Belowground mechanisms involved in plant competition are still poorly understood. Since plant species are differently affected by soil organisms, changes in soil community composition might affect interspecific competition with consequences for plant community structure. We studied whether soil community composition affects competition between the grass Holcus lanatus L. and the legume Lotus corniculatus L. We established three different soil communities by adding no soil organisms (control), microorganisms 30 μm, AMF and nematodes to gamma-sterilized soil. Nodulation and aboveground biomass of Lotus was decreased in the sterilized control soil and in the presence of Holcus. Contrastingly, the grass grew better in the presence of the legume than in monoculture and was not affected by soil community composition. Legume monocultures tended to produce the greatest aboveground biomass of the plant combinations when soil microorganisms were present, while the root biomass in legume monocultures was the lowest. Then, in a second experiment, we used natural (not sterilized) soil and added activated carbon to test whether the reduced nodulation of Lotus in interspecific competition is caused by allelopathic compounds of Holcus. In the natural soil, nodulation and flowering of Lotus was reduced, but the aboveground biomass was not affected by the competition with Holcus. Contrary to our expectations, activated carbon had a strong negative effect on the nodulation, growth and flowering of Lotus and shifted the interspecific competition in favour of Holcus. Probably, activated carbon impeded the nodulation by disrupting the communication between the legume and N2-fixing bacteria. We suggest that interruption of plant-microbe communications by activated carbon might be widespread and will confound interpretations on the role of allelopathy. Generally, we observed that the symbiosis of the legume with N2-fixing bacteria plays a crucial role in the grass-legume competition. When the symbiosis was deterred, the legume was outcompeted by the grass.