Nitrogen-fixing trees contribute a large amount of new nitrogen (N) to ecosystems in which they are present. The symbiosis between a plant and the bacteria that fix nitrogen is generally thought of as a trade between carbon (C) and N: the plant delivers C gained by photosynthesis to the bacteria, while the bacteria deliver fixed N to the plant. Assessing the costs and benefits of this symbiosis requires an understanding of the relationship between available N and photosynthetic rates. While there is much global data demonstrating that higher levels of N in plants generally lead to higher rates of photosynthesis, our understanding of this foliar N-photosynthesis relationship in N-fixing plants is less clear. We grew eight species of N-fixing trees (4 rhizobial, 4 actinorhizal) in a greenhouse experiment, assessing changes in rates of N fixation and photosynthesis (Asat) across a fine-scale 15N-labeled fertilizer gradient. We also included an inoculation treatment: for each species, half of the plants at each fertilization level were given access to their symbiotic bacteria, while the other half were not.
Results/Conclusions
When N-fixing trees do not have access to their symbiotic bacteria, they mirror the common relationship between plant N and photosynthesis: as N fertilization levels increase, photosynthetic rates increase as well. This was true for both rhizobial and actinorhizal fixers in our study. However, when N-fixing trees do have access to their symbiotic bacteria, several different patterns emerge. The most dominant pattern, shown in five of the eight species examined here, was an inverse relationship between N and photosynthesis: photosynthesis rates were highest at the lowest levels of fertilization, and decreased as N fertilization levels increased. In these species, photosynthetic rates were similar between inoculated and uninoculated plants at the highest levels of N fertilization. These results suggest that the symbiotic bacteria associated with N-fixing plants cause changes in plant resource allocation and photosynthetic trait expression, likely influencing the cost-benefit analysis of the C-for-N trade in which these species engage.