Root exudates-microbe interactions promote nodulation in legume-based intercropping

The classical nitrogen competition hypothesis posits that intercropping enhances legume nodulation as a passive response to reduced soil N availability. We propose a complementary mechanism whereby nitrogen competition serves as an ecological trigger, while root exudate-mediated microbial restructuring may amplify symbiotic interactions. In a meta-analysis of 157 studies, we found that intercropping selectively altered flavonoid and organic acid exudation in a crop-combination-dependent manner, with baseline exudation levels of monocultured legumes emerging as an important negative moderator. Intercropping consistently promoted arbuscular mycorrhizal fungal (AMF) colonization and enriched key plant growth-promoting rhizobacteria (PGPR) taxa, particularly Pseudomonas and Sphingomonas, with stronger microbial responses under low soil nitrogen. Co-inoculation with AMF and rhizobia or multiple PGPR strains generally produced greater nodulation benefits than single-strain inoculation, although these benefits declined with increasing intrinsic nodulation potential, indicating host-imposed constraints on symbiotic enhancement. Integrating these patterns with mechanistic insights, our synthesis indicates that reciprocal interactions between root exudates and rhizosphere microbes may reinforce rhizobial infection, phytohormone signalling, and nutrient mobilization, thereby sustaining and amplifying nodulation. This framework extends the resource-competition paradigm and provides a basis for developing crop combinations and microbial management strategies to enhance symbiotic N inputs in sustainable intercropping systems.