Effects of seaweed fertilizer on the Malus hupehensis Rehd. seedlings growth and soil microbial numbers under continue cropping

We investigated the effects of seaweed fertilizer on the physiological features of apple seedlings under continues cropping conditions. The six-leaf Malus hupehensis Rehd. seedlings were planted in replant soil with seaweed fertilizer applied at three rates: 5, 20, and 40 g kg− 1, and one control group with no seaweed fertilizer application. The results showed that the addition of seaweed fertilizer obviously increased the seedling plants growth. The rate of photosynthesis, chlorophyll (a + b) content, and activities of protective enzymes (superoxide dismutase, ascorbate peroxidase, and catalase), were 78%, 20%, 23%, 45% and 144% higher, respectively, in the seedlings treated with 40 g kg− 1 of seaweed fertilizer than in the control group. Furthermore, plants in the 40 g kg− 1 seaweed fertilizer treatment group had H2O2, O2−, malondialdehyde and proline contents that were 32%, 60%, 56% and 38% lower, respectively, than in the control group. Finally, the number of bacteria, fungi, and the ratio of bacteria:fungi in the replant soil with 40 g kg− 1 of seaweed fertilizer applied were 172%, 67% and 150% higher, respectively, than in the soil without seaweed fertilizer application. Overall, seaweed fertilizer application promoted plant growth, increased the rate of photosynthesis, activated antioxidant enzymes, decreased lipid peroxidation, improved the soil nutrient, and regulated the soil microorganisms. As a result of these changes, the treated seedlings did not exhibit the symptoms of apple replant disease.