Paddy fields are an important source of atmospheric CH₄, the second most important greenhouse gas. There is a strong concern that the increasing atmospheric CO₂ concentration ([CO₂]) and global warming are further stimulating CH₄ emissions, but the magnitude of this stimulation varies substantially by study, and few open-field evaluations have been conducted. Here we report results obtained at a Japanese rice free-air CO₂ enrichment (FACE) site under water and soil temperature elevation during two growing seasons. Our objectives were to evaluate the effects of high [CO₂] (ambient + 200 μmol mol⁻¹) and elevated soil temperature (+ 2 °C) on CH₄ emissions under completely open-field conditions. We found about 80% enhancement in total seasonal emissions by the additive effects of FACE and warming, indicating a strong positive feedback effect of global warming. The enhancement in CH₄ emission from the FACE-effect alone (+ 26%) was statistically non-significant (P = 0.19). Nevertheless, observed positive correlations between CH₄ emissions and rice biomass agreed well with previous studies, suggesting that higher photosynthesis led to greater rhizodeposition, which then acted as substrates for methanogenesis. Soil warming increased the emission by 44% (P < 0.001), which was equivalent to a Q₁₀ of 5.5. Increased rice biomass by warming could only partly explain the enhanced CH₄ emissions, but stoichiometric analysis of the electron budget indicated that even a moderate enhancement in organic matter decomposition due to soil warming can cause a large increase in CH₄ production under conditions where Fe(III) reduction, which was little affected by soil warming, dominates electron-accepting processes. At later rice growth stages, advanced root senescence due to elevated temperature probably provided more substrate for methanogenesis. Our stoichiometric evaluation showed that in situ Fe reduction characteristics and root turnover in response to elevated temperature should be understood to correctly predict future CH₄ emissions from paddy fields under a changing climate. Challenges remain for determination of in situ root-exudation rate and its response to FACE and warming

Quantification of rhizodeposition (root exudates and root turnover) represents a major challenge for understanding the links between above-ground assimilation and below-ground anoxic decomposition of organic carbon in rice paddy ecosystems. Free-air CO 2 enrichment (FACE) fumigating depleted 13 CO 2 in rice paddy resulted in a smaller 13 C/ 12 C ratio in plant-assimilated carbon, providing a unique measure by which we partitioned the sources of decomposed gases (CO 2 and CH 4 ) into current-season photosynthates (new C) and soil organic matter (old C). In addition, we imposed a soil-warming treatment nested within the CO 2 treatments to assess whether the carbon source was sensitive to warming. Compared with the ambient CO 2 treatment, the FACE treatment decreased the 13 C/ 12 C ratio not only in the rice-plant carbon but also in the soil CO 2 and CH 4 . The estimated new C contribution to dissolved CO 2 was minor (ca. 20%) at the tillering stage, increased with rice growth and was about 50% from the panicle-formation stage onwards. For CH 4 , the contribution of new C was greater than for heterotrophic CO 2 production; ca. 40-60% of season-total CH 4 production originated from new C with a tendency toward even larger new C contribution with soil warming, presumably because enhanced root decay provided substrates for greater CH 4 production. The results suggest a fast and close coupling between photosynthesis and anoxic decomposition in soil, and further indicate a positive feedback of global warming by enhanced CH 4 emission through greater rhizodeposition.

Canada bluejoint grass [Calamagrostis canadensis (Michx.) Beauv., referred to as bluejoint below] is a competitive understory species widely distributed in the boreal region in North America and builds up a thick litter layer that alters the soil surface microclimate in heavily infested sites. This study examined the effects of understory removal, N fertilization, and litter layer removal on litter decomposition, soil microbial biomass N (MBN), and net N mineralization and nitrification rates in LFH (the sum of organic horizons of litter, partially decomposed litter and humus on the soil surface) and mineral soil (0-10 cm) in a 13-year-old white spruce [Picea glauca (Moench.) Voss] plantation infested with bluejoint in Alberta, Canada. Removal of the understory vegetation and the litter layer together significantly increased soil temperature at 10 cm below the mineral soil surface by 1.7 and 1.3°C in summer 2003 and 2004, respectively, resulting in increased net N mineralization (by 1.09 and 0.14 mg N kg -1 day -1 in LFH and mineral soil, respectively, in 2004) and net nitrification rates (by 0.10 and 0.20 mg N kg -1 day -1 in LFH and mineral soil, respectively, in 2004). When the understory vegetation was intact, nitrification might have been limited by NH 4 + availability due to competition for N from bluejoint and other understory species. Litter layer removal increased litter decomposition rate (percentage mass loss per month) from 2.6 to 3.0% after 15 months of incubation. Nitrogen fertilization did not show consistent effects on soil MBN, but increased net N mineralization and nitrification rates as well as available N concentrations in the soil. Clearly, understory removal combined with N fertilization was most effective in increasing rates of litter decomposition, net N mineralization and nitrification, and soil N availability. The management of understory vegetation dominated by bluejoint in the boreal region should consider the strong effects of understory competition and the accumulated litter layer on soil N cycling and the implications for forest management.

Paddy fields are an important source of atmospheric CH₄, the second most important greenhouse gas. There is a strong concern that the increasing atmospheric CO₂ concentration ([CO₂]) and global warming are further stimulating CH₄ emissions, but the magnitude of this stimulation varies substantially by study, and few open-field evaluations have been conducted. Here we report results obtained at a Japanese rice free-air CO₂ enrichment (FACE) site under water and soil temperature elevation during two growing seasons. Our objectives were to evaluate the effects of high [CO₂] (ambient+200 μmol mol⁻¹) and elevated soil temperature (+2 °C) on CH₄ emissions under completely open-field conditions. We found about 80% enhancement in total seasonal emissions by the additive effects of FACE and warming, indicating a strong positive feedback effect of global warming. The enhancement in CH₄ emission (+26%, P = 0.19) from the effect of FACE alone was similar to that in rice biomass, suggesting that higher photosynthesis led to greater rhizodeposition, providing substrates for methanogenesis. Soil warming increased the emission by 44% (P < 0.001), which was equivalent to a Q₁₀ of 5.5. Increased rice biomass by warming could only partly explain the enhanced CH₄ emissions, but stoichiometric analysis of the electron budget indicated that even a moderate enhancement in organic matter decomposition due to soil warming can cause a large increase in CH₄ production under conditions where Fe(III) reduction, which was little affected by soil warming, dominates electron-accepting processes. At later rice growth stages, advanced root senescence due to elevated temperature probably provided more substrate for methanogenesis. Our stoichiometric evaluation showed that in situ Fe reduction characteristics and root turnover in response to elevated temperature should be understood to correctly predict future CH₄ emissions from paddy fields under a changing climate. Challenges remain for determination of in situ root-exudation rate and its response to FACE and warming

Azolla filiculoides is a floating aquatic fern growing in tropical and temperate freshwater ecosystems. As A. filiculoides has symbiotic nitrogenfixing 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 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 2 ] (380 ppm for ambient and 680 ppm for elevated [CO 2 ]) in controlled-environment chambers. In 2008, we grew A. filiculoides in four controlled-environment chambers at two [CO 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 2 ], temperature, and P level (all P \ 0.01), with a significant interaction between elevated [CO 2 ] and added P (P \ 0.01). Tissue N content was decreased by elevated [CO 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 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 2 ].