REGULAR ARTICLE Interactions between elevated CO 2 and N 2 -fixation determine soybean yield—a test using a non-nodulated mutant Shimpei Oikawa & Kay-May Miyagi & Kouki Hikosaka & Masumi Okada & Toshinori Matsunami & Makie Kokubun & Toshihiko Kinugasa & Tadaki Hirose Received: 19 May 2009 / Accepted: 1 October 2009 / Published online: 14 October 2009 # Springer Science + Business Media B.V. 2009 Abstract Elevated CO 2 increases seed production more in plant species that form a symbiotic associa- tion with N 2 –fixing bacteria than in species without such association. We studied the mechanism of the increase of seed production with elevated CO 2 using nodulated soybean (Glycine max cv. Enrei) and its non-nodulated isogenic line (cv. En1282). Increase in seed production with elevated CO 2 was observed in nodulated Enrei but was not in non-nodulated En1282. The increase in seed production in Enrei was explained by the increase in the rate of dry mass production during the reproductive period. This increase was associated with the increase in N assimilation in the reproductive period and the seed N concentration that remained the same as that at ambient CO 2 . Dry mass production and nitrogen assimilation did not increase in the vegetative phase in both lines. These results accorded with the amount of nodules in Enrei that increased at elevated CO 2 especially after flowering. We conclude that the increase in N assimilation in the reproductive period would be the key for increasing soybean yield in the future high-CO 2 world. Plant Soil (2010) 330:163–172 DOI 10.1007/s11104-009-0189-5 Responsible Editor: Alfonso Escudero. S. Oikawa Center for Bioresource Field Science, Kyoto Institute of Technology, Saga, Kyoto 616-8354, Japan K.-M. Miyagi : K. Hikosaka Graduate School of Life Sciences, Tohoku University, Sendai, Miyagi 980-8578, Japan M. Okada Faculty of Agriculture, Iwate University, Morioka, Iwate 020-8550, Japan T. Matsunami Akita Prefectural Agriculture, Forestry and Fisheries Research Center, Yuuwa, Akita 010-1231, Japan M. Kokubun Graduate School of Agricultural Science, Tohoku University, Sendai, Miyagi 981-8555, Japan T. Kinugasa Faculty of Agriculture, Tottori University, Koyama-Minami, Tottori 680-8553, Japan T. Hirose Department of International Agricultural Development, Tokyo University of Agriculture, Setagaya, Tokyo 156-8502, Japan Present Address: S. Oikawa (*) Department of Plant Biology, University of Illinois, Urbana, IL 61801, USA e-mail: oikawaDX@gmail.com