Management Impacts on Soil Microbial Community Dynamics in Virginia Apple Orchards

Introduction

Soil health is fundamental to the long-term sustainability and productivity of perennial fruit production systems. Unlike annual cropping systems, apple orchards (Malus domestica) represent a multi-decade investment in which the soil's biological infrastructure directly influences tree longevity, nutrient uptake, and overall fruit quality (Montanaro et al., 2017; Crews et al., 2018). For Virginia growers, maintaining this ecosystem is not merely an environmental goal but a critical management strategy for ensuring orchard resilience across multiple growing seasons.

The functionality of this foundational ecosystem is primarily dictated by the soil microbiome—a complex community of bacteria and fungi that mediate essential biochemical processes (Pandey & Saharan, 2025). These microorganisms are active participants in nutrient cycling, particularly in the mineralization of nitrogen and the mobilization of phosphorus, making these elements available for root uptake. Furthermore, a diverse microbial community serves as a biological defense mechanism, suppressing soil-borne pathogens and improving soil structural stability, which enhances water infiltration and retention. However, the assembly and stability of these microbial assemblages are continuously shaped by environmental stressors—such as drought and salinity—as well as anthropogenic interventions, including the overarching farming system and management intensity.

Beyond environmental factors, the tree’s genetic composition plays a pivotal role in shaping the rhizosphere microbiome. The interaction between the scion (above-ground cultivar) and the rootstock (below- ground system) determines the quantity and quality of root exudates—organic compounds like sugars and organic acids secreted into the soil (Chai et al., 2022).

These exudates serve as specific signaling molecules and carbon sources that selectively recruit and sustain unique microbial populations. Research has demonstrated that different scion-rootstock combinations can significantly alter the biological environment of the root zone, thereby influencing nutrient availability and the tree’s inherent response to both biotic and abiotic stressors.

While the genetic template established by the host tree provides a localized biological signature, the broader management regimes serve as sustained external drivers of microbial structure and diversity. Conventional management regimes typically prioritize the use of synthetic fertilizers, pesticides, and targeted herbicides to maximize immediate yields and manage competition. In contrast, organic management systems rely on organic amendments, such as compost and mulch, to build the soil’s organic matter and stimulate biological activity. Research indicates that these contrasting management styles selectively recruit different microbial taxa, often shifting the ratio of bacterial to fungal dominance (Banerjee et al., 2022).

Understanding these shifts is essential for apple growers to predict how specific inputs or cultural practices will impact the long-term biological health of the rhizosphere.

This study specifically focuses on the interaction between farming systems and scion genotypes to identify how biological health in apple orchards is modulated by both human intervention and plant genetics. Identifying the specific microbial groups and functional shifts triggered by these variables allows for a more nuanced approach to orchard maintenance.

Experimental Methods

Experiments were conducted at paired orchard sites in Winchester, Virginia, utilizing a certified organic site (39.115780, -78.284624) and a nearby conventional site (39.146230, -78.274400) with long-term, distinct management histories. The study employed a factorial design to evaluate two primary factors: management system (organic vs. conventional) and scion genotype, specifically comparing Liberty and Enterprise apple cultivars grafted onto standardized MM.111 rootstock. All trees were six years old and selected for uniform vigor and health status to ensure that observed microbial differences were attributable to management and scion genetics rather than to tree age or health variability.

Rhizosphere samples were collected during the 2025 growing season at the fruit-set stage, specifically when fruit diameter reached 20–25 mm, representing a period of high physiological demand. Sampling was conducted at three equidistant positions around the drip line of each replicate tree, approximately 4.9ft from the trunk, where fine feeder roots were excavated to a depth of 10 cm.

The rhizosphere was isolated by shaking feeder roots to dislodge bulk soil, followed by careful collection of adhered soil using sterile instruments. To ensure biological representativeness and prevent contamination, all sampling tools underwent a rigorous multi-stage sterilization protocol involving sodium hypochlorite, ethanol, and deionized water rinses between biological replicates.

Total genomic DNA was extracted from homogenized rhizosphere samples. The microbial communities were characterized through high-throughput sequencing of the 16S rRNA gene for bacteria and the Internal Transcribed Spacer (ITS) region for fungi. Statistical analysis focused on both alpha and beta diversity to identify shifts in community richness and composition across the management regimes and scion genotypes.

For core hypothesis testing, a balanced dataset was utilized to isolate the effects of organic versus conventional systems and their interaction with the Liberty and Enterprise genotypes (for more information, please visit our recent publication at Microbial Ecology (https://doi.org/10.1007/s00248-026-02774-7).

Results and Discussion

Management systems and microbial diversity

The analysis of microbial community metrics revealed that long-term management intensity is a primary determinant of the biological infrastructure in Virginia orchard soils. As shown in Table 1, organic management practices resulted in a significant expansion of the soil microbial communities compared to conventional regimes. Total microbial richness, measured by the number of unique Amplicon Sequence Variants (ASVs), was almost 21% higher in organic blocks (2574) than in conventional blocks (2042). This increase suggests that the consistent employment of organic farming strategies provides a broader spectrum of microbial taxa than the targeted synthetic inputs used in conventional systems.

Community complexity and stability were further assessed using the Shannon diversity index, which accounts for both the number of species present and the evenness of their distribution. Bacterial diversity was significantly higher under organic management (5.15) compared to conventional management (4.75; Table 1). Even more pronounced shifts were observed in the fungal communities, where diversity increased from 3.6 in conventional blocks to 4.15 in organic blocks (Table 1). These shifts indicate that organic practices foster a more balanced and complex microbial neighborhood. In perennial systems such as apple orchards, higher diversity is often associated with increased functional redundancy, meaning the soil ecosystem is better equipped to maintain essential processes, such as nutrient mineralization, even under environmental stressors (Delgado-Baquerizo et al., 2023; Yu et al., 2024).

The relationship between the fungal and bacterial components of the soil—expressed here as the Diversity Balance (F: B Ratio)—also shifted significantly according to management style. The organic system maintained a higher balance (0.81) compared to the conventional system (0.76, Table 1). In perennial ecosystems, a shift toward higher fungal complexity relative to bacteria is a key indicator of soil maturation and stable organic matter cycling (Siles et al., 2023).

This improved balance in organic orchards suggests a rhizosphere environment that favors slow-release nutrient cycling and enhanced soil structural stability.

On the other hand, beyond broad diversity metrics, the functional composition of the soil microbiome revealed a robust presence of beneficial microbial groups across both management regimes. The relative abundance of beneficial guilds—which include organisms specialized for biocontrol, nitrogen fixation, and phosphate solubilization—was maintained at 25.3 ± 2.4% in conventional soil and 23.0 ± 1.5% in organic soil (Table 1). While the conventional system demonstrates a slightly higher cumulative abundance, this numerical footprint is driven by a more restricted taxonomic range. As discussed in later sections, this suggests that conventional management favors a high-output specialization strategy. In contrast, organic management builds functional capacity through a broader, more redundant assemblage of microbial partners. By maintaining these high levels of beneficial recruitment, both systems leverage the soil’s inherent biological potential through fundamentally different ecological configurations.

^1. Fungal: Bacterial diversity balance is calculated as the ratio of the fungal Shannon index to the bacterial Shannon index.

^2. Beneficial Guilds include microbial groups identified for biocontrol, nitrogen fixation, and phosphate solubilization based on curated literature databases.

The Scion effect on rhizosphere recruitment

A critical finding of this research is that the apple scion itself acts as a selective filter for soil life, creating a unique biological signature in the root zone. Our analysis of the beneficial workforce reveals that even when grown under identical management practices and similar rootstock, Liberty and Enterprise cultivars recruit different microbial partners (Fig. 1). This recruitment is driven by scion-specific root exudates— chemical signals like sugars and acids that invite specific microbes to the rhizosphere (Chai et al., 2025). Our data show that Enterprise has a strong affinity for nitrogen-fixing bacteria, with significantly higher recruitment of Rhizobium, Sinorhizobium, and Mesorhizobium than Liberty (Fig. 1).

Additionally, Enterprise fostered a higher abundance of the specialized biocontrol fungus Metarhizium and Clonostachys (Fig. 1). In contrast, the Liberty cultivar demonstrated a superior capacity for recruiting Talaromyces and Penicillium, which are vital for nutrient mineralization (Chandra et al., 2025).

What this shows is that cultivar selection is not just about fruit quality or harvest timing; it is also a decision about which biological services we want to prioritize in our soil. For a grower, this is important because it allows them to match their variety to specific orchard challenges. If the site has a history of soil-borne pests, choosing a variety like Enterprise may provide a natural boost in disease suppression. On the other hand, if they are looking to maximize the availability of existing soil minerals or improve water uptake, a variety like Liberty offers a biological advantage in nutrient efficiency.

Understanding these signatures allows growers to leverage the specific workforce each variety recruits to ensure long-term orchard productivity.

Functional roles: Biocontrol and nutrient cycling

Beyond identifying microbes in the soil, this study also investigated what these microbes are doing. By categorizing the community into functional guilds, we compared the capacity of organic and conventional systems to provide essential ecosystem services: Biocontrol, Nitrogen Fixation, and Phosphate Solubilization (Figs. 2 and 3).

A critical observation from our data is the difference in stability and specialization between the two systems.

While conventional management occasionally shows high spikes in beneficial groups, these populations exhibit high variability (large error bars, Fig. 2). In soil science, this suggests an unpredictable workforce that may perform well in one season but fail during environmental stress (Griffiths & Philippot, 2013)

In contrast, organic management fosters a significantly more stable and specialized microbial community (Fig. 3). The organic system demonstrates a clear advantage in Phosphate Solubilization, where fungal partners consistently exhibit activity (Fig. 2 and 3). This organic advantage is driven by the recruitment of a functional elite—a group of microbes that are not just present in high numbers but are specialized to thrive in a low-input environment. By fostering these specialized fungal networks, organic systems create a soil environment that is more efficient at unlocking tied-up minerals and maintaining a natural shield against pathogens.

Strategic soil management: Dominance vs. Redundancy

To provide a more comprehensive examination of the observed patterns, this section explores the ecological architecture underlying these functional groups, elucidating the specific structural shifts in microbial community organization (Fig. 3). Our data indicate that while the essential services—such as nitrogen fixation and biocontrol—are maintained across all management types, the strategy used to achieve these results differs significantly between conventional and organic systems. As illustrated in Figure 3, the conventional management system follows a specialization strategy. In functional categories such as Biocontrol and Nitrogen Fixation, the Workforce Size (Abundance percentage, Fig. 3) is often high, but this output is driven by a significantly lower Workforce Diversity (Number of Genera, Fig. 3). In this system, management selects for a few, highly specialized genera—such as Streptomyces and Rhizobium (data not shown, for more information please visit our recent publication at Microbial Ecology (Zarrabian and Sherif, 2026))—that dominate the rhizosphere. This architecture is built for efficiency. It creates a lean, high-performing team that can respond powerfully to specific inputs. However, because the system relies on so few types of microbes, it lacks biological backup, making the functional output more sensitive to environmental changes or management shifts (Wagg et al., 2019).

Conversely, the organic management system utilizes a diversity-driven redundancy strategy. As illustrated in Figure 3, while the relative abundance of specific functional guilds remains comparable between systems, the taxonomic richness—represented by the number of genera (Fig. 3, bottom panels)—is almost higher in organic soils across every functional guild. This provides mechanistic evidence of functional redundancy. In organic soils, essential ecosystem functions are not centralized within one or two dominant taxa but are distributed across a diverse multi-taxa assemblage of various genera working in concert (Griffiths & Philippot, 2013). This architecture provides a robust ecological insurance effect: when specific genera are suppressed by environmental stressors, the presence of alternative genera helps maintain critical ecosystem processes (Wagg et al., 2019). This strategy is built for resilience, ensuring that nutrient cycling and pathogen defense continue predictably throughout the season (Delgado-Baquerizo et al., 2023; Yu et al., 2024).

Conclusion

This study demonstrates that both conventional and organic management systems possess the native biological potential to support a healthy orchard, though they utilize fundamentally different ecological configurations.

For systems utilizing conventional management, the soil microbiome functions as a specialized workforce of dominant, highly abundant taxa. In these scenarios, maintaining high-output efficiency relies on protecting these keystone microbial groups. Conversely, organic management systems leverage a redundant microbial workforce, where high taxonomic diversity serves as a mechanism for ecological resilience. This study indicates that the stability and self-sufficiency of organic soils are directly supported by the wide diversity of microbial partners, probably fostered through the maintenance of soil organic matter.

Ultimately, the soil’s biological potential remains a powerful asset regardless of the overarching management philosophy. By understanding these distinct strategies and pairing them with the specific recruitment power of the chosen scion—such as the nutrient-mineralizing signature of Liberty or the biocontrol-intensive signature of Enterprise—the integration of management and variety selection ensures the long-term resilience and profitability of the orchard ecosystem.

Acknowledgements

The authors wish to express their gratitude to Ms. Diane Kearns for providing valuable insights and access to her organic and conventional orchards for this study.

Sincere thanks are also extended to Mr. Gerald Michaels III and Ms. Katherine Furcho for their assistance in collecting the rhizosphere soil and extracting microbiome DNA.

References