Effects of single-root-grafting, double-root-grafting and compost application on microbial properties of rhizosphere soils in Chinese protected cucumber (Cucumis sativus L.) production systems

• Grafting influenced rhizosphere soil microbial community diversity and composition.
• Compost application (CA) increased soil microbial diversity in the early spring season.
• Double-root grafting increased total fruit yields compared to single-root grafting.
• The combination of double-root grafting and CA showed the highest total fruit yields.

The grafting and compost application generally have positive effects on plant growth in continuous cropping systems, which may be related to root-associated microbial properties that are important for plant growth. A field study was conducted to investigate the effects of grafting and compost application on improvement of degraded soil in a continuous cucumber cropping system in Beijing, China. Self-root (SR), single-root-grafted (SRG) and double-root-grafted (DRG) seedlings were respectively transplanted to the soils treated with the chicken manure (CM) or CM compost (CMC), and rhizosphere soil microbial properties, plant biomass and fruit yields were determined in two cropping seasons after treatments. Rhizosphere soil microbial properties were evaluated with a combination of microbial populations measured by quantitative PCR, microbial community composition analyzed by denaturing gradient gel electrophoresis and 454 pyrosequencing, and sole-carbon utilization capacity by microbial communities. In general, DRG increased microbial (bacterial and fungal) abundance but decreased microbial diversity compared to SR, and increased total fruit yield compared to SRG. SRG decreased microbial abundance and diversity, but increased total dry matter compared to SR. When compared to CM, CMC increased bacterial abundance, microbial diversity and cucumber fruit yield, but decreased fungal abundance and microbial diversity in the early-spring (ES) season. Reverse trends were found in the autumn–winter (AW) season. The highest total fruit yield was found under the treatment DRG-CMC. Principal component (PC) analysis of soil microbial properties showed that both PC1 and PC2 were significantly correlated with TDM (both P < 0.01). These results suggest that DRG-CMC is a beneficial practice for increasing cucumber yields. In addition, there are close but complex relationships between rhizosphere soil microbial properties and total dry matter.