Nitrogen research at watershed scale: a bibliometric analysis during 1959–2011

Because of enhanced anthropogenic nitrogen input, eutrophication, hypoxia, and acidification threaten the health of aquatic ecosystems. To better understand the current state of research and emerging trends in this area, a bibliometric approach was applied to quantitatively evaluate global nitrogen research at the watershed scale. Using 9,748 articles selected from among 10,163 returned by a search in the Science Citation Index Expanded (SCI-Expanded) database from 1900 to 2011, spatial and temporal characteristics of the articles, authors, institutions, countries, and keywords are presented, and focal research areas are derived. Compared with the annual increase in all articles in the SCI-Expanded (4.5 %), the studies on nitrogen in watersheds increased more quickly (11.2 %), indicating an increasing interest in this area. The relationship between authors and their output was evaluated by a two-step function, in which 6,074 authors (26.8 %) publishing on this topic were key scientists who contributed 56.4 % of the total articles. Based on the number of authors, first authors, international collaborators, and citations, four types of authors were analyzed using cluster methods. The influence of the authors, institutions and countries was also analyzed in terms of publication and citation, and a co-occurrence analysis was used to assess cooperation among countries and research hotpots. The keywords were compared among countries to assist our understanding of interests of research and modes. From the analysis of the primary subjects and the co-occurrence of keywords, studies involving nitrogen’s environmental effects, the nitrogen process and models are increasing, which indicates that they are likely to become a primary research focus in the near future.