Nitrate leaching and nitrous oxide emissions related to bacteria and not to archaea in nitrogen rich grassland soils

Abstract

The oxidation of ammonia (NH₃) to nitrate (NO₃⁻) 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 NO₃⁻ leaching. We investigated the relationships between nitrification, NO₃⁻ leaching and N₂O emissions with ammonia oxidizing bacteria (AOB) and archaea (AOA) in nitrogen rich grassland soils. Both AOA and AOB were detected in large numbers in these grassland soils. The AOB abundance grew by 3.2 to 10.4 fold and activity increased by 177 fold in response to the addition of a urine-N substrate, and the AOB growth was significantly inhibited by a nitrification inhibitor, dicyandiamide (DCD). However, neither the AOA abundance, nor activity, increased with the application of an ammonia substrate. DCD decreased NO₃⁻ leaching by 59% and decreased N₂O emissions by 64% from animal urine-N patches. Significant quantitative relationships were found between the AOB abundance and the nitrification rate, NO₃⁻ -N leaching losses, and N₂O emissions, whereas no such relationships were found with AOA. These findings suggest that nitrification and thus NO₃⁻ leaching and N₂O emissions are driven by bacteria rather than archaea in these nitrogen rich grassland soils.