Population Dynamics of Ammonia Oxidizing Bacteria and Archaea and Relationships with Nitrification Rate in New Zealand Grazed Grassland Soils

Abstract

The oxidation of ammonia (NH₃) to nitrate (NO₃ ⁻) in different ecosystems is a key process in the global nitrogen (N) cycle which has major ecological and environmental implications both in influencing nitrous oxide (N₂O) emissions and nitrate leaching. We investigated the population dynamics of ammonia oxidising bacteria (AOB) and ammonia oxidising archaea (AOA) under controlled laboratory conditions using quantitative polymerase chain reaction (qPCR or real-time PCR) in six different intensively managed dairy grassland soils sampled from across New Zealand. The AOA amoA gene copy numbers varied from 1.27×10⁷ to 3.85×10⁶. The AOA to AOB ratio varied from 10.7 to 0.2. While the AOB population grew by 3.2 to 10.4 fold in response to the addition of a urine-N substrate and were significantly inhibited by a nitrification inhibitor, dicyandiamide (DCD), the AOA population remained largely unchanged irrespective of the urine-N and DCD treatments. A significant exponential quantitative relationship was found between the AOB population and the nitrification rate, whereas no such relationship was found with AOA. These findings suggest that nitrification rate is more closely related to the dynamics of AOB than to that of AOA in these grassland soils with high nitrogen loads.