Biochar and vermicompost improve the soil properties and the yield and quality of cucumber (Cucumis sativus L.) grown in plastic shed soil continuously cropped for different years

Organic amendments such as biochar and vermicompost have been reported to improve soil and plant properties. However, whether the co-application of the two amendments has a synergistic effect on the alleviation of vegetable continuous cropping obstacles in plastic shed cultivation is still unclear. This study investigated the single and joint effects of biochar and vermicompost on the improvement of soil quality and cucumber yield and quality in plastic shed soil that had been continuously cultivated for 0, 5, and 20 years in northern China. The experiment was conducted in a completely randomized design with four treatments: control, without the addition of biochar (CK); 15 t/ha biochar (BC); 15 t ha−1 vermicompost (VC); and 7.5 t ha−1 biochar + 7.5 t ha−1 vermicompost (BV). The results showed that the single and joint application of biochar and vermicompost generally improved soil properties and increased cucumber yield and quality. Soil pH was most significantly increased (1.24–2.46%) by BC treatments, followed by BV treatments, which generally performed the best in decreasing soil electrical conductivity (EC) and increasing dissolved organic carbon (DOC) and the amounts of fungi and bacteria. The BV treatments also most significantly improved the fruit quality and increased cucumber yields by 29.2–56.0%. The results of structural equation modeling showed that soil pH, DOC, available phosphorus, available potassium, and the amounts of fungi and bacteria positively contributed to cucumber fruit yield and quality, whereas soil EC had a negative effect. These properties also interacted with each other (e.g., there were strong correlations among soil EC, pH, DOC, nutrients, and the amounts of fungi and bacteria), thereby indirectly affecting cucumber yield and quality. The co-application of biochar and vermicompost is therefore recommended for the alleviation of continuous cropping obstacles in plastic shed cultivation.