Contributions of Ammonia-oxidizing Archaea and Bacteria to the Nitrification Potential of an Agricultural Soil under Contrasting Nitrogen Management

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Jeanette Norton


Nitrification is the biological oxidation of reduced forms of nitrogen (N) such as ammonium, to nitrate. In agricultural soils, nitrate is mobile and susceptible to loss by leaching and denitrification. Therefore, it is desirable to manage agricultural systems to reduce nitrification and increase N fertilizer use efficiency. In this study, our goal is to determine the effects of conventional and organic N management on nitrification and nitrifying organisms. The effects of N source for corn were compared using a randomized block design with 4 treatments, control (no additional N), ammonium sulfate (AS, 100 & 200 kg N ha-1), and compost (200 kg N ha-1) over 3 seasons. Soil N pool sizes and nitrification potential were determined in August annually. Ammonium pool size was not significantly affected, while the AS200 treatment soil had the highest nitrate pools and nitrification potential rates. Nitrifying organsims were examined using DNA extracted from the soil and quantification of the gene (amoA) encoding an enzyme responsible for ammonium oxidation. Quantification of amoA with real-time PCR revealed that ammonia oxidizing archaea (AOA) had similar abundance in the differently treated soils and were more abundant than ammonia oxidizing bacteria (AOB). In contrast, the AOB populations were strongly affected by the different N treatments, AOB were more abundant in soils from the fertilizer treatments (AS200 and AS100) compared to control and compost treated soils.. Studies using specific inhibitors of nitrification suggest that although the AOA are found in higher abundance (100 x) they are responsible for approximately 50% of nitrifying activity in the control and compost treated soils. In the ammonium fertilized soils the AOA while more abundant (20-30 X) contribute only about 30% while AOB dominated the nitrification potential activity (70%). Understanding the differential response of the microbial community to N sources will inform our ability to manage N more efficiently in agriculture.

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