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 (CO2) 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 [CO2] (380 ppm for ambient and 680 ppm for elevated [CO2]) in controlled-environment chambers. In 2008, we grew A. filiculoides in four controlled-environment chambers at two [CO2] levels and two temperature levels (34/26C (day/ night) and 29/21C). We found that biomass and C assimilation by A. filiculoides were significantly increased by elevated [CO2], temperature, and P level (all P\0.01), with a significant interaction between elevated [CO2] and added P (P\0.01). Tissue N content was decreased by elevated [CO2] 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 [CO2] but was significantly stimulated by P addition. N-fixation activity decreased at higher temperatures (34/26C), indicating that 29/21C 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 [CO2].
