemolithoautotrophic ammonia-oxidizing bacteria and archaea (AOB and AOA) and nitrite-oxidizing bacteria (NOB), until the discovery of complete ammonia-oxidizing bacteria (comammox) that oxidize ammonia completely to nitrate. Comammox bacteria are seemingly ubiquitous, and they are hypothesized to compete in environments with low ammonia concentrations and, thus, may play an increasing role in N cycling in places like the Adirondack Mountains that are experiencing a decline in nitrogen inputs. The research question for this study is: How do site, soil horizon, and soil nutrient concentrations affect comammox abundance and competitiveness over well-studied nitrifiers? Soil samples were taken from two paired subwatersheds (S14 and S15) in Newcomb, NY, that are known to differ in environmental calcium and nitrate concentrations. Soil chemistry was analyzed, and bacterial groups were quantified using quantitative polymerase chain reaction, targeting the ammonia monooxygenase genes (amoA) specific to comammox, AOA, and AOB.
Results/Conclusions
Overall, comammox amoA gene quantities differed significantly between S14 and S15, with a p-value of 0.01191. The average gene quantities per gram were 1.89x107 (std. dev. = 4.41x107 ) in S14 and 3.99x106 (std. dev. = 4.52x106) in S15 . There was not a significant linear relationship between comammox gene quantities and concentrations of NH3, Ca, Cu, Fe, or P within either site, nor did gene quantity differ significantly between soil horizons of a site. S14 exhibited a significantly greater quantity of AOB amoA genes than S15 (p-value 0.023622), with average gene quantities of 9.85x103 and 1.58x103, respectively. The variances of comammox genes were 7.18x108 for S14 and 3.65x107 for S15. AOB amoA genes were significantly less numerous than those of comammox B across both sites, with a p-value of 0.00016. These results suggest that comammox are more dominant nitrifiers in this system than the typically dominant AOB. Although future research will also analyze abundance of AOA, these results suggest that recently-identified comammox bacteria may play a more significant role in forest soil N cycling than initially thought.