Effects of film mulching on allelopathic material accumulation and fungal community characteristics in rhizosphere soil of apple in Longdong dryland

Abstract: 【Objective】Long-term film mulching in the Longdong dry plateau area exacerbated the degradation of rhizosphere soil, resulting in a decrease in the content of organic matters in the soil. The community structure and activity of soil microorganisms were closely related to the changes of rhizosphere soil microecological environment, root exudates and soil nutrients. Exploring the effects of different mulching years on the enzyme activity, root exudates and fungal community characteristics of the rhizoosphere soil of apple trees is helpful to lay a theoretical foundation for the improvement and rational implementation of water-saving and water-preserving technology in apple orchards in the dry land of Northwest Loess Plateau.【Methods】This study used 18-year old Nagano Fuji No. 2 apple trees as the experimental object. High performance liquid chromatography (HPLC) and Illumina high- flux sequencing technology were used to investigate the effects of clean tillage (Control, CK), film mulchingfor 2 years (2Y), film mulching for 4 years (4Y) and film mulching for 6 years (6Y) on the root-sphere surface soil (0-20 cm) and subsurface soil (＞20-40) in apple orchard. Meanwhile, soil physical properties, carbon and nitrogen accumulation, enzyme activities related to carbon and nitrogen metabolism, root biomass and growth distribution characteristics of different soil layers were monitored.【Results】 The results showed that the urease activity of apple rhizosphere soil decreased with the increase of mulching years, while the enzyme activity of soil sucrase existed the opposite trend. Short-term mulching (2Y and 4Y) treatments effectively improved the activities of soil catalase, sucrase, alkaline phosphatase, β-1, 4-xylosidase, β-1, 4-glucosidase and cellobiohydrolase. Compared with CK, the enzyme activities increased by 19.67%, 18.46%, 29.39% and 84.4%, 89.01% and 83.45%, respectively, which contributed to the increase of soil organic carbon, microbial biomass carbon and total nitrogen, which were significantly increased by 15.00%, 8.29%, 50.16% and 31.44%, respectively by 4Y treatment relative to CK. Favorable rhizosphere environment was conducive to the growth and development of apple roots. The growth of apple roots under 4Y treatment was better than under the other mulch treatments, and the root biomass, root surface area and root volume in 4Y were 54.62 g, 10 921.66 cm2 and 220.56 cm3 , respectively. In the surface soil, soil ventilation decreased with the increase of mulching years. Compared with CK, 2Y, 4Y and 6Y treatments had a soil ventilation decreased by 3.23%, 24.22% and 18.51%, respectively. Therefore, the root biomass and root surface area of CK, which were 20.63 g and 4 956.95 cm2 , respectively, were obviously higher than those of the mulching treatments. The contents of main allelochemicals in soil (ferulic acid, benzoic acid, phloroside and vanillin) revealed a decreasing trend with the increase of film mulching years, but p- hydroxybenzoic acid did not appear in CK and 2Y treatment, but increased with mulching years. The contents of p-hydroxybenzoic acid and cinnamic acid in apple rhizosphere soil under 6Y treatment reached the highest level, while coumaric acid reached the highest content at 4Y. Coumaric acid might be the product accumulated under the specific ecological environment of long-term mulch. The dominant fungal groups in the rhizosphere soil of apple with different mulching years were Basidiomycota (5%-19%), Mortierellomycota (5%-22%), Aphelidiomycota (22%-44%) and Aphelidiomycota (1%-7%), with the dominant fungal genera being Mortierella (9%-39%), Solicoccozyma (9%-33%), Petriella (1%-16%) and Fusarium (1%-3%). The characteristics of fungal communities in different soil layers were distinctive. In the surface soil, Chao1 index and Shannon index of the fungal community increased with the increase of mulching years, while in the subsurface soil, Chao1 index of the fungal community decreased with the increase of mulching years, while the Shannon index appeared an opposite trend. With the increase of mulching years, the abundance of Fusarium in rhizosphere soil increased significantly. Compared with CK, the contents of soil water content, organic carbon, microbial biomass carbon and total nitrogen in rhizosphere soil under 4Y treatment were significantly improved by 5.65%-15.00%, 3.72%-8.29%, 50.16%-64.31% and 14.19%-31.44%, respectively. The enzyme activities of β-1, 4-xylosidase, β-1, 4-glucosidase and cellobiohydrolase were significantly increased by 78.71%-84.4%, 89.01%-97.43% and 83.45%-87.68%, respectively, compared with CK. Relative to CK, apple root biomass, root surface area and root volume increased by 489.84%, 533.87% and 542.39%, respectively. The activities of soil urease and β-1, 4-glucosidase, and vanillin content in root exudates were the main environmental factors affecting soil fungal community in rhizosphere ecological environment.【Conclusion】In conclusion, mulching measures can improve the enzyme activity and microbial metabolic activity of apple rhizosphere soil to a certain extent, and promote the carbon cycling process in the soil. The increase of mulching years reduces the accumulation of allelochemicals in the soil, but too long mulching may have adverse effects on the soilfungal community structure and promote the accumulation of Fusarium, which is harmful to the healthy development of rhizosphere soil and roots. Therefore, 4 years of film mulching is the appropriate continuous mulching years for apple orchards in the northwest dry land.