Nitrous oxide emissions from grazed grassland as affected by a nitrification inhibitor, dicyandiamide, and relationships with ammonia-oxidizing bacteria and archaea

Nitrous oxide (N2O) is a potent greenhouse gas and, in grazed grassland systems where animals graze outdoor pastures, most of the N2O is emitted from animal urine nitrogen (N) deposited during grazing. Recently, ammonia-oxidizing archaea (AOA) were found to be present in large numbers in soils as well in the ocean, suggesting a potentially important role for AOA, in addition to ammonia-oxidizing bacteria (AOB), in the nitrogen cycle. The relationship between N2O emissions and AOB and AOA populations is unknown. The objective of this study was to determine the quantitative relationship between N2O emissions and AOB and AOA populations in nitrogen-rich grassland soils. We determined N2O emissions and the abundance and activity of AOB and AOA in four different soils as affected by the applications of animal urine N and dicyandiamide (DCD) as a nitrification inhibitor. Relationships were then determined between N2O emissions and the AOB and AOA populations. Total N2O emissions from the urine N applied at 1,000 kg N ha(-1) varied significantly in these soils, from 13.9 to 39.8 kg N2O-N ha(-1). The DCD treatment reduced these N2O emissions to between 2.8 and 15.3 kg N2O-N ha(-1). The average emission factor of the urine N (EF3) was 2.2% and this was decreased to 0.8%, representing a 64% reduction. Total N2O emissions were significantly related to the amoA gene copy numbers of the AOB population and to the amount of nitrate-N in the soil, but not related to the abundance of AOA. These results demonstrate that the N2O flux is mainly associated with the dynamics of the AOB population rather than the AOA in these nitrogen-rich grassland soils, and therefore, it is the AOB that should be the target of inhibition when mitigating N2O emissions using nitrification inhibitors.