Symbiotic nitrogen (N) fixation is a major input of N to ecosystems, but it is energetically intensive. Theory suggests that low light should inhibit symbiotic N fixation, which may explain the exclusion of N-fixing trees from low-light understories of mature temperate forests. N-fixing trees provide symbiotic bacteria with a carbon food source, giving them the energy necessary to fix atmospheric N into plant-available form. Rates of symbiotic N fixation, therefore, should depend on the availability of labile carbon within a plant, which is produced through photosynthesis. We hypothesized that N-fixing tree seedlings exposed to a reduction in light intensity would decrease photosynthesis and N fixation rates. To test this, we decreased day-time light availability in potted seedlings of the temperate N-fixing tree Robinia pseudoacacia. To calculate relative N fixation rates, we conducted non-destructive, weekly measurements of nitrogenase activity within entire seedlings using Acetylene Reduction Assays by Cavity-ring down Absorption Spectroscopy (ARACAS), and we tracked changes in N fixation rates of individual seedlings over time. Whole plant carbon flux data were also collected weekly for the same seedlings, and we compared nitrogenase activity and net photosynthesis.
Results/Conclusions
Robinia pseudoacacia seedlings exhibited variable responses to a decrease in day-time light availability. Some seedlings decreased N fixation from an average of 21.0 to <1.0 µg N fixed plant-1 hr-1 within 5 weeks, consistent with the expectation that a decrease in carbon supply would inhibit N fixation. Other seedlings, however, increased N fixation over the same 5-week period from an average of 25.7 to 39.8 µg N fixed plant-1 hr-1. The seedlings that increased N fixation maintained high rates of photosynthesis and used fixed N to increase leaf area by producing new foliage. As such, net photosynthesis was positively correlated with N fixation across all seedlings (R2 = 0.70, p < 0.01). We speculate that seedlings are able to subsidize the N needed in new leaves by trading stored carbohydrates for fixed N, thereby investing in future carbon gain. The ability to use stored carbohydrates to promote N fixation may mean that rates of fixation are regulated by plant demand for future growth rather than by current photosynthetic rates.