Objectives Apple replanting disease (ARD) is a major factor that restricts the sustainable development of apple industry. In this research, biochar and organic fertilizer were simultaneously used to improve the resistance of apple seedlings to replanting disease, expecting to provide a theoretical support for the prevention and control of ARD for the renewal of aged orchards.

Methods A pot experiment was carried out to study the effects of biochar, organic fertilizer, and their combined application on the apple seedlings and soil environment suffered from ARD. The seedlings of Malus hupehensis Rehd. were planted in pots with apple replant soil. The experiment included 5 treatments, which were no biochar nor organic fertilizer (CK), 2% biochar (B), 2% organic fertilizer (OF), 2% biochar + 2% organic fertilizer (BOF) and methyl bromide sterilization (F). The biomass, root respiration rate and indicators of Malus hupehensis Rehd. seedlings were measured by conventional method. The number of microorganism, fungal group structure and the Fusarium oxysporum numbers were detected by Illumina high-throughput sequencing technology on Miseq platform and a real-time quantitative PCR detection system.

Results Compared with the control, the treatment B, OF, and BOF all significantly enhanced the plant height, ground diameter, fresh and dry weight. B, OF, and BOF treatments increased the root respiration rate significantly, which were 1.3, 1.2 and 1.5 times of that in control, respectively. The treatments of B, OF, and BOF increased the root total length, root surface area, root volume significantly, with the highest in BOF treatment, they were 2.3, 4.8 and 3.4 times of those in control. The three treatments also increased the soil enzyme activates, with the best effect in BOF, in which the soil urease, invertase and phosphatase activities were 2.1, 2.9 and 2.9 times of those in control. The three treatments ameliorated the soil fungal community structure in different degrees. BOF significantly improved the soil fungi richness and diversity, which were significantly different from other three treatments. Compared with the control, the combined application of biochar with organic fertilizer and methyl bromide sterilization reduced the gene copy numbers of Fusarium oxysporum, which explained that the harmful fungi number decreased significantly in replanting soils.

Conclusions The combined application of biochar and organic fertilizer shows better result to improve the condition in apple replant soil than the solely application of biochar or organic fertilizer. The combined application improved the Malus hupehensis Rehd. seedlings biomass, root respiration rate, soil enzyme activity, reduced the Fusarium oxysporum gene copy numbers, and optimized the soil fungi community. Therefore, the combined application of biochar and organic fertilizer is a better practice to prevent and control the ARD in replanting soil.