Patterns of Growth Costs and Nitrogen Acquisition in Cytisus striatus (Hill) Rothm. and Cytisus balansae (Boiss.) Ball are Mediated by Sources of Inorganic N

Nitrogen-fixing shrubby legumes in the Mediterranean area partly overcome nutrient limitations by making use of soil N and atmospheric N₂ sources. Their ability to switch between different sources lets them adjust to the carbon costs pertaining to N acquisition throughout the year. We investigated the utilization of different inorganic N sources by Cytisus balansae and Cytisus striatus, shrubby legumes under low and a sufficient (5 and 500 µM P, respectively) levels of P. Plants grew in sterile sand, supplied with N-free nutrient solution and inoculated with effective Bradyrhizobium strains; other treatments consisted of plants treated with (i) 500 µM NH₄NO₃; and (ii) 500 µM NH₄NO₃ and inoculation with effective rhizobial strains. The application of NH₄NO₃ always resulted in greater dry biomass production. Carbon construction costs were higher in plants that were supplied with mineral and symbiotic N sources and always greater in the endemic C. striatus. Photosynthetic rates were similar in plants treated with different sources of N although differences were observed between the two species. Non-fertilized inoculated plants showed a neat dependence on N₂ fixation and had more effective root nodules. Results accounted for the distribution of the two species with regards to their ability to use different N sources.