Plant and Soil 222: 191–202, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 191 Effects of increased atmospheric CO 2 , temperature, and soil N availability on root exudation of dissolved organic carbon by a N-fixing tree (Robinia pseudoacacia L.) Shauna M. Uselman 1 , Robert G. Qualls 1,∗ and Richard B. Thomas 2 1 Department of Environmental and Resource Sciences, University of Nevada, Reno, NV 89557, USA and 2 Department of Biology, West Virginia University, Morgantown, WV 26506, USA Received 13 July 1999. Accepted in revised form 15 March 2000 Key words: CO 2 enrichment, carbon storage, climate change, dissolved organic carbon, nitrogen fixation, root exudate Abstract Root exudation has been hypothesized as one possible mechanism that may lead to increased inputs of organic C into the soil under elevated atmospheric CO 2 , which could lead to greater long-term soil C storage. In this study, we analyzed exudation of dissolved organic C from the roots of seedlings of the N-fixing tree Robinia pseudoacacia L. in a full factorial design with 2 CO 2 (35.0 and 70.0 Pa) × 2 temperature (26 ◦ and 30 ◦ C during the day) × 2 N fertilizer (0 and 10.0 mM N concentration) levels. We also analyzed the decomposition rates of root exudate to estimate gross rates of exudation. Elevated CO 2 did not affect root exudation of organic C. A 4 ◦ C increase in temperature and N fertilization did, however, significantly increase organic C exudation rates. Approximately 60% of the exudate decomposed relatively rapidly, with a turnover rate of less than one day, while the remaining 40% decomposed more slowly. These results suggest that warmer climates, as predicted for the next century, may accelerate root exudation of organic C, which will probably stimulate rapid C cycling and may make a minor contribution to intermediate to more long-term soil C storage. However, as these losses to root exudation did not exceed 1.2% of the net C fixed by Robinia pseudoacacia, root exudation of organic C appears to have little potential to contribute to long-term soil C sequestration. Introduction Although many plant species respond to doubled at- mospheric CO 2 levels with increased growth (e.g. Ceulemans and Mousseau, 1994), this response may not be sustained in natural ecosystems which are N-limited (Eamus and Jarvis, 1989; Kramer, 1981; Norby et al., 1986). However, Norby (1987) has hy- pothesized that N-fixing plants may be able to sustain increased growth under elevated CO 2 conditions. If increased growth is more likely to be sustained on a long-term basis by N-fixing trees, forests with these trees may have a greater potential for C storage. Under elevated CO 2 levels, carbon allocation among plant tissues is often altered. In most stud- ∗ FAX No: (775) 784-4789. E-mail: qualls@equinox.unr.edu ies, total root biomass increases (Eamus and Jarvis, 1989; Lewis et al., 1994; Norby et al., 1986; Norby et al., 1987; Norby et al., 1995; Rogers et al., 1994), and, generally, tree species exhibit a slight increase in R:S ratio (Norby, 1994). In addition, increased alloca- tion of 14 C-labeled photosynthate (Hodge and Millard, 1998; Norby et al. 1987) and carbohydrate concentra- tion (Lewis et al., 1994; and see reviews by Norby, 1994 and Rogers et al., 1994) has been found in roots. This increased input of C to roots, in the form of in- creased biomass or carbohydrate concentration, could stimulate greater root respiration and/or higher rates of exudation of soluble organic compounds (Norby, 1994). Researchers have either hypothesized (Ceule- mans and Mousseau, 1994; Rogers et al., 1994; van Veen et al., 1991) or observed an increase in C inputs