Abstract: 【Objective】 The experiment was conducted to explore the effects of biochar and chitin combined application on the plant biomass, root respiration rate, root protective enzymes and soil environment under replanting conditions, and to provide a basis for prevention of apple replanting disease (ARD). 【Method】 The Malus hupehensis Rehd. seedlings were planted in pots with apple replanting soil. The experiment included 5 treatments, which were the control (CK), 1‰ chitin (T1), 2% biochar (T2), 1‰ chitin + 2% biochar (T3) and methyl bromide sterilization (T4). The biomass, root respiration rate and protective enzymes activities of M. hupehensis seedlings were measured by conventional method. The number of microorganism, fungal group structure and the Fusarium oxysporum numbers were detected by terminal restriction fragment length polymorphism (T-ＲFLP) and a real-time quantitative PCR detection system. 【Result】 The results showed that compared with the control, T3 and T4 significantly enhanced the plant height, ground diameter, fresh and dry weight, and that in T3 increased by 44.6%, 33.0%, 76.8% and 77.1%, and in T4 increased by 73.1%, 76.9%, 117.0% and 123.7%, respectively. T1, T2, T3 and T4 treatments made the root respiration rate increased significantly, which were 1.37, 1.70, 1.87 and 2.02 times more than the control. The four different treatments also increased the root activity of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT), and the increase reached a significant difference. The SOD, POD and CAT activities of T4 treatments were 1.34, 2.68 and 2.64 times compared with the control, and T3 were 1.21, 2.52 and 2.18 times, respectively. T2 and T3 improved the numbers of bacteria, enhanced the ratio of bacteria and fungi and made the soil become “bacterial soil”. The ratios of T3 and T4 were 1.7 and 2.2 times as large as the control, methyl bromide sterilization made the numbers of bacteria and fungi decreased significantly, and the fungi numbers decreased more than bacteria’s, so the bacteria/fungi ratio was higher than the control. T4 had the highest Shannon diversity index, evenness index and richness index, increased by 24.4%, 41.3% and 13.6% compared with CK, and the second was the T3 treatment. Compared with the control, T3 and T4 reduced the gene copie’s number of F. oxysporum, which explained that the F. oxysporum harmful fungi number decreased significantly in replanting soils. Compared with CK, the concentration of phlorizin, phloretin, cinnamic acid, p-hydroxybenzoic acid and phloroglucinol reduced by 28.1%, 30.6%, 50.8%, 33.6% and 46.2% in T3, respectively.【Conclusion】The biochar and chitin combined application improved the condition in apple replanting soil better than solely biochar or chitin addition. The combined application improved the M. hupehensis. seedlings biomass, root respiration rate and root activity of protective enzymes. And the combined application optimized the soil fungi community structure, increased the soil bacteria/fungus ratio, and reduced the F. oxysporum gene copy numbers and contents of phenolic acids. Therefore, biochar and chitin combined application can better relieve ARD.

Key words: apple replant disease, biochar, chitin, Malus hupehensis Rehd. seedlings