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 N(2) 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 NH(4)NO(3); and (ii) 500 µM NH(4)NO(3) and inoculation with effective rhizobial strains. The application of NH(4)NO(3) 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 N(2) 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.