Abstract
The perennial forbs Centaurea jacea and C. scabiosa have relatively similar morphological traits, but different distributions and local abundance. Both occur in ruderal sites and in calcareous grasslands, but C. jacea may dominate in calcareous grasslands, while C. scabiosa is a subordinate species. We hypothesised that: (1) C. jacea is dependent on the presence of the root-associated microbial community of a calcareous grassland (CG inoculum), as opposed to the microbial community from a ruderal site (old field inoculum, OF), while C. scabiosa is not; and (2) if there is a positive response of C. jacea to CG inoculum, it will be more evident in oligotrophic soil conditions. In our experiment the two Centaurea species were grown with two different inocula (CG, OF) at three different nutrient levels. CG inoculation considerably increased the leaf number of C. jacea as compared with the OF inoculation. This response was interpreted as an investment into competitive ability in the future. CG inoculum resulted in a significantly higher root biomass in C. scabiosa, while the biomass of C. jacea did not vary among differentially inoculated plants. This was interpreted as an increased allocation to storage in C. scabiosa, to improve performance in stress conditions. Thus, compared with ruderal inoculum (OF), grassland (CG) inoculum enhanced 'competitive behaviour' in C. jacea and 'stress-tolerating behaviour' in C. scabiosa. Both responses were more evident at high nutrient levels. Fertilisation decreased the shoot to root ratio in both species, which may represent an 'element of stress-tolerating behaviour', much more common among rhizomatous perennial plants than is usually recognized.