Effects of elevated CO 2 concentration on rhizodeposition from Lolium perenne grown on soil exposed to 9 years of CO 2 enrichment S. Bazot a , L. Ulff b , H. Blum c , C. Nguyen a , C. Robin a, * a Rhizosphere group, UMR INPL-INRA Agronomie et Environnement ENSAIA, 54505 Vandoeuvre-les-Nancy, France b Department of Soil Sciences, Division of Plant Nutrition and Soil Fertility, Swedish University of Agricultural Sciences, S-75007 Uppsala, Sweden c Institute of Plant Sciences, Swiss Federal Institute of Technology (ETH), 8092 Zu ¨rich, Switzerland Received 9 February 2005; received in revised form 22 June 2005; accepted 22 June 2005 Available online 27 July 2005 Abstract The effects of enriched CO 2 atmosphere on partitioning of recently assimilated carbon were investigated in a plant-soil-microorganism system in which Lolium perenne seedlings were planted into cores inserted into the resident soil within a sward that had been treated with elevated CO 2 for 9 consecutive years, under two N fertilisation levels (Swiss FACE experiment). The planted cores were excavated from the ambient (35 Pa pCO 2 ) and enriched (60 Pa pCO 2 ) rings at two dates, in spring and autumn, during the growing season. The cores were brought back to the laboratory for 14 C labelling of shoots in order to trace the transfer of recently assimilated C both within the plant and to the soil and microbial biomass. At the spring sampling, high N supply stimulated shoot and total dry matter production. Consistently, high N enhanced the allocation of recently fixed C to shoots, and reduced it to belowground compartments. Elevated CO 2 had no consequences for DM or the pattern of C allocation. At the autumn sampling, at high N plot, yield of L. perenne was stimulated by elevated CO 2 . Consistently, 14 C was preferentially allocated aboveground and, consequently belowground recent C allocation was depressed and rhizodeposition reduced. At both experimental periods, total soil C content was similar in all treatments, providing no evidence for soil carbon sequestration in the Swiss Free Air CO 2 Enrichment experiment (FACE) after 9 years of enrichment. Recently assimilated C and soil C were mineralised faster in soils from enriched rings, suggesting a CO 2 -induced shift in the microbial biomass characteristics (structure, diversity, activity) and/or in the quality of the root-released organic compounds. q 2005 Elsevier Ltd. All rights reserved. Keywords: Carbon sequestration; Elevated CO 2 ; Microbial biomass; Mineralisation; Nitrogen fertilisation; Rhizodeposition; Rhizosphere respiration 1. Introduction Living roots continually release organic carbon into the rhizosphere, the soil volume in the vicinity of roots (Rovira, 1969), with the amount representing a significant part of the total plant C budget. This process, referred to as rhizodeposition (Whipps and Lynch, 1983), is affected qualitatively and quantitatively by a wide range of factors including plant species, phenology and environmental conditions (Rovira, 1959; Hale and Moore, 1979; Curl and Truelove, 1986; Grayston et al., 1996). Within the global change context, the predicted rise of atmospheric CO 2 concentration is an important environmental change with crucial implications for terrestrial ecosystems. Therefore, the responses of plants and rhizosphere micro-organisms to rising atmospheric CO 2 are particularly important to quantify if the consequences of rising atmospheric CO 2 for soil C dynamics and nutrient cycling are to be predicted. Most studies aiming to quantify the consequences of elevated CO 2 on C fluxes from plants to the rhizosphere and various soil compartments have been conducted under controlled conditions that usually optimize plant growth. Few field studies have been reported, despite their obvious relevance for C dynamics. A survey of the literature shows that for both controlled environment and field conditions, elevated CO 2 frequently increases dry matter yield (Kimball, 1983; Ross et al., 1995, 1996; Hebeisen et al., 1997; Daepp et al., 2000; Suter et al., 2002), in the absence of limiting factors such as the availability of nitrogen. This CO 2 -induced yield increase is attributed to a stimulation of net CO 2 Soil Biology & Biochemistry 38 (2006) 729–736 www.elsevier.com/locate/soilbio 0038-0717/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2005.06.023 * Corresponding author. Tel.: C33 3 83 59 58 56; fax: C33 3 83 59 57 99. E-mail address: christophe.robin@ensaia.inpl-nancy.fr (C. Robin).