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Issue Title: Aquatic Invasions and Relation to Environmental Changes: Proceedings of the 12th International Symposium on Aquatic Weeds, European Weed Research Society / Guest Editors: A. Pieterse, S. Hellsten, J. Newman, J. Caffrey, F. Ecke, T. Ferreira, B. Gopal, J. Haury, G. Janauer, T. Kairesalo, A. Kanninen, K. Karttunen, J. Sarvala, K. Szoszkiewicz, H. Toivonen, L. Triest, P. Uotila, N. Willby
Azolla filiculoides is a floating aquatic fern growing in tropical and temperate freshwater ecosystems. As A. filiculoides has symbiotic nitrogen-fixing cyanobacteria (Anabaena azollae) within its leaf cavities, it is cultivated in rice paddies to improve N availability and suppress other wetland weeds. To understand how C assimilation and N accumulation in A. filiculoides respond to elevated atmospheric carbon dioxide concentration (CO^sub 2^) in combination with P addition and higher temperatures, we conducted pot experiments during the summer of 2007 and 2008. In 2007, we grew A. filiculoides in pots at two treatment levels of added P fertilizer and at two levels of [CO^sub 2^] (380 ppm for ambient and 680 ppm for elevated [CO^sub 2^]) in controlled-environment chambers. In 2008, we grew A. filiculoides in four controlled-environment chambers at two [CO^sub 2^] levels and two temperature levels (34/26°C (day/night) and 29/21°C). We found that biomass and C assimilation by A. filiculoides were significantly increased by elevated [CO^sub 2^], temperature, and P level (all P < 0.01), with a significant interaction between elevated [CO^sub 2^] and added P (P < 0.01). Tissue N content was decreased by elevated [CO^sub 2^] and increased by higher temperature and P level (all P < 0.01). The acetylene reduction assay showed that the N-fixation activity of A. filiculoides was not significantly different under ambient and elevated [CO^sub 2^] but was significantly stimulated by P addition. N-fixation activity decreased at higher temperatures (34/26°C), indicating that 29/21°C was more suitable for A. azollae growth. Therefore, we conclude that the N accumulation potential of A. filiculoides under future climate warming depends primarily on the temperature change and P availability, and C assimilation should be increased by elevated [CO^sub 2^].[PUBLICATION ABSTRACT]