Research progress on the influence of orchard green manure on fruit tree- soilmicrobe system

Abstract: Fruit tree planting area and total output in our country are at top rank in the world. By 2019, orchard area in China had reached 122.766 8 million hm2. In order to maintain high yield of fruit trees, large quantities of chemical fertilizers are applied. Long-term use of chemical fertilizers causes soil compaction, soil quality loss, tree growth suppression, fruit quality reduction, and serious pests and dis- eases. Green manure coverage is an important measure to improve soil quality, reduce soil erosion, in- crease soil nutrient content, and improve soil microbial community in orchard. China has begun to im- plement organic fertilizers in orchards and tea gardens to partly replace chemical fertilizers. Intercrop- ping with green manure affects the soil properties, photosynthesis of fruit trees, and root growth and distribution, and increases the activities of stress resistance related enzymes in fruit trees, which results in promotion of fruit tree growth with increased tree height, crown size, water use efficiency and leaf nutri- ent content. However, green manure has both positive and negative effects on the yield and quality of fruit trees. In actual production, the characteristics of fruit tree at different stages, green manure, and regional climate should be taken into consideration for application of green manure. In order to prevent  green manure plants from competing with fruit trees for water and nutrients in the soil, the overlap of the growth period of green manure with the period of flowering and fruit setting of fruit trees and other periods of high nutrient and water demand should be avoided. Intercropping with green manure changes the content of antagonistic bacteria against pathogens in the soil, increases the diversity and abundance of other beneficial microorganisms in the soil, and competes with pathogens for resources such as space and nutrients. The compounds produced by the decomposition of green manure inhibit the growth of pathogens in the soil, and thereby promote the growth of fruit trees, improve the overall health and resis- tance of fruit trees, and reduce the occurrence of fruit tree diseases. However, the use of green manure is mostly for covering and overwintering. Plant residues provide nutrients and a living environment for pathogens to survive and overwinter, and thus increase the accumulation of pathogens in the orchard and the risk of diseases. Green manure improves soil aggregate structure, mechanical stability, porosity and erosion resistance, reduces soil bulk density, and changes soil water content, pH, and nutrient content. The growth conditions of fruit trees are improved through these effects. The current research on the influence of green manure on the composition of the microbial community in orchard soil mainly fo- cuses on the effect on growth-promoting bacteria or symbiotic bacteria, bacteria related to nutrient de- composition and cycling, and soil- borne pathogens. Intercropping with green manure increases the abundance of growth-promoting bacteria or symbiotic microbe, and bacteria related to nutrient decom- position and cycling in orchard soil, which is beneficial to increase the ability of plants to absorb nutri- ents, promote plant growth, improve plant resistance, and accelerate the decomposition of green manure crop residue. Soil microbe promotes nutrient circulation in the orchard and improves the productivity of the orchard. The content of pathogen in the soil is closely related to the health of plants. Intercropping with green manure in orchards usually reduces the infection of plant by soil-born pathogens. So far, scholars at home and abroad have carried out series of studies of the effect of green manure on fruit tree growth and fruit products, fruit tree diseases, soil nutrients and physical and chemical properties. Some new technologies, such as high-throughput technologies, have been used in the study of soil microbial community structure, soil beneficial microorganisms and soil pathogen. Intercropping with green manure reduces diurnal temperature differences, increases air humidity, improves the growth environment of fruit trees, which helps to form a microclimate with the characteristics of the fruit tree-green manure intercropping system and directly affects the growth and yield of fruit trees. These studies have effectively promoted scientific research and application of green manure in orchards. At present, application of green manure in orchards in China is still at a small scale. There are differences in suitable green manure varieties and planting patterns in different regions, and there is still a lack of supporting production and utilization technologies. It is important to clarify the effects of green manure intercropping in orchards on fruit tree and to explore the interaction mechanism between fruit trees and green manure. This is of great significance for making full use of green manure to increase orchard soil nutrients and organic matter, optimize the structure of soil microbial community, reduce diseases, promote fruit tree growth and improve the quality of fruit products. It is also conducive to the development and promotion of planting technologies for different regions, and provide reference for promoting the green ecological development of high-quality orchards in the future.