Effects of increased atmospheric CO2, temperature, and soil N availabilityon root exudation of dissolved organic carbon by a N-fixing tree (Robinia pseudoacacia L.)

Root exudation has been hypothesized as one possible mechanism that may lea d to increased inputs of organic C into the soil under elevated atmospheric CO2, 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 CO2 (35.0 and 70.0 Pa) x 2 temperature (26 degrees and 30 degrees C durin g the day) x 2 N fertilizer (0 and 10.0 mM N concentration) levels. We also analyzed the decomposition rates of root exudate to estimate gross rates o f exudation. Elevated CO2 did not affect root exudation of organic C. A 4 d egrees C increase in temperature and N fertilization did, however, signific antly increase organic C exudation rates. Approximately 60% of the exudate decomposed relatively rapidly, with a turnover rate of less than one day, w hile the remaining 40% decomposed more slowly. These results suggest that w armer climates, as predicted for the next century, may accelerate root exud ation of organic C, which will probably stimulate rapid C cycling and may m ake 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.