Analysis of differences in physicochemical properties and bacterial community characteristics of rhizosphere soil in apple orchards under different cover treatments

【Objective】Cover cropping is a widely adopted soil management technique used in fruit tree cultivation. By planting specific vegetation to cover the ground, this practice significantly influences nutrient cycling, microbial activity, and biodiversity within the soil ecosystem. Thus, cover cropping plays a critical role in maintaining soil health and enhancing fruit quality. This study explored the impact of the grass cover (GC) and rice straw mulch (RSM) on the physicochemical properties of rhizospheric soil and the structure of bacterial communities in apple orchards.【Methods】The soil samples were collected from the apple orchards subjected to both GC and RSM treatments. We compared the physico-chemical properties of the soils under each treatment, focusing on key indicators such as alkaline nitrogen (AN), soil moisture (SM), available potassium (AK), soil organic matter (OM), effective phosphorus (AP), pH level, soil bulk density (SBD), and soil porosity (SP). Additionally, enzyme activities were measured, specifically for urease (UA) and alkaline phosphatase (APA). The structure and functionality of the bacterial communities were analyzed using the high-throughput sequencing of 16S rRNA. The core microbiomes and specific biomarkers were identified through LEfSe analysis and random forest methods. Moreover, the redundancy analysis (RDA) and Pearson correlation analysis were performed to explore the relationships between the key microbial abundances and soil physicochemical properties. 【Results】The significant differences were observed in the physicochemical properties of the soil under different treatments. The GC treatment led to an increase in AN levels, while the RSM treatment enhanced SM and AK. However, no significant differences were detected between the two treatments concerning OM, AP, pH, SBD, and SP. Furthermore, the enzyme activities of UA and APA in the soil with GC treatment were significantly higher than those in the soil with RSM treatment (p ＜ 0.05), with increases of 76.81% and 45.24% , respectively. Conversely, there were no significant differences (p ＞ 0.05) in the sucrase activity (SA) and catalase activity (CA) between the two types of mulched soils. This suggested that the distinct mulching treatments would have contrasting effects on the soil enzyme activities, with UA and APA exhibiting greater sensitivity to these treatments. The hierarchical clustering analysis based on unweighted UniFrac distances revealed that inter- group differences among the soil samples from the different treatments were greater than intra-group differences. Following the two treatments, the rhizospheric bacterial communities in the apple orchards comprised 38 phyla, 100 classes, 254 orders, 417 families, and 777 genera. At the phylum level, Proteobacteria was the dominant phylum, accounting for 32.41% in the GC sample and 42.37% in the RSM sample. At the genus level, the dominant taxa in the soil with GC treatment were MND1 (5.41% ), RB41 (5.07% ), Ralstonia (4.62% ), and Raoultella (3.42%). In the soil with RSM treatment, they were MND1 (9.10%), Ralstonia (7.11%), RB41 (2.00%), and Pseudarthrobacter (1.94%). Although the dominant bacterial groups at both the phylum and genus levels were largely similar between the soils treated with GC and RSM, the richness of the rhizospheric bacteria was considerably higher in the RSM sample. Moreover, the alpha diversity indices indicated greater bacterial diversity in the RSM soil, although the difference from the GC sample was not statistically significant. Through LEfSe analysis and random forest methods, we identified five core microbial taxa (RB41, MND1, Ralstonia, Raoultella, and Pseudarthrobacter), as well as five specific biomarkers (Ellin6067, Sphingomonas, Nocardioides, Subgroup 10, and RB41). The RDA and Pearson analyses revealed strong correlations between these microbial abundances and physicochemical soil properties, with AN and AK emerging as the primary factors that would influence the structure of the rhizosphere bacterial communities.【Conclusion】In this study, we discovered significant differences in the physicochemical properties of the rhizospheric soils with GC and RSM treatments in apple orchards. These disparities led to substantial variations in the bacterial community structure. Our findings indicated that GC enhanced the soil alkaline nitrogen content and enzymatic activities, while RSM improved the soil moisture and available potassium levels. The dominant bacterial phyla and genera remained broadly similar in the soils treated with the two treatments, but notable differences in relative abundance were observed, with RSM exhibiting higher bacterial richness than GC. There was a close correlation between the soil physicochemical properties and key bacterial abundances, with AN and AK acting as major influencing factors. Specifically, the bacterial community associated with RSM exhibited a significant positive association with AK levels, while that associated with GC correlated positivelywith AN expression. This research would provide a theoretical basis for optimizing cultivation practices and enhancing soil quality and microbial functionality in apple orchards located in the cool highland regions of Southwestern China.