پاسخ سرخس آبزی شناور Azolla filiculoides به سطوح افزایش یافته دما، فسفر و co2 Response of the floating aquatic fern Azolla filiculoides to elevated CO2, temperature, and phosphorus levels

Introduction Azolla is a floating aquatic fern that grows in tropical and temperate freshwater ecosystems. As Azolla has symbiotic N-fixing cyanobacteria (Anabaena azollae) within its leaf cavities, it has been cultivated for many centuries in rice paddies in southern China and northern Vietnam as ‘‘green manure’’ to improve rice N availability (Watanabe & Liu, 1992; Wagner, 1997). Even though chemical N fertilizers have been substituted for Azolla as an N source, Azolla is still cultivated by organic farmers, especially in rice- fish-Azolla or rice-duck-Azolla multiple eco-production systems in China and Japan (Watanabe, 2006). In addition to providing N, Azolla is known to modify the physical, chemical, and biological properties of soil and the soil–water interface in rice fields and for mobilizing fixed phosphates, retarding the NH3 volatilization that accompanies the application of chemical N fertilizer, and suppressing aquatic weeds in flooded rice fields (Mandal et al., 1999; Biswas et al., 2005). Depending on population growth and energy use scenarios, atmospheric CO2 concentration (CO2) is expected to rise from its current level of 380 ppm to between 485 and 1000 ppm by 2100 (Intergovernmental Panel on Climate Change (IPCC), 2007). Increases of CO2 and other greenhouse gases (methane and nitrous oxide) are predicted to cause an average global warming of 1.1–۶٫۴C by 2100 (IPCC, 2007). The stimulative effect of atmospheric CO2 enrichment on plant growth and development has been predicted to increase vegetative productivity, with large variations between species (Kimball et al., 2002; Ainsworth & Long, 2005). The variations in growth and photosynthetic enhancements under elevated [CO2] may be associated with the differential responses of species to other limiting factors, such as temperature, nutrients, light, and water stress (Kimball et al., 2002). As nitrogen (N) already limits productivity in most ecosystems and because tissue N content is a major determinant of photosynthesis, the CO2 fertilization effect often decreases with increasing exposure to elevated [CO2] as a result of downregulation of photosynthetic capacity under elevated [CO2] (Luo et al., 2004; Reich et al., 2006). In contrast, N-fixing plant species often show a larger growth response to elevated [CO2] than nonfixing species if other nutrients are not deficient (Lee et al., 2003). After N, phosphorus (P) is the other most frequently limiting nutrient for terrestrial and aquatic plant growth (Kobayashi et al., 2008), especially for N-fixing plants (Singh & Singh, 1988; Vitousek et al., 2002; Cˇ erna´ et al., 2009). Our objective was to understand how the floating aquatic fern Azolla responds to elevated [CO2] in combination with P addition and higher temperatures; and how these changes in climate parameters and P levels affected the N-fixation activity of Azolla filiculoides. Since A. azollae symbiotically fixes atmospheric N and supplies fixed N to Azolla, we hypothesize that Azolla growth would be increased by elevated [CO2], and the stimulatory effect of elevated [CO2] would be enhanced by P addition and increased temperature. We tested this hypothesis by examining growth of the biomass, C assimilation, and N accumulation by two experiments using controlled-environment chambers in the summers of 2007 and 2008. Materials and methods Experimental design and controlled-environment chambers We conducted two separate pot experiments during the summer seasons in 2007 and 2008 at the National Institute for Agro-Environmental Sciences, Tsukuba, Japan (36010 N, 140070 E). We used four controlledenvironment chambers (Climatron; Shimadzu, Kyoto, Japan) to maintain the two [CO2] and two temperature treatments. Each chamber was 4 m 9 2 m 9 2 m (L 9 W 9 H) and could hold 72 pots. The pots were used to grow rice (Oryza sativa L.), Azolla filiculoides, and some aquatic weeds, included Monochoria vaginalis and Barnyardgrass (Echinochloa crus-galli) during 2007 and 2008 summer season. We have used these chambers since 1996 to carry out elevated [CO2] experiments with rice, and the chambers have performed well in controlling atmospheric CO2 concentration and temperature (Cheng et al., 2001, 2006; Sakai et al., 2001, 2006). During the 2007 season, we used only two chambers to study how elevated [CO2] and P nutrient levels affected the growth of A. filiculoides. During the 2008 season, all four chambers were used to study interaction effects of elevated [CO2] and high temperature on growth of A. filiculoides. Details of the controlled-environment chamber systems have been described by Sakai et al. (2001). Azolla filiculoides inocula (IRRI code FI 1001) were provided by Dr. Y. Kishida of Okayama University, Japan.