Wheat cover crop alters soil microbial community and increases cucumber yield under different potassium regimes

Cover crops can promote the subsequent plant growth and improve soil potassium (K) fertility. However, whether and how wheat cover crop benefits cucumber growth and yield under different K regimes and their functional roles are still unclear. In this study, a two-year greenhouse experiment from spring 2017 to autumn 2018 was conducted to evaluate the effects of wheat cover crop on cucumber plant growth, nutrition and yield. Moreover, a litterbag experiment was carried out in the greenhouse in spring 2018 to investigate the nutrient dynamics during decomposition of wheat shoot and root residues under the two K regimes. Soil bacterial and fungal communities during residues decomposition were analyzed by high-throughput amplicon sequencing, and the abundances of total bacterial, fungal, Bacillus and Pseudomonas spp. communities were also estimated by quantitative PCR. The responses of cucumber growth to changes in the soil microbial communities induced by wheat cover crop were assessed by plant-soil feedback experiment. Wheat cover crop increased cucumber yield except for in spring 2017 and plant growth and K concentration at 70 d in spring 2018, and reduced K application did not weaken the beneficial effects of wheat on cucumber growth and yield. Decomposition rate and nutrient release of wheat shoot residue differed from root residue, but no differences in shoot or root residue decomposition rate and nutrient release between the two K regimes. Wheat shoot and root residues differently changed the abundances, compositions and diversities of soil bacterial and fungal communities. Wheat shoot residue increased total bacteria, Bacillus and Pseudomonas spp. community abundances compared with root residue irrespective of K regimes, and Bacillus spp. community abundance was higher in shoot residue with reduced K application than that with normal K application from 220 to 240 d. Wheat shoot residue decreased the Shannon and inverse Simpson diversities of soil bacterial and fungal communities compared with root residue from 200 to 240 d. Moreover, there was significant difference in these diversities of soil fungal community of root residue between the two K regimes except for 240 d. In addition, wheat cover crop caused more stable co-occurrence network with more positive and negative total links and wheat shoot residue had higher these links than root residue, while reduced K application decreased these links of both shoot and root residues. Feedback effects of changes in soil microbial communities induced by wheat cover crop on cucumber growth were positive, and such effect was stronger in shoot residue than in root residue irrespective of K regimes. Overall, wheat cover crop can alter soil microbial communities and enhance cucumber productivity through positive plant-soil feedbacks mediated by soil biota, and reduced K application did not weaken such beneficial effects. • Reduced K did not weaken the beneficial effects of wheat on cucumber productivity. • Wheat cover crop increased microbial interactions and reduced K decreased such effects. • Wheat residues decomposition shifted keystone species to bacteria-dominated ones. • Soil biota-mediated feedback effects of wheat on cucumber growth were positive.