Effects of Waxy Maize Relay Intercropping and Residue Retention on Rhizosphere Microbial Communities and Vegetable Yield in a Continuous Cropping System

Intercropping and residue retention contribute to high yield and quality of crops. However, their coupled effects on rhizospheric microbial communities under a continuous vegetable cropping system have not been adequately addressed. The objective of the present study was to assess the effects on soil microbial community and yields of waxy maize (Zea mays L.) intercropped with or without residue retention in a continuous broccoli (Brassica oleracea L.) cropping system, i.e., relay intercropping of broccoli and waxy maize (B/M-B), relay intercropping of broccoli and waxy maize with residue retention (B/MR-B), and broccoli monoculture (B-B). The biomass yields of spring and autumn vegetables in B/MR-B were 16.3%-32.5% and 30.1%-46.1% higher than those of B-B, respectively. Autumn vegetable economic yields of B/MR-B were 28.2%-40.3% higher than B-B. The average well color development followed the order: B/MR-B > B/M-B > B-B. The Shannon index, Simpson index, and McIntosh index were higher in B/MR-B than under monoculture. A principal component analysis showed that microbial communities of B/MR-B soils differed from those of B/M-B and B-B soils. Carbon (C) sources utilized by the rhizosphere microorganisms were mainly carbohydrates, carboxylic acids, amino acids, and polymers; however, the C sources for the soil microbial community differed between intercropping and monoculture. The communities from B/MR-B preferred amino acids and polymers. Available nitrogen (N), potassium (K), and phosphorus (P) had an obvious impact on soil microbial community. Additionally, the C source utilization by microorganisms was significantly affected by pH and available K and P. Cropping system diversification through relay intercropping and residue retention effectively improved the functional diversity of the soil microbial communities and increased the yields of vegetables.