Biochar enhances cucumber production by modulating rhizosphere microbiota and soil metabolites under continuous cropping systems.

Biochar, a soil amendment with diverse regulatory functions, has been widely applied to enhance soil conditions. However, its underlying mechanism for alleviating continuous cropping obstacles, from the perspective of rhizosphere microbe-metabolite-plant coupling, remains to be further elucidated. Using cucumber (Cucumis sativus L.) as the model crop, this study explored the rhizosphere-mediated effects of biochar application under continuous cropping conditions via the analytical methods of metagenomics and metabolomics. Six biochar application rates (0, 5, 10, 20, 30, and 40 t ha-¹) were tested. All biochar treatments significantly improved cucumber yield by 20%-50%, with the C30 and C40 treatments producing the most pronounced yield enhancement. C10, C20, C30 and C40 treatments had a positive effect on cucumber quality, soil physicochemical properties and enzymatic activities. Vitamin C and soluble protein peaked in C20, whereas some sugar indicators decreased across all biochar treatments. Urease activity was significantly elevated under C20, C30, and C40 treatments. Notably, the C40 treatment led to marked increases in total nitrogen, available phosphorus, and sucrase activity. Biochar amendments also enriched key bacterial phyla involved in carbon and nitrogen cycling, including Actinobacteria, Bacteroidetes, Chloroflexi, and Bacillota. Medium to high application rates (C20, C30, C40) upregulated various secondary metabolic pathways associated with biotic stress resistance, including the biosynthesis pathways of phenylpropanoids, various alkaloids, and the metabolic pathway of phenylalanine. High biochar application rate (C40) characterized lipid metabolism as the core responsive pathway and significantly downregulated galactose metabolism. This study reveals that biochar application represents a promising strategy to mitigate continuous cropping obstacles of cucumber by enhancing nutrient cycling, enzyme activities, soil metabolite composition, and the rhizosphere microbial community in facility systems of the cold and arid northern regions of China.