Impact of elevated CO2 on metal homeostasis and the actinorhizal symbiosis in early successional alder shrubs

The increasing atmospheric CO2 concentration could stimulate terrestrial ecosystem and create an important carbon (C) sink that could slow down climate change due to anthropogenic activities. The extent of this enhanced growth will strongly depends upon the availability of nitrogen (N) to plant. In alder, the predominant N2 fixing tree in boreal forest, the ability to establish actinorhizal symbiosis will thus be key. How high CO2 concentration and exogenous N impact actinorhizal symbiosis remains poorly known. In this study on speckled alder and green alder, we evaluated the effect of CO2 and exogenous N availability on (i) the efficiency and development of the actinorhizal symbiosis and (ii) on the homeostasis of eleven elements, including essential nutrients for N2 fixation such as phosphorus (P) and molybdenum (Mo). We report that Frankia sp. infection is the primary factor controlling nutrient homeostasis (allocation and concentration) in plant. P and Mo, and in a less extend magnesium (Mg) and manganese (Mn), are the elements which homeostasis was the most affected by Frankia sp. infection. Exogenous N and CO2 concentrations had limiting or no impact on the homeostasis of most elements. We also show that actinorhizal symbiosis could be critical to alder response to elevated CO2. In absence of Frankia sp. infection neither speckled alder nor green alder responded to elevated CO2. On the contrary, in presence of Frankia sp., the biomass production was significant stimulated by increasing CO2 and independent to exogenous N.