Differential growth costs and nitrogen fixation in Cytisus multiflorus (L'Hér.) Sweet and Cytisus scoparius (L.) Link are mediated by sources of inorganic N.

Shrubby legumes in Mediterranean-type ecosystems face strong nutrient limitations that worsen in summer, when water is absent. Nitrogen-fixing legumes are likely to be able to switch between soil N and atmospheric N (N2 ) sources to adjust the C costs of N acquisition in different seasons. We investigated the utilisation of different inorganic N sources by two indigenous shrubby legumes (Cytisus multiflorus and Cytisus scoparius). Plant performance in terms of photosynthesis and biomass production was also analysed. Plants were cultivated in sterile river sand supplied with Hoagland nutrient solution, grown in N-free nutrient solution and inoculated with effective rhizobial strains from nodules of adult plants of the same species. A second treatment consisted of plants given 500 μm NH4 NO3 added into the nutrient solution. In a third treatment, plants were watered with another source of N (500 μm NH4 NO3 ) as well as being inoculated with effective rhizobial strains. The application of NH4 NO3 to the legumes resulted in a larger increase in plant dry matter. Carbon construction costs were higher in plants supplied with mineral and symbiotic N sources and always higher in the endemic C. multiflorus. Differences in photosynthesis rates were only observed between species, regardless of the N source. Non-fertilised inoculated plants had more effective root nodules and a clear dependence on N2 fixation. We propose that the ability of C. scoparius to change N source makes it a plastic species, which would account for its broader distribution in nature.