Positive effects of plant association on rhizosphere microbial communities depend on plant species involved and soil nitrogen level

How plant species associations affect the rhizosphere microbiota, in comparison to each plant species, has been overlooked. Here we investigated how bi-species plant associations affect total bacterial and N-cycling microbial communities (nitrate reducers and ammonia-oxidizers), as an example of functional communities, at low and high soil N levels. At low N level, the total bacterial abundance in the rhizosphere of the bi-species associations was significantly higher than that of either plant on its own for half of the bi-species associations. For the other associations, the bacterial abundance was not different from the corresponding mono-species cultures. The abundance of several N-cycling guilds was also enhanced by some of the bi-species associations with increases of up to 125% and no negative effects were recorded, which highlights the importance of plant diversity in agro-ecosystems.