Soil Biology & Biochemistry 39 (2007) 2208–2221 Elevation of atmospheric CO 2 and N-nutritional status modify nodulation, nodule-carbon supply, and root exudation of Phaseolus vulgaris L. Susan Haase a , Gu¨nter Neumann b , Angelika Kania b , Yakov Kuzyakov c , Volker Ro¨mheld b , Ellen Kandeler a, a Institute of Soil Science and Land Evaluation, Soil Biology Section, University of Hohenheim, Emil-Wolff-StraX e 27, 70599 Stuttgart, Germany b Institute of Plant Nutrition, University of Hohenheim, Fruwirthstr. 20, 70593 Stuttgart, Germany c Department of Agroecosystem Research, University of Bayreuth, Dr. Hans-Frisch-StraX e 1-3, 95440 Bayreuth, Germany Received 11 December 2006; received in revised form 7 March 2007; accepted 12 March 2007 Available online 19 April 2007 Abstract Increased root exudation and a related stimulation of rhizosphere-microbial growth have been hypothesised as possible explanations for a lower nitrogen- (N-) nutritional status of plants grown under elevated atmospheric CO 2 concentrations, due to enhanced plant- microbial N competition in the rhizosphere. Leguminous plants may be able to counterbalance the enhanced N requirement by increased symbiotic N 2 fixation. Only limited information is available about the factors determining the stimulation of symbiotic N 2 fixation in response to elevated CO 2 . In this study, short-term effects of elevated CO 2 on quality and quantity of root exudation, and on carbon supply to the nodules were assessed in Phaseolus vulgaris, grown in soil culture with limited (30 mg N kg 1 soil) and sufficient N supply (200 mg N kg 1 soil), at ambient (400 mmol mol 1 ) and elevated (800 mmol mol 1 ) atmospheric CO 2 concentrations. Elevated CO 2 reduced N tissue concentrations in both N treatments, accelerated the expression of N deficiency symptoms in the N- limited variant, but did not affect plant biomass production. 14 CO 2 pulse-chase labelling revealed no indication for a general increase in root exudation with subsequent stimulation of rhizosphere microbial growth, resulting in increased N-competition in the rhizosphere at elevated CO 2 . However, a CO 2 -induced stimulation in root exudation of sugars and malate as a chemo-attractant for rhizobia was detected in 0.5–1.5 cm apical root zones as potential infection sites. Particularly in nodules, elevated CO 2 increased the accumulation of malate as a major carbon source for the microsymbiont and of malonate with essential functions for nodule development. Nodule number, biomass and the proportion of leghaemoglobin-producing nodules were also enhanced. The release of nod-gene-inducing flavonoids (genistein, daidzein and coumestrol) was stimulated under elevated CO 2 , independent of the N supply, and was already detectable at early stages of seedling development at 6 days after sowing. r 2007 Elsevier Ltd. All rights reserved. Keywords: Elevated CO 2 ; Rhizosphere; Phaseolus vulgaris L.; Root exudation; Malate; Malonate; Rhizobia; Nitrogen fixation; Signal flavonoids 1. Introduction Higher atmospheric CO 2 concentrations alter the func- tioning of soil ecosystems mainly due to changes in plant–soil interactions. Greater photosynthetic assimila- tion rates under elevated CO 2 can increase root and shoot biomass, and a fraction of the additional fixed carbon (C) may be released into the rhizosphere by root exudation. An increase in root exudation is a common (Cheng and Johnson, 1998; van Ginkel et al., 2000; Allard et al., 2006) but not a general response to elevated CO 2 (Hodge and Millard, 1998; Bazot et al., 2006). Enhanced overall root exudation was caused either by a stimulation of root ARTICLE IN PRESS www.elsevier.com/locate/soilbio 0038-0717/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2007.03.014 Corresponding author. Tel.: +49 711 45924220; fax: +49 711 45923117. E-mail address: kandeler@uni-hohenheim.de (E. Kandeler).