ISMESS 2009 • RESEARCH ARTICLE Nitrous oxide emissions from grazed grassland as affected by a nitrification inhibitor, dicyandiamide, and relationships with ammonia-oxidizing bacteria and archaea Hong J. Di & Keith C. Cameron & Rob R. Sherlock & Ju-Pei Shen & Ji-Zheng He & Chis S. Winefield Received: 8 November 2009 / Accepted: 16 December 2009 / Published online: 15 January 2010 # Springer-Verlag 2010 Abstract Purpose Nitrous oxide (N 2 O) is a potent greenhouse gas and, in grazed grassland systems where animals graze outdoor pastures, most of the N 2 O 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 N 2 O emissions and AOB and AOA populations is unknown. The objective of this study was to determine the quantitative relationship between N 2 O emissions and AOB and AOA populations in nitrogen-rich grassland soils. Materials and methods We determined N 2 O 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 N 2 O emissions and the AOB and AOA populations. Results and discussion Total N 2 O 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 N 2 O-N ha -1 . The DCD treatment reduced these N 2 O emissions to between 2.8 and 15.3 kg N 2 O-N ha -1 . The average emission factor of the urine N (EF 3 ) was 2.2% and this was decreased to 0.8%, representing a 64% reduction. Total N 2 O emis- sions 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. Conclusions These results demonstrate that the N 2 O flux is mainly associated with the dynamics of the AOB popula- tion 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 N 2 O emissions using nitrifi- cation inhibitors. Keywords Ammonia-oxidizing archaea (AOA) . Ammonia-oxidizing bacteria (AOB) . Dicyandiamide . Grazed grassland . Greenhouse gas . Nitrification inhibitor . Nitrous oxide 1 Introduction Nitrification is a key biogeochemical process of the nitrogen (N) cycle which results in the oxidation of ammonia (NH 3 ) to nitrite (NO 2 - ) and then to nitrate (NO 3 - ). This process has major environmental and ecolog- ical consequences because it releases nitrous oxide (N 2 O) which is a potent greenhouse gas and NO 3 - which can be leached to contaminate groundwater and surface waters. The long-term global warming potential of N 2 O is 310 times that of carbon dioxide (CO 2 ) (IPCC 1995). N 2 O is Responsible editor: Caixian Tang. H. J. Di (*) : K. C. Cameron : R. R. Sherlock Centre for Soil and Environmental Research, Lincoln University, P.O. Box 84, Lincoln 7647 Christchurch, New Zealand e-mail: hong.di@lincoln.ac.nz J.-P. Shen : J.-Z. He Research Centre for Eco-Environmental Sciences, Chinese Academy of Sciences, 18 Shuangqing Road, Beijing 100085, People’ s Republic of China C. S. Winefield Faculty of Agriculture and Life Sciences, Lincoln University, P.O. Box 84, Lincoln 7647 Christchurch, New Zealand J Soils Sediments (2010) 10:943–954 DOI 10.1007/s11368-009-0174-x