Elevated CO2 stimulates associative N2 fixation in a C3 plant of the Chesapeake Bay wetland

In this study, the response of N-2 fixation to elevated CO2 was measured in Scirpus olneyi, a C-3 sedge, and Spartina patens, a C-4 grass, using acetylene reduction assay and N-15(2) gas feeding, Field plants grown in PVC tubes (25 cm long, 10 cm internal diameter) were used. Exposure to elevated CO2 significantly (P < 0.05) caused a 35% increase in nitrogenase activity and 73% increase in N-15 incorporated by Scirpus olneyi, In Spartina patens, elevated CO2 (660 <plus/minus> 1 mu mol mol(-1)) increased nitrogenase activity and N-15 incorporation by 13 and 23%, respectively. Estimates showed that the rate of N-2 fixation in Scirpus olneyi under elevated CO2 was 611 +/- 75 ng N-15 fixed plant(-1) h(-1) compared with 367 +/- 46 ng N-15 fixed plant(-1) h(-1) in ambient CO2 plants. In Spartina patens, however, the rate of N-2 fixation was 12.5 +/- 1.1 versus 9.8 +/- 1.3 ng N-15 fixed plant(-1) h(-1) for elevated and ambient CO2, respectively. Heterotrophic non-symbiotic N-2 fixation in plant-free marsh sediment also increased significantly (P < 0.05) with elevated CO2. The proportional increase in N-15(2) fixation correlated with the relative stimulation of photosynthesis, in that N-2 fixation was high in the C-2 plant in which photosynthesis was also high, and lower in the C-4 plant in which photosynthesis was relatively less stimulated by growth in elevated CO2, These results are consistent with the hypothesis that carbon fixation in C-3 species, stimulated by rising CO2, is likely to provide additional carbon to endophytic and below-ground microbial processes.