Effects of different intercropping patterns on soil physicochemical properties and carbon-nitrogen cycling in apple orchards in the Longdong arid plateau

【Objective】This study explores the mechanisms and optimization strategies for improving soil quality under different intercropping patterns in apple orchards located on the arid Loess Plateau of eastern Gansu, a region characterized by fragile ecological environments and limited natural resource availability. The sustainability of orchard systems in such regions depends heavily on soil health, which is influenced by physical, chemical, and biological processes. Recognizing the urgent need for effective and ecologically sound agricultural practices, this research focused on how various intercropping systems can restore and enhance soil functions in apple orchards for a long time.【Methods】Utilizing a comprehensive assessment framework, the study integrated data on soil physicochemical properties, carbon and nitrogen (C-N) cycling, and microbial enzyme activities to provide a holistic understanding ofsoil system responses. Three representative intercropping treatments—natural grass cover, onion intercropping, and rapeseed intercropping—were implemented in comparison with traditional clean tillage, which involved long-term maintenance of bare soil devoid of vegetative cover. Field experiments were conducted over three consecutive apple growing seasons in a representative orchard on the eastern Loess Plateau, where climatic conditions were semi- arid, precipitation was low and uneven, and soil degradation due to prolonged cultivation became a widespread concern. Each treatment was monitored continuously, and a multi-index evaluation approach was adopted to track the effects of the different systems on soil quality parameters.【Results】All three intercropping patterns exerted beneficial effects on soil physical structure, albeit through different mechanisms. The natural grass cover system (C) gradually improved soil structure through a combination of aboveground canopy shading and the formation of a dense and fibrous root network in the subsurface soil. These roots contributed to aggregate formation and stabilization, which were crucial for enhancing soil porosity and resistance to erosion. Quantitative data revealed that the sand content in the 0-20 cm and 20-40 cm soil layers decreased by 21.10% and 10.40%, respectively, under this treatment, suggesting a shift towards finer, more cohesive soil particles. More importantly, this shift led to improved water- holding capacity and nutrient retention in the root zone. During periods of drought, water content in the 20- 40 cm subsurface soil was 25%- 30% higher than that in clean tillage plots (CK), with a relative increase in water retention capacity of 77.15%. These improvements were critical for supporting the physiological needs of apple trees during water-limited periods, offering a potential strategy for mitigating drought stress in orchard systems. In contrast, scallion (A) and rapeseed (B) intercropping primarily improved soil structure through biological drilling effects created by their deep and vigorous taproots. These roots penetrated the compacted layers, forming natural channels that facilitate water infiltration and gas exchange. Consequently, these systems were associated with a reduction in soil bulk density by 7.97% to 8.70% and an increase in total soil porosity by 8.66% to 9.74%, resulting in significantly better aeration and drainage. Such changes created a more favorable environment for root respiration and microbial colonization, supporting overall soil ecosystem functioning. In terms of nutrient dynamics, particularly the carbon and nitrogen cycles, scallion intercropping showed marked enhancements in the accumulation and transformation of organic matter. Compared to the clean tillage control, the total organic carbon (TOC), soil organic matter (SOM), and microbial biomass carbon (MBC) under onion intercropping increased by 16.73% , 16.78%, and 21.58%, respectively. The soluble organic nitrogen (SON) content also rose significantly by 16.00% , indicating greater nitrogen availability for plant uptake and microbial utilization. These changes not only reflected improvements in the base fertility of the soil but also suggested that onion roots may influence rhizosphere processes that accelerated organic matter mineralization and nutrient release. Microbial activity, another key indicator of soil health, was significantly stimulated under natural grass cover, particularly in relation to enzymes involved in the decomposition of plant litter and the cycling of carbon. Enzyme assays revealed dramatic increases in the activities of β- 1, 4- glucosidase (365.78%), β-1, 4-N-acetylglucosaminidase (115.38%), and β-1, 4-xylosidase (145.30%) relative to the clean tillage treatment. These enzymes are essential for breaking down complex carbohydrates such as cellulose and chitin, releasing simpler carbon compounds that are readily used by soil microbes. The enhancement of enzymatic activity under natural grass indicated a more dynamic and responsive soil microbial community, which accelerated the turnover of organic matter and facilitated the formation of a resilient“plant- soil-microbe”feedback loop. Further correlation analysis confirmed that soil physicochemical traits—such as bulk density, porosity, moisture content, and particle composition—had signifi-cant impacts on the distribution, transformation, and stability of carbon and nitrogen fractions. Notably, the response mechanisms differed between surface (0-20 cm) and subsurface (20-40 cm) soil layers, indicating vertical heterogeneity in how intercropping influenced soil function. This spatial variability suggested that intercropping strategies should be tailored to address specific constraints within soil profiles. For example, in orchards where the surface compaction is severe and air–water balance is disrupted, onion or rapeseed intercropping may be more effective due to their root system architecture. Conversely, in drought-prone regions with poor water retention, natural grass cover offers a low-input and ecologically aligned solution for maintaining stable moisture conditions throughout the growing season. Additionally, in terms of improving carbon and nitrogen transformation efficiency, onion intercropping contributes by increasing the soil’s capacity to generate labile organic fractions, thereby building longterm fertility. Meanwhile, natural grass promotes faster organic matter decomposition through enzyme activation, reinforcing the carbon cycling loop and ensuring a continuous supply of microbial energy substrates. While each strategy offers unique benefits, both contribute meaningfully to enhancing soil fertility and ecological stability.【Conclusion】In conclusion, this study demonstrates that different intercropping systems exert distinct, yet complementary, effects on soil physical properties, nutrient cycling, and biological activity in apple orchards under arid conditions. By elucidating the ecological mechanisms underlying these effects, the research provides a scientific basis for designing targeted soil management strategies tailored to specific environmental constraints and production goals. These findings offer valuable insights for the development of sustainable orchard practices in water-limited regions and contribute to broader goals of ecological restoration and agricultural sustainability.