Legume-bacteria-soil interaction networks linked to improved plant productivity and soil fertility in intercropping systems

Intercropping with legumes is an effective strategy to increase plant productivity, reduce nitrogen (N) inputs, and maintain soil fertility in an industrial monoculture forest. However, the facilitation/complementarity mechanism of the interactions between legumes (keystone species in intercropping), bacteria and soil are still unknown, and the contribution of this triple interaction to plant growth and ecosystem function is still difficult to predict. Taking the introduction of Dalbergia odorifera (D. odorifera), into eucalyptus plantations as an example, we evaluated the changes in nutrient uptake, phytohormone contents, N metabolism-related enzyme activities, and root-associated bacteria of D. odorifera under different N applications (0, 3, 6 and 12 g pot−1) and different planting patterns (monoculture and intercropping), and investigated the regulatory mechanism of these cascaded changes on productivity and soil fertility in response to intercropping. The results showed that intercropping increased the total biomass by 5.90–56.29% compared to monoculture; in particular, intercropping in combination with 6 g N pot−1 had superiority in promoting the gibberellin content, glutamine synthetase activity, nitrogenase activity, soil fertility, and bacterial diversity, as well as the relative abundance of bacteria with the potential to undergo aerobic chemoheterotrophy, N fixation and N respiration. The differentially expressed genes identified via transcriptome analysis were mainly involved in metabolic pathways, signal transduction, and the tricarboxylic acid cycle. To integrate the associations between plant traits, bacterial communities, and soil properties, we built a multivariable network based on multiple plant traits, soil properties, differentially accumulated genera, and differentially expressed genes in monoculture and intercropping. These results led to the identification of candidates related to intercropping advantages, including two genes (acidic endo chitinase, Na+/K+ transporter) and four bacterial genera (Bacteroides, Coxiella, Pseudomonas, SJA-15). More specifically, moderate fertilization of eucalyptus/D. odorifera intercropping systems create a more stable interaction network with additional module hubs. These results highlight the importance of bacterial diversity and the plant-bacteria-soil interaction network in maintaining soil fertility and plant productivity, which may help fertilization practices in eucalyptus plantations and provide guidance for the ecological sustainability of introducing D. odorifera into industrial forests.