A test of a potential short cut in the nitrogen cycle: The role of exudation of symbiotically fixed nitrogen from the roots of a N-fixing tree and the effects of increased atmospheric CO2 and temperature

N-fixing trees facilitate the growth of neighboring trees of other species. These neighboring species benefit from the simple presence of the N fixati on symbiosis in their surroundings. Because of this phenomenon, it has been hypothesized that a change in atmospheric CO2 concentration may alter the role of N-fixing trees in their environment. It is thought that the role of N-fixing trees in ecosystems of the future may be more important since the y may help sustain growth increases due to increased CO2 concentration in n itrogen limited forests. We examined: (1) whether symbiotically fixed N was exuded from roots, (2) whether a doubled atmospheric CO2 concentration wou ld result in increased organic N exudation from roots, and (3) whether incr eased temperature or N availability affected N exudation from roots. This s tudy analyzed exudation of dissolved organic N from the roots of seedlings of the N-fixing tree Robinia pseudoacacia L. in a full factorial design wit h 2 CO2 (35.0 and 70.0 Pa) x 2 temperature (26 or 30 degrees C during the d ay) x 2 N fertilizer (0 and 10.0 mM N concentration) levels. Trees with no other source of N except N fixation exuded about 1% to 2% of the fixed N th rough their roots as dissolved organic N. Increased atmospheric CO2 concent rations did not, however, increase N exudation rates on a per gram belowgro und biomass basis. A 4 degrees C increase in temperature and N fertilizatio n did, however, significantly increase N exudation rates. These results sug gest that exudation of dissolved organic N from roots or nodules of N-fixin g trees could be a significant, but minor, pathway of transferring N to nei ghboring plants in a much more rapid and direct way than cycling through de ath, decomposition and mineralization of plant residues. And, while exudati on rates of dissolved organic N from roots were not significantly affected by atmospheric CO2 concentration, the previously observed 'CO2 fertilizatio n effect' on N-fixing trees suggests that N exudation from roots could play a significant but minor role in sustaining increases in forest growth, and thus C storage, in a CO2 enriched atmosphere.