Competing Bradyrhizobia strains determine niche occupancy by two native legumes in the Iberian Peninsula

Abstract

The effect of soil pH on the competitive abilities of two rhizobial strains was investigated in two sterile systems. The two strains come from wild plants of the endemic shrubby legumes Cytisus multiflorus (cmu) and Cytisus balansae (cba). Strains were used to infect seedlings of C. multiflorus, C. balansae, and Ornithopus compressus, grown in soil and hydroponics with acidic and neutral pH. All seedlings were inoculated with a single-strain inoculum containing 106 total cells of one of the two test strains or with a mixed inoculum (1:1 cmu:cba). Controls consisted of non-inoculated seedlings. At harvest, nodule occupants were determined by PCR. The majority of nodules (>95 %) formed on plants grown in acidic soil were occupied by cmu strain. This pattern of nodule occupancy changed in neutral pH both in soil and in hydroponics. When cmu was paired with cba, the former formed 78 % of the nodules in the acidic media and the number of nodules formed by cba was higher in neutral media (8 %). When nodule occupancy was dominated by cmu, the total nitrogen and biologically fixed nitrogen were higher in C. multiflorus and O. compressus. The different nodule occupancy percentages indicate a correlation between the preferred growing conditions of both host plants and Bradyrhizobia strains. Results indicate that soil pH can influence which symbiotype will competitively nodulate C. multiflorus and C. balansae in the field which accounts for the current distribution of these two plants.