Grass-legume mixtures enhance forage production via the bacterial community

Forage monoculture and grass (Gramineae)-legume mixtures are the two most commonly used planting patterns for establishing farming grasslands, but the effects of these planting patterns on the rhizosphere bacterial community and forage production are still unclear. In this study, we carried out a field experiment on the Tibetan Plateau, and observed that grass-legume mixtures significantly increased forage production over monocultures (P < 0.05). We investigated the effects of these two planting patterns on rhizosphere and bulk soil bacterial communities, and found that the grass-legume mixtures altered the community structure and composition of the grass rhizosphere bacterial community, while the bacterial community of bulk soils remained unchanged. Source tracker analysis demonstrated that 50.77% and 34.72% of the changes in mixture rhizosphere of two gramineous grasses were derived from the legume, illustrating that the legume greatly influenced the grass rhizosphere in grass-legume planting. The molecular ecological network analysis demonstrated that network vulnerability and robustness of the mixtures were significantly higher than that of monoculture, indicating that the grass-legume mixtures provided a favorable condition for microbial interactions. Besides, by using structural equation modeling, it was found that the stability and complexity of the bacterial community network had significant positive correlations with forage production, while the soil physicochemical properties had no significant direct correlations, suggesting the rhizosphere bacterial community played a critical role in enhancing plant growth in grass-legume mixtures. This study provides a new insight into the mechanism behind the enhancement of forage production in the grass-legume mixture planting pattern.