Chris Mwangi

Soil nitrification, or conversion of ammonia (NH4+) to nitrite (NO2-) and nitrate (NO3-) with gaseous intermediates by ammonia-oxidizing archaea (AOA) and bacteria (AOB), regulates the bioavailability of nitrogen (N) in soils. The relative contribution of AOA and AOB to these processes is not well understood, especially in resources limited drylands. To better understand relative contribution of AOA and AOB to nitrification in deserts we measured soil NH4+ oxidation and N2O production potentials and related them to AOA and AOB community structure in four soils along a management-induced soil organic carbon (SOC) content gradient in the Negev Desert. We used 1-octyne specific inhibition to separate between AOA and AOB driven NH4+ oxidation and quantitative PCR (qPCR) of amoA gene for AOA and AOB community structure assessment. In addition, we measured total archaeal and bacterial population of this desert soils by qPCR of 16S rRNA genes. We found that increasing SOC accelerated NH4+ oxidation and N2O production potentials together with an increase in the relative contribution of AOB to soil nitrification. AOB contributed between 69±11 to 89±7 % and 72±9 to 80±5 % to NH4+ oxidation and N2O emission potentials, respectively, in all but the soil with the lowest SOC content. The ratio of AOA to AOB, similarly decreased with SOC increase while general archaea to bacteria ratio in soils showed opposite trend. AOB activity correlated positively with amoA copies number, AOA activity, however showed little connection to gene copies. Overall, results suggest that SOC content of desert soils, a) positively affected both AOA and AOB abundance and activity, b) had stronger effect on relative contribution of AOB to nitrification and soil N2O emissions, and c) community structure measured by gene abundance wasn’t good predictor for measured nitrification activity of soil.