Synergy and comprehensive evaluation of biochar-based biogas residue fertilizer for enhancing yield and quality of greenhouse cucumber

The application of engineered organic amendments is crucial for advancing sustainable greenhouse vegetable production.This study evaluated the effects of a biochar-based biogas residue fertilizer (BRC) on cucumber (Cucumis sativus L.) in a greenhouse system.Eight fertilization treatments were applied, including different N levels and combinations with chemical fertilizers.These included a control group with no fertilization (CK), conventional chemical fertilizer (CF), biogas residue from cow manure (BR), biochar-based biogas residue without chemical fertilizer (BRC), biogas residue mixed with chemical fertilizer (CB), and biochar-based biogas residue combined with chemical fertilizer (CBC).The results indicated that BRC significantly enhanced cucumber physiological performance: It increased the average growth rate by 21.95% compared to the unamended control and improved root activity by up to 46.34% relative to CF.Although the highest cucumber yield was achieved under sole high-rate biogas residue (BR-34), BRC treatments increased yield relative to conventional chemical fertilization (CF-34) and markedly improved N use efficiency by 109.27%.Notably, BRC improved fruit quality by elevating vitamin C and soluble protein contents while significantly reducing nitrate accumulation.A fuzzy mathematics-based comprehensive evaluation confirmed that CBC-34 (combined BRC and mineral fertilizer treatment) achieved the highest overall fruit quality score of 0.73.The synergistic effect between biochar and biogas residue likely moderated nutrient release and improved rhizosphere conditions.This work reveals that sole BRC application delivers outstanding N retention capacity, whereas CBC-34 balances nutrient supply, crop growth and fruit quality; thus CBC-34 is the optimal integrated fertilization regime for sustainable greenhouse cucumber production.