## 4 Discussion  
Legumes are able to change the sources of N they use to meet their metabolic needs [40; 41]. The two studied species in the present research confirm this fact, and the reported behavior in terms of N use matches their current distribution in nature. Under control conditions, we tested the responses of _C. balansae_ and _C. striatus_ to changes in P and N supplies as well as the role of rhizbial inoculation in plant growth and biomass allocation. Both species show shifts from organic to inorganic forms of N when P in the growing media is present, which allows them to adjust to changing environmental conditions. Strong differences in plant performance under \(-\)N+R, +N\(-\)R, +N+R with significantly greater biomass production compared with plants under the \(-\)N\(-\)R treatment prove the strong dependence of the two species on N and P availability. Similarly, the dependence of both species on N was clearly moderated by the micro-molar concentrations of P in the growing media. Under sufficient levels of phosphorus in the growing media, both _C. balansae_ and _C. striatus_ were more efficient at incorporating NH\({}_{4}\)NO\({}_{3}\) than at fixing atmospheric N. This can be explained by the fact that it is less expensive to acquire mineral sources of N than to fix them from the atmosphere [42]. The immediate result is a noticeable increase in biomass production when N and P are sufficient in the media. That would translate into a profuse colonization of soils by either of the two species. However, not all species are likely to colonize all soils because the amounts of nutrients needed for satisfactory plant growth would vary from one species to another. Differences in nutritional needs are linked to the legume-rhizobia combination as well as the inorganic source of N [42; 43]. Similarly, legumes differ in their P requirements and in their ability to assimilate P from the soil [44; 45], which correlates with their colonization status [46]. Most legumes from Western Australia would be killed by P concentration in soils from the Iberian Peninsula (toxic effect), whereas the latter would show P deficiencies if grown in the P-improverished soils from Western Australia [47]. In our study, plants of cst grown under low P produced greater amounts of biomass and fixed more N\({}_{2}\) than those of cba. As all plants from the two species were experimentally maintained under exactly the same glasshouse conditions and nutrient availability, we explain the greater biomass production by cst in terms of greater efficiency of the legume-rhizobia interaction [42], which can simultaneously explain its ability to effectively colonize soils beyond its natural area of distribution [48; 49]. It has been demonstrated that legumes under low or zero concentrations of P and N are forced to acquire N through symbiotic N\({}_{2}\) fixation; on the other hand, when N is present in the soil, legumes avoid the expensive process of N\({}_{2}\) reduction [50; 51]; this very same scheme is depicted by cba and cst in this experiment, which resembles the behavior of the Fynbos legume _Vigiglia divariatica_[7]. Nevertheless shifts in the use of N are actually mediated by levels of P. Under limiting levels of P for plant growth, both atmospheric N\({}_{2}\) and NH\({}_{4}\)NO\({}_{3}\) supplies contribute to increase the C sink strength of cba and cst plants in order to maintain enough carbon in the plant tissues to maintain both N fixation and soil N acquisition. Concomitantly, carbon costs and root allocations of plants in the +N+R treatments were the greatest amongst all treatments; that fact proves that the plants of the two species have to maintain the structures for N acquisition [7; 50; 51]. We observed differences in plant biomass accumulation under low P and N supply between the two species, with a marked biomass production by cba, which we explain in terms of carbon sink strength and lower photosynthetic rate of _C. balansae_.