Abstract:

In order to clarify the effects of Bacillus megaterium on the growth of apple（Malus × domestica）trees and the ecological effects of the soil，a field experiment was conducted to explore the effects of B. megaterium application on the growth of new branches，the yield and quality of apple fruits，and the characteristics of soil microbial community using 16 years-old‘Red Fuji’apple trees. The results showed that the growth rate of new branches in 2021 and 2022 were increased by 3.45% and 17.01% within compared with the control 30 d，respectively（P < 0.05），and by 2.58% and 6.71% within 60 d，respectively（P < 0.05），after application of B. megaterium inoculant to apple trees in spring. Moreover，the yield increased by 7.17% and 7.01% respectively（P < 0.05）；the fruits number per plant were increased by 4.95% in 2021（P < 0.05），in the treatment group of 2021 and 2022，but application of B. megaterium inoculant had no significant effect on the fruit shape index，hardness of fruits，and the contents of soluble sugar，soluble solids，and vitamin C in apple fruits. In 2022，the fruit color related indexes of the treatment group，Red-Green Index a^* is lower and Chroma angle h is higher than those of the control group，and there were no significant differences in 2021. Furthermore，the number of fungi in the soil layer of 0-20 cm and 20-40 cm decreased，while the number of bacteria showed the opposite trend. There was no significant difference in α-diversity of soil fungal and bacterial communities in the soil layer of 20-40 cm. For the soil fungal community at genus level，the relative abundances of Trichocladium，Solicocozyma，Naganishia and Ilyoctria in the treatment 1.87，1.92，2.91 and 3.38 times higher than those in the control；The relative abundances of Tausonia and Cephalotrichum were 76.44% and 67.96% of that in the control，respectively（P < 0.05）. For the soil bacterial community at genus level，the relative abundances of Streptomyces and Gemmatimonas in the treatment were 2.65- and 1.33-fold of that in the control，but the relative abundance of Chujaibacter was 48.88% lower than that of the control（P < 0.05）. The number of bacteria and fungi OTU in the rhizosphere of the treatment changed significantly compared with the control；Additionally，the contents of soil available phosphorus and potassium in the rhizosphere of the treatment were 21.87% and 20.81%；5.14% and 5.31%，respectively（P < 0.05），which are higher than those in the control（P < 0.05）. In summary，the application of B. megaterium inoculant to apple trees in spring can inhibit harmful soil fungi，improve the structure and quantity of soil microbial community，mobilize soil phosphorus and potassium，and promote the growth of apple tree and apple fruits yield.

Key words: apple, Bacillus megaterium, yield, quality, soil microorganism, soil nutrients