Seasonal variation and controlling factors of anaerobic ammonium oxidation in freshwater river sediments in the Taihu Lake region of China Yongqiang Zhao a,b , Yongqiu Xia a , Todd M. Kana c , Yucheng Wu a , Xiaobo Li a,b , Xiaoyuan Yan a,⇑ a State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China b University of Chinese Academy of Sciences, Beijing 100049, China c Horn Point Laboratory, The University of Maryland Center for Environmental Science, Cambridge, MD 21613, USA highlights  We measured 29 N 2 and 30 N 2 using membrane inlet mass spectrometry.  Potential anammox rates varied spatially and temporally in the studied rivers.  Water temperature and NO  3 in sediments were the main controls of anammox activity.  Candidatus Kuenenia was the dominated anammox species in the river sediments. graphical abstract article info Article history: Received 13 May 2013 Received in revised form 18 July 2013 Accepted 22 July 2013 Available online xxxx Keywords: Anammox Denitrification Membrane inlet mass spectrometry Isotope-pairing 16S rRNA River systems abstract Anaerobic ammonium oxidation (anammox) has been recently recognized as an important pathway for the removal of fixed nitrogen (N) from aquatic systems. However, the functions of anammox in freshwa- ter river systems remain uncertain. In this study, we evaluated the occurrence of anammox activity in two rivers in the Taihu Lake region in China during a seasonal survey. Homogenized sediments were incubated with 15 N-labeled NO  3 and NH þ 4 amendments to determine the potential importance of the anammox process relative to canonical denitrification. Production of 29 N 2 and 30 N 2 in slurries was determined using membrane inlet mass spectrometry. Potential anammox rates in the two river sedi- ments ranged from 0.11 ± 0.07 to 6.79 ± 1.28 lmol N m 2 h 1 and the remove of N by anammox accounted for 0.8 ± 0.00% to 10.7 ± 0.03% of total N 2 production. Potential anammox rates varied spatially and temporally in the two rivers, with the highest and lowest mean anammox rates appearing during summer and early autumn and during winter, respectively. The variation of thepercentage of anammox to total N 2 production displayed the same trend with potential anammox rates. Water temperature and NO  3 content in sediments were the main factors affecting anammox activity. Anammox bacteria were detected in sediment samples using barcode pyrosequencing. The 16S rRNA anammox gene sequences in the river sediments were affiliated with Candidatus Kuenenia, Candidatus Jettenia, and Candidatus Scalindua, among which C. Kuenenia dominated the anammox bacterial communities. Our results confirmed the presence of anammox bacteria but their role is relatively small in removing fixed N from freshwater river systems. Ó 2013 Elsevier Ltd. All rights reserved. 0045-6535/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2013.07.063 ⇑ Corresponding author. Tel.: +86 25 86881530; fax: +86 25 86881000. E-mail address: yanxy@issas.ac.cn (X. Yan). Chemosphere xxx (2013) xxx–xxx Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere Please cite this article in press as: Zhao, Y., et al. Seasonal variation and controlling factors of anaerobic ammonium oxidation in freshwater river sediments in the Taihu Lake region of China. Chemosphere (2013), http://dx.doi.org/10.1016/j.chemosphere.2013.07.063