Research trends and hotspots related to ammonia oxidation based on bibliometric analysis

Ammonia oxidation is the rate-limiting and central step in global biogeochemistry cycle of nitrogen. A bibliometric analysis based on 4314 articles extracted from Science Citation Index Expanded database was carried out to provide insights into publication performances and research trends of ammonia oxidation in the period 1991-2014. These articles were originated from a wide range of 602 journals and 95 Web of Science Categories, among which Applied and Environmental Microbiology and Environmental Sciences took the leading position, respectively. Furthermore, co-citation analysis conducted with help of CiteSpace software clearly illustrated that ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA), and anaerobic ammonia oxidation (anammox) were three dominant research themes. A total of 15 landmark works identified with the highest co-citation frequencies at every 8 years were extracted, which demonstrated that the establishments of culture-independent molecular biotechnologies as well as the discoveries of anammox and AOA played the most significant roles in promoting the evolution and development of ammonia oxidation research. Finally, word cluster analysis further suggested that microbial abundance and community of AOA and AOB was the most prominent hotspot, with soil and high-throughput sequencing as the most promising ecosystem and molecular biotechnology. In addition, application of anammox in nitrogen removal from wastewater has become another attractive research hotspot. This study provides a basis for better understanding the situations and prospective directions of the research field of ammonia oxidation.