COS 228-4 - How nitrogen fixation makes plants more water use efficient, both directly and through higher leaf N concentrations

Abstract: To better understand and model global climate change, we must understand the relationships among water, carbon (C), and nitrogen (N) in plants. One such relationship is the degree to which N mediates plant water use efficiency (WUE, the ratio of carbon gained to water lost during photosynthesis). Decades of research show that WUE increases with leaf N, but recent work has asked how the connection between WUE and N varies across plant functional groups that access N in different ways. Specifically, N-fixing plants, which acquire N from the atmosphere through a symbiosis with bacteria, might operate differently than other plants. Using a greenhouse experiment we tested the mechanisms driving the relationship between WUE and N in N-fixing versus non-fixing plants. We asked: how do soil N supply, N fixation, and the presence of symbiotic bacteria affect the relationship between leaf N and WUE?

We grew seven N-fixing tree species across a gradient in soil N supply. Half the plants were inoculated and the other half were left uninoculated. Using a structural equation model we assessed the direct and indirect effects of inoculation, N fixation (%Ndfa), fertilization, leaf mass per area, and leaf N per mass on δ13C (used to infer WUE integrated through time). %Ndfa was the dominant direct and indirect (through its effects on leaf N) driver of δ13C. Compared to a hypothetical inoculated plant that did not fix any N, a plant fixing 100% of its N had a 1.4‰ higher δ13C. The indirect effect was also strong: fixing 100% of N made leaves 50% more N rich, and increased δ13C by 0.75‰. Surprisingly, fertilization had negligible effects on δ13C, even among uninoculated plants. Also, inoculation itself led to a 0.95‰ decrease in δ13C, independent of N fixation. Overall, when these costs and benefits are considered together, our results suggest that N fixers are more water use efficient, in part because of having greater leaf N and in part for other reasons. Given that elevated CO2 can intensify N limitation and climatic drought, our results suggest a critical role for N-fixing plants in future ecosystems.