Chapter 1 Introduction
1.1 Research Background
Wild orchids are a rare group of plants with high ecological value and are in danger of extinction worldwide. Due to their narrow range of distribution and specific life histories, wild orchid populations are among the important protected wild plants in the world. Due to the high requirements of the microenvironment, the habitat is very easy to be damaged and fragmented; therefore, it is urgent to learn about the ecology of these species to survive in weak ecosystems. The rhizosphere soil microenvironment is one of the main factors affecting the growth of these plants, and it is also an important area for many belowground biological and biochemical interactions. This microenvironment forms the foundation for the development of orchids; that is, it provides a place for seed germination through necessary symbiosis, and it can efficiently obtain nutrients from a small area of soil and maintain the two types of reproduction, asexual and sexual. The structure and function of the rhizosphere microenvironment are closely related to the stability of the population of wild orchids. In recent years, with changes in the paradigm of soil ecology, the research on rhizosphere microflora has been transformed from a reductionist approach that focuses on one single type of microbe to one that looks at the whole. A new theory has been developed to explain that synergistic enzymatic cascades, cross-feeding metabolic networks and quorum-sensing regulatory circuits all serve to maintain soil and nutrients in the rhizosphere of plants. Based on the above reasons, we will study how the cooperation and competition of microflora affect the stability of fragile orchid habitats and try to solve this problem. To know the overall functional result of the rhizosphere microbiome better than just looking at the physiology of isolated strains, we can better predict the resilience of the environment and formulate reasonable conservation plans for endangered plants.
1.2 Research Objectives and Significance
In order to systematically unravel the complicated mechanism of the rhizosphere ecosystems of wild orchids, this paper proposes a two-level framework of research objectives. First, to characterise the structural differences in the microbial community of the rhizosphere and non-rhizosphere of wild orchids, establish a taxonomic and quantitative foundation for identifying specialized microbial assemblages that are recruited by orchid roots; Second, to study the interaction patterns of various types of rhizosphere microorganisms and how they influence the cycles of carbon, nitrogen, phosphorus, and potassium in the soil. Based on the functions of the keystone microorganisms, we will study how the metabolites of the microorganisms work together to promote the mineralisation and bioavailability of the necessary macronutrients. The third goal is to analyse how the microflora network's stability is affected by a small-scale disturbance in the habitat and the continuity of the nutrient cycle. Assess the topological robustness of the rhizosphere co-occurrence network and its resistance to changes in biogeochemical fluxes due to environmental fluctuations.
Theoretical significance of this research is to increase the knowledge of the theory of orchid rhizosphere ecology. In this paper, through multidimensional studies of community assembly, inter-species relationships, and nutrient mobilisation, it is proposed to go beyond the description of microbiology to provide a mechanism for the symbiosis of mycorrhizal fungi and saprophytic fungi in some plant-soil systems. Also, by quantifying the stability thresholds of microbial networks, we can advance the theory of belowground biodiversity and ecosystem multifunctionality. In terms of application, the results of this study will help provide a basis for the development of targeted protection measures for the in-situ wild orchid populations. Based on the study of indicator microorganisms and the problem of microflora-nutrient coupling, it can be transformed from general habitat conservation to targeted ecological engineering. By inoculating with beneficial microorganisms or improving the soil environment, we can activate and maintain the degraded rhizosphere, thus ensuring the survival and reproduction of wild orchid populations in their original habitat.
1.3 Research Scope and Technical Route
The research scope of this paper is to study the rhizosphere microflora and nutrient cycle of wild orchids in subtropical forest ecosystems. Select two representative ecologically significant species, Cymbidium goeringii and Cymbidium sinense, to study their mycorrhizal dependence and nutrient absorption patterns in their native environment. In terms of space, the sampling range is only in the 0-20cm soil layer, which is the area where orchid roots are mainly distributed, and mycorrhizal colonization and active nutrient movement occur here. There are three types of objects for analysis: soil physicochemical properties, the structure of microbial communities, and changes in nutrient cycles. Through this all-encompassing coverage, we have been able to systematically analyse the intricate relationship between abiotic soil properties and biotic microbial agents.
To ensure the reliability and reproducibility of the methods, a logical sequence of operations will be adopted. First, standardised field sample collection is carried out to obtain paired rhizosphere and bulk soil samples, avoiding the influence of environmental factors. Then, the experimental detection protocol is used to determine the soil physicochemical indexes and measure the main enzymatic activities involved in the carbon, nitrogen and phosphorus cycles. Simultaneously, high-throughput sequencing data processing is conducted, targeting both bacterial 16S rRNA and fungal ITS gene amplicons. This bioinformatic pipeline consists of steps such as the raw read quality filtering, chimera removal, amplicon sequence variant deduction and taxonomic classification to obtain the architecture of the rhizosphere microbiome. After multiple pieces of data are obtained, statistical modelling is used to study these complex interactions. Multivariate analysis, such as redundancy analysis and structural equation modelling, is used to explore the causal links between changes in microbial communities and changes in soil physicochemical properties and nutrient transformation rates. Finally, cross-validation of the sequencing output with empirical enzyme and chemical assays is performed to verify the results and confirm the mechanism of microbial-assisted nutrient uptake in orchids. The whole process is to provide a more reliable and complete explanation of the rhizosphere microecosystem.
1.4 Structural Arrangement of the Paper
The structure of the paper is as follows: after briefly explaining the theory of wild orchid rhizosphere soil microflora and nutrient cycling, it will be empirically verified and explained in detail. The structure will be well-organised with good logic and will cover the literature review, experimental verification, mechanism analysis and conclusion.
The first part will introduce the topic of the study, based on a review of previous research on orchid mycorrhizal associations and rhizosphere microecology. It will also point out the lack of single-factor correlation analysis in previous research and state the research objectives.
The second part is about the methodology chapter, which will provide detailed information on the design of field sampling and metagenomic sequencing experiments and establish standardised procedures to investigate the structural characteristics of microbial communities and changes in physicochemical nutrient parameters in the rhizosphere.
The third section is about the results, and it will be based on the data on microbial community structure and soil nutrient levels that have been obtained from experiments, and various statistical methods will be used to show the distribution patterns of these two aspects.
In addition, the subsequent section will present data on microbial community structure and soil nutrient pools, and then a multivariate statistical method will be used to study their distribution.
The first part of the discussion will be more than just descriptive statistics; it will also be used to delve deeper into the mechanism. At this time, the internal logic of the thesis is no longer in a state of disorganisation; it is gradually being clarified. That is to say, we can see how some particular microorganisms, especially mycorrhizal fungi and associated bacteria, control the carbon, nitrogen and phosphorus cycles through their influence on biogeochemical cycles. The main innovation of this paper is that, in contrast to previous research that only considered single-factor correlation, it uses an integrated interaction framework to study how the microflora's coexistence and competition affect nutrient mobilisation and orchid adaptation.
In the last part, according to the conclusions drawn from the above analysis and experiments, we will discuss some of the causes and their influence on the wild orchid and its special microflora, and raise some problems in the research method and future directions. The structure of the paper is still in good order, and all parts work together to achieve a full and detailed explanation of how rhizosphere microflora interact with and influence nutrient cycling.
Chapter 2 Theoretical Foundation and Literature Review
2.1 Core Concept Definition
2.1.1 Wild orchid rhizosphere microhabitat
The wild orchid rhizosphere microhabitat is defined as a narrow soil region that is within 1-3 millimeters of the root surface and is in close proximity to the physiological and biochemical activity of the root epidermis of orchids. Due to the limitation of space, it is also divided from the bulk soil of non-rhizosphere, and the microbial community and physicochemical properties there are not significantly affected by root metabolism. Moreover, the rhizosphere microhabitat of wild orchids is different from that of general non-mycorrhizal plants; it does not contain many simple carbohydrates and amino acids released by root exudates, and rather has a special functional structure due to its own evolutionary history.
The lack of nutrients in this microhabitat is one of its characteristics. Wild orchids often grow in environmentally degraded areas such as humid forests, and the main components of the soil are humus and other organic matter that are hard to be directly absorbed by plants. Therefore, the Orchid Rhizosphere is in a state of severe nutrient deficiency, which affects the biogeochemical processes there.
In order to alleviate this shortage, there is a special background root secretion in the microhabitat. Orchid roots secrete a variety of secondary metabolites, such as specific phenolic compounds and organic acids, which are not only used as carbon sources but also act as signalling substances that regulate the composition of surrounding microorganisms. The exudate profile is a very selective environment filter; it is different from that of ordinary non-mycorrhizal plants.
Above all, the rhizosphere environment of wild orchids has a strong relationship with symbiotic bacteria, especially mycorrhizal fungi. Because there is a lack of bioavailable nutrients, and the type of exudates of orchids is specific, the metabolic functions in the 1-3mm area are dependent on fungal enzymes to break down various organic substances in the soil and promote the transport of minerals. Due to the need for symbiosis, the microhabitat is not just a simple contact area between plants and soil but has become a highly integrated part of the ecosystem. Based on the above limitations, we will narrow down the scope; that is to say, we will create a set of constraints for structure, nutrition and biology.
2.1.2 Rhizosphere soil microflora interaction network
Rhizosphere soil microflora interaction network refers to a symbiotic, competitive, and synergistic relationship system among bacteria, fungi, and other microorganisms in the rhizosphere microhabitat due to material exchange and signalBE signal transmission. Moving(07) transcends the simple description of microbial species composition used in early related studies, and this network paradigm no longer views the rhizosphere as a static list of separate taxa but rather as a dynamic network. The operation paths of the system are maintained by a series of intricate biochemical signalling cascades and cross-feeding mechanisms, with root exudates acting as the first carbon sources for coordinating metabolic dependencies. Quorum sensing and the production of secondary metabolites can work together; for example, they can provide nutrients to each other cooperatively and antagonistically, such as antibiosis and niche exclusion. To quantify the above complex topologies, modern microbial ecology has adopted the following basic evaluation indicators: the node degree of a certain taxon in the network, which is the number of edges connected to it; modularity, indicating how much the network is divided into functionally separate microbial groups that respond to different rhizosphere stimuli; and keystone taxa abundance, which is the number of highly connected microorganisms that largely maintain the integrity of the structure and function of the network. The practical significance of building a wild orchid nutrient cycling interaction network is very significant. Orchids are heavily dependent on mycorrhizal symbiosis to obtain carbon and phosphorus at the beginning of life, and this dependence cannot be explained simply by the richness of species. Based on the model of the rhizosphere as an interaction network, it is hoped that the mediating effect of keystone taxa on the element turnover process and changes in network characteristics on the host's fitness under environmental changes can be investigated. This conceptual development has realised the mechanical understanding of rhizosphere change and offered the necessary theoretical foundation to analyse the role of microflora in orchid nutrition.
2.1.3 Coupling mechanism of microflora and nutrient cycle
The coupling mechanism of microflora and the nutrient cycle is basically defined as a two-way process, that is, in the process of soil carbon, nitrogen, phosphorus, potassium and other elements, Rhizosphere microflora promotes the decomposition, transformation, and turnover of these nutrient elements; at the same time, the supply of these nutrients will adjust the structure of the microbial community. It is not in the direction of the traditional one-way; instead, it is a reciprocal feedback circuit. Rhizosphere microorganisms, through the secretion of extracellular enzymes and the operation of special metabolic pathways, serve as the main biological engines that mineralise organic matter and carry out the biogeochemical cycle of macro-elements. On the other hand, the heterogeneity of nutrient availability in time and space has a significant selection pressure, leading to the assembly and functional redundancy of the microbiome. The paper focuses more on the non-linear dynamic coupling process and does not provide a simple one-way causal analysis. In the rhizosphere of wild orchids, there are emergent properties among mycorrhizal fungi, endophytic bacteria, and saprotrophs that cannot be analysed by linear reduction. Non-linear dynamics in this situation are the phenomena of threshold effects, synergistic co-metabolism, and multiplicative feedback loops; in other words, small changes in nutrient stoichiometry may cause large changes in the microbial community structure, and conversely. Based on this core idea, the theoretical framework has been established that nutrient cycling and microbial community structuring are intertwined state variables in a single complex adaptive system. Knowing the above coupling mechanism can help us better understand the ecological adaptability of wild orchids in poor soil and clarify how mutualistic networks use and recycle limited resources to maintain systemic stability in the face of environmental disturbances. Therefore, to study this two-way and non-linear relationship, we need to develop a mechanism-based, predictive model for soil-microbe-plant continuous dynamics in the field of rhizosphere ecology.
2.2 Related Theoretical Basis
2.2.1 Rhizosphere priming effect theory
Rhizosphere Priming Effect (RPE) theory is one of the basic theories in soil biogeochemistry and is used to explain how root exudates affect the rate of decomposition of native soil organic matter (SOM). The RPE in principle is based on the multiple trophic relationships among plant carbon sources and soil microbes. According to its basic idea, the continuous release of simple organic matter such as sugars, amino acids and organic acids will alter the metabolism of heterotrophic bacteria. The change mainly has a positive priming effect, meaning that when easy-to-use rhizodeposits are released, it can activate microorganisms to produce more extracellular enzymes. Oxidase and hydrolase are the main enzymes that catalyse the depolymerisation of the original organic matter that is difficult to degrade, thus promoting its mineralisation rate. Exudates may preferentially direct the microbial substrate utilisation away from the original SOM and temporarily suppress its decomposition, resulting in a negative priming effect. Therefore, the regulatory system of root exudate input on SOM decomposition is subject to the stoichiometry of resource allocation, microbial growth rate and the specific enzymes generated in the rhizosphere microenvironment. Since it was first put forward, RPE theory has moved from the early empirical observations of nutrient flux to a more refined knowledge about the physiological responses of microorganisms at the molecular level; therefore, it is now being applied to various ecosystems such as agricultural fields and natural forest areas. Given the importance of the RPE theory in the study of wild orchid rhizosphere soil microflora and nutrient cycling, the differences in structure and function of rhizosphere and non-rhizosphere bulk soil will be investigated based on this. Wild orchids have peculiar mycorrhizal associations and distinctive root exudation profiles, thus they exert a strong localised influence on the microorganisms in the rhizosphere. Researchers can use RPE theory to study how the particular exudates of orchids affect the enzymatic oxidation of native organic carbon and explain why there is a difference in the rate of organic carbon mineralisation between the biologically active rhizosphere and the relatively inert bulk soil. Therefore, this theoretical model is important for the study of carbon turnover and nutrient availability in wild orchid habitats.
2.2.2 Microbial functional trait theory
Microbial functional trait theory holds that the ecological functions of microbial communities are in fact visible, measurable phenotypic and genotypic traits that can be characterized and predicted to some extent. Different from the past taxonomy that mainly used phylogenetic markers, it now focuses on the actual functions in biogeochemical processes. The main characteristics are extracellular enzyme secretion ability, nutrient utilization preference, and stress resistance; together, these determine how the microbes' metabolic pathways work and interact with the environment. Through the operation of these characteristics, we can examine the community's function in an organised way and thus change our focus from species to functions.
According to the nutrient cycle, microbial functional taxa are grouped by their roles in biogeochemical processes, not by their phylogenetic relationships. For example, there are four types of taxa: carbon decomposers, nitrogen fixers, ammonia oxidizers and phosphate solubilizers. The reason for this classification is that different nutrients have different types of nutrient cycling modules, such as the affinity for a certain type of organic matter and the level of expression of key extracellular enzymes like cellulase, phosphatase and urease in different nutrient cycles. Trait-based grouping can help us better understand how microbes influence the change in nutrients around plant roots and identify redundant functions and ecological niches in the soil microflora.
For the wild orchid rhizosphere, this theory can predict nutrient transformation functions better. Orchids have a special need for mycorrhizal associations and free-living rhizosphere microbes to obtain nutrients; thus, they thrive in low-nutrient environments. By measuring the functional traits of mycorrhizal partners, such as the amount of phosphatase secreted, or the cellulolytic preference of saprotrophs, we can predict how quickly essential nutrients will be released in the environment. Therefore, the theory of microbial functional traits can connect the composition of microflora with the function of ecosystems and help explain how rhizosphere microbial communities meet the strict nutrient requirements of wild orchids.
2.2.3 Soil nutrient stoichiometry balance theory
Soil nutrient stoichiometry balance theory is used to study how different necessary nutrients in the environment are distributed and interconnected. The stoichiometric ratio of carbon, nitrogen, phosphorus and potassium is in the form of a mole ratio; that is to say, mass concentrations of the respective elements are converted into moles and then express their relative proportions. These are the elemental ratios to know how much nutrients are available, what is the composition of organic matter, and how quickly it mineralises. Ecologically, the C:N ratio can show how much carbon and nitrogen are in a certain amount of organic matter, and then calculate how much organic matter is suitable for decomposition. At the same time, the ratio of N to P is usually used to determine the type of nutrient limitation in terrestrial ecosystems; K is often considered as a determinant of the rate of enzymes being activated and how much osmotic regulation occurs at the special plant-soil interface.
The stoichiometric balance will have an important effect on the growth and metabolism of the rhizosphere microflora. Microorganisms have a relatively stable and consistent stoichiometric ratio in their bodies, and this ratio is generally 60:7:1 (C:N:P). When the environmental nutrient ratio is not at this metabolic optimum, the microbial community will adjust its physiological processes or change in composition to reduce the stoichiometric imbalance. For example, when there is an increase in the C:N ratio, nitrogen deficiency will occur in the environment and cause the microorganisms to produce enzymes that can obtain nitrogen from the outside; at this time, the rate of carbon mineralisation will also decrease. On the other hand, if the stoichiometric alignment is appropriate, it can help the microorganisms grow efficiently, speed up the decomposition of organic matter, and optimise the stoichiometry of enzymes, thereby promoting the co-cycling of various nutrients.
For the wild orchid rhizosphere microhabitat, this theory has important guiding significance. Wild orchids are highly dependent on mycorrhiza and live in areas with poor nutrients; their soil environment is thus relatively poor. By using the stoichiometry of C, N, P and K, we can accurately determine what kind of nutrient deficiency is limiting the development of orchids in the rhizosphere. In addition, it can explain how these stoichiometric limitations affect the metabolic relationship between mycorrhizal fungi and host plants, and thus determine the speed and direction of rhizosphere nutrient cycling. Therefore, soil nutrient stoichiometry balance theory is an essential analytical tool for exploring the mechanisms of biogeochemical cycling in the rhizosphere of wild orchids.
2.3 Domestic and Overseas Research Status
2.3.1 Overseas research progress on orchid rhizosphere microbial communities
Foreign research on orchid rhizosphere microbial communities has made progress in recent years, with many high-throughput sequencing and multi-omics technologies being utilised. There have been many studies on the diversity of orchid mycorrhizal fungi (OMF). Based on the phylogenetic tree of orchids, it has been shown that their mycorrhizal fungi are highly conserved and are generally close relatives of Serendipitaceae and Tulasnellaceae. Representative research teams that studied the comparative analysis of tropical, temperate and subtropical orchid habitats have obtained a large amount of empirical data showing that the diversity of OMF is largely driven by host phylogeny and ecological specialisation. Temperate terrestrial orchids generally have a wider OMF assemblage for the purpose of acquiring nutrients, while tropical epiphytic species depend on a small number of specialised, narrow-range fungal partners at the time of life stages vulnerable to changes in carbon and water needs.
At the same time, some foreign studies have been conducted on the distribution of rhizosphere bacterial communities. According to overseas studies, there are many different kinds of bacteria in the roots of orchids. The main taxonomic groups in all regions are Proteobacteria and Actinobacteriota. Bacteria are necessary for life; they fix nitrogen and solubilise phosphate and produce phytohormones that help with the growth of orchids and participate in the nutrient cycle in the rhizosphere.
In addition, the way in which orchid roots selectively enrich certain microbial species from heterogeneous bulk soil is now a standard topic in research. Experimental data show that the chemical signalling of root exudates is very important for niche differentiation. Orchid roots actively adjust the composition of their exudates to attract beneficial microorganisms and inhibit antagonistic ones, forming a good rhizosphere microenvironment suitable for their own growth and development.
The main technical route used by foreign scholars to study these complex interactions has changed from the traditional Sanger sequencing and culture-dependent isolation to the use of an integrated meta-omics framework. By combining metagenomics, metatranscriptomics and stable isotope probing, modern international research has obtained more detailed data on the structure and function of the rhizosphere microbiome of orchids, so it has provided a basis for the preservation of orchids and the recycling of nutrients in the earth.
2.3.2 Domestic research achievements on wild orchid habitat nutrient cycles
In recent years, the basic concept of the orchid rhizosphere as a very special micro-ecological zone has been developed in research on the physicochemical properties of wild orchid rhizosphere soil at home and abroad. Chinese scholars have shown systematically that the rhizosphere soil of wild orchids is acidic, lacks available nutrients and has special organic matter accumulation. Physicochemical properties are jointly controlled by symbiotic mycorrhizal fungi and a particular litter-decomposition pathway; this is how the nutrient cycle in the environment works. To perform this function, researchers will follow the standardised rhizosphere soil sampling method, use various chemical fractionation techniques to separate different parts and measure their contents, etc.
Based on the above reasons, regarding the changes in the amount of nutrients in the habitat seasonally, there have been a lot of data collected recently in various areas of China, especially in subtropical and temperate biodiversity hotspots, that show that nitrogen, phosphorus and potassium have certain periods. Based on the data from multiple sources, it has been shown that nutrient availability is at its highest during the rainy season and closely related to the phenological stage of orchid vegetative growth and flowering, and large amounts of nutrients are trapped by fungi in dry seasons. This time division shows that seasonal changes are an important reason why wild orchids' small-scale ecosystem remains stable.
In addition, in recent years, a lot of research in China has focused on how the soil's nutritional state affects the reproduction of wild orchid populations. Empirical evidence suggests that the microhabitat of the rhizosphere, which is relatively poor in nutrients but rich in microorganisms, has a relatively high rate of seed germination and protocorm survival; thus, some degree of nutrient deficiency can promote the establishment of mycorrhizal fungi and later seedlings.
The above are all domestic research results and will serve as the basis for this paper. Based on the previous data of regional physicochemical profiles, seasonal nutrient trends, and regeneration-nutrient feedback loops, a complete analytical system will be built in this paper. It can be used to study the regularities of rhizosphere microflora and nutrient cycling in wild orchid habitats systematically.
2.3.3 Research gaps and pending key issues
The present research has only been used to examine the composition of the microbial community in the wild orchid rhizosphere and determine the static physicochemical properties of the habitat nutrient pool. Although many descriptive studies have been conducted on mycorrhizal specificity and general enzymatic activity, they are still in their infancy in terms of theory and method: there is no targeted analysis of the dynamic interaction networks among different microbial groups and no quantitative study of their synergistic driving effects on the whole nutrient cycle. The rhizosphere is a miniature ecosystem in which cross-kingdom and interspecies metabolic communication control the biogeochemical cycles. Without clarifying these interaction mechanisms, it will be impossible to determine why a particular community can support the ecosystem and thus predict nutrient mobilization. Therefore, to solve the problem of the disconnection between the stability of the community and its functions, three key problems that need to be addressed are as follows. First, it is still unknown exactly which keystone microbial taxa are present in the orchid rhizosphere; it is necessary to find these highly connected nodal organisms in order to know the structure of the interaction network and how it regulates nutrient uptake at its foundation. Second, there is a serious lack of quantitative calculations for the efficiency of nutrient cycle regulation; a reasonable quantitative standard needs to be established to express how much various microbial communities influence the turnover rate of limiting elements, particularly carbon, nitrogen and phosphorus, so that we can move from qualitative correlation to mechanistic explanation. Third, the coupling relationship between microflora and nutrient cycling will change significantly due to disturbances in the habitat; many kinds of environmental changes have occurred in natural orchid habitats, but the threshold dynamics and direction of change of this coupling structure under external stress have not yet been investigated empirically. Clarifying the resilience and plasticity of this coupling system under disturbance will help us develop more reasonable conservation plans. To address these related problems is important for building an overall, mechanistic theory of wild orchid rhizosphere ecology.
2.4 Summary of the Chapter
In this chapter, we have built a theoretical system and given some basic information about wild orchid rhizosphere soil microflora and how nutrients are cycled in the environment. First, the basic concepts of the three core areas were presented: the rhizosphere microflora, which is a dynamic microbial group that is influenced by orchid root exudates in both space and time; the nutrient biogeochemical cycle, which is a systemic transformation and flow of necessary elements; and the plant-microbe interaction, which is the complex molecular and metabolic signalling exchange between orchid roots and soil microbes. Second, the chapter presented three supporting theories: the theory of rhizosphere effect to explain the obvious changes in physicochemical and biological properties of soil near the root system, the theory of ecological stoichiometry to provide the mathematical constraints on the proportional balance of elemental cycling and microbial resource allocation, and ecological niche theory to explain the spatial and functional partitioning of microbial communities in the orchid rhizosphere. Third, a critical review of domestic and overseas research progress has shown that although many studies have mapped the mycorrhizal associations in orchid systems, they have not focused on all fungal symbionts or bacteria and archaea of the whole microflora. At present, most of the studies on microbial community structure have been conducted by static taxonomic methods; thus, few links have been made between the expression of dynamic microbial functional genes and the actual rate of nutrient turnover. The above research gaps show that the taxonomic presence of wild orchid rhizosphere microorganisms does not align with the function of the ecosystem. In response to these deficiencies, this study seeks to move beyond description and explore the reasons for the mobilisation of nutrients. Therefore, this chapter provides a solid theoretical basis and literature support for the subsequent experimental design, and it will be used as the analytical tool for the first study to examine how rhizosphere microflora regulate nutrient cycling.
Chapter 3 Materials, Methods and Preliminary Analysis of Wild Orchid Rhizosphere Soil Properties
3.1 Sample Site Selection and Sample Collection
3.1.1 Setting of wild orchid distribution sampling plots
The selection of sampling plots is based on ecological representativeness and statistical accuracy, so the soil microflora and nutrient data collected can reflect the rhizosphere interaction of wild orchids. The designated sampling areas are in the pristine subtropical broadleaf-evergreen forests of the Wuyi Mountains; their approximate centre is located at around 27.45°N latitude and 117.60°E longitude. The altitude range of the selected sites is between 600 and 1200 meters above sea level, covering the best altitude belt for the target epiphytic and terrestrial orchid populations. The main type of vegetation is a mature forest of Castanopsis and Schima; there is also a well-developed understory of ferns and broad-leaf shrubs. There were 3.5 wild orchid individuals per square metre in these sites, and they were clustered.
To consider the internal population heterogeneity and ensure that the operation can be completed, each target wild orchid species will have three sets of 10m by 10m sample plots. The replication principle is based on the minimum number needed for statistical purposes for calculating variance and guaranteeing that subsequent inferential studies of microbial community structure and nutrient cycling are reliable. 10m × 10m is the most suitable size in terms of operation; it is large enough to cover the local root architecture areas and mycorrhizal hyphal networks of grouped orchids, but small enough to reduce the difference in microclimate within the plot.
To determine the biochemical and microbial changes in the wild orchid rhizosphere, some plots with the same habitat were set up as controls. These control plots do not have orchids growing under the forest; instead, they are home to different herbaceous plants with similar root systems and physiological characteristics. Using the aspect, soil parent material and canopy cover to pair plots, the interference due to differences in habitat background can be effectively eliminated. Therefore, the difference between the orchid plot and the non-orchid plot can be attributed to the specific plant-soil-microbe feedback mechanism in the wild orchid rhizosphere, and not to other pedological or climatic factors. To study the subtle changes in nutrient cycling caused by orchid-mycorrhizal symbiosis, it is necessary to be strict in our methods.
3.1.2 Hierarchical collection of rhizosphere and non-rhizosphere soil samples
A hierarchical collection of rhizosphere soil and non-rhizosphere soil is a necessary first step in the study of the special microflora and nutrient cycling of wild orchid roots. The rhizosphere is the small area of soil that is directly affected by the secretions from the roots; as such, its microorganisms and physical and chemical characteristics are different from those in the surrounding bulk soil. Therefore, a strict and stratified sampling system must be used to collect the samples. First, excavate the root system of the wild orchid with a small shovel to reduce damage to the soil-root interface. After exposure, loosely adhering to the soil will be shaken off. Then, take the rhizosphere soil that is firmly attached to the surface of the root by a distance of 1mm. Micro-spatial division is necessary since it is the core area where plants and microorganisms communicate and exchange nutrients. At the same time, non-rhizosphere control samples are taken from a depth of 5 to 10 cm and a horizontal distance of 20 cm from the orchid root system. Because of this spatial separation, baseline soil data can be obtained that do not have an immediate impact on root exudates, making it possible to compare the nutrient pools of rhizosphere soil and bulk soil. To ensure the reliability and repeatability of statistics, six independent biological replicates will be set up for each sample group. All samples were quickly placed in a low-temperature ice box after extraction to avoid unintended microbial metabolism and to preserve their nucleic acids during transport. After arriving at the laboratory, the samples will be split into two groups; one is used for testing the general physical and chemical properties, and the other is for high-throughput microbial sequencing. A hierarchical and binary processing mode is required to relate the environmental factors in the soil with the structure and function of wild orchid rhizosphere microflora.
3.2 Determination of Soil Physicochemical Properties and Microflora Sequencing
3.2.1 Detection of core soil nutrient indicators (C, N, P, K)
The detection of the four indicators, namely carbon, nitrogen, phosphorus and potassium, is necessary to know the nutrient conditions of wild orchid roots and how they cycle nutritionally. The main energy source for heterotrophic microbial metabolism is TOC, and the potassium dichromate oxidation external heating method is used to determine its concentration. The first method is to use a high-temperature and high-acid environment for the oxidation of organic matter with dichromate ions; according to the stoichiometric reaction, the amount of carbon can be calculated accurately. Total nitrogen (TN), one of the reasons why symbiotic fungi can improve plant growth, was analysed using the Kjeldahl method. In order to obtain the basic content of nitrogen in organic and ammonium salts, the Kjeldahl method is used to convert these substances into ammonium salts through acid digestion, and then alkaline distillation and titration are conducted for the analysis. The total phosphorus (TP) was measured by the molybdenum antimony anti-colorimetric method. Based on the fact that phosphate reacts with molybdate and antimony tartrate in an acidic medium to form a heteropoly acid and is then reduced to form a characteristic phosphomolybdenum blue complex, this spectrophotometric method has high sensitivity and accuracy for the determination of trace phosphorus. Flame photometry was employed to determine the amount of total potassium, which is a strong atomic emission method that quantifies potassium ions based on their different spectral emission intensities in a high-temperature flame. To understand how soon these macronutrients can be used by the orchid roots and the micro-organisms in the rhizosphere, the available nitrogen, available phosphorus and available potassium were determined. These fractions are the dynamic mobilisation nutrient pools that directly participate in plant-microbe interaction. To guarantee the absolute accuracy and scientific authenticity of the comprehensive detection data set, strict quality control measures will be taken during the whole process of analysis. Certified standard reference soil samples were systematically added to each analytical batch; this continuous examination of methodological accuracy will help reduce errors in the procedure and thus enhance the stability of the empirical basis for the next experiments.
3.2.2 16S rRNA and ITS high-throughput sequencing of soil microflora
To understand the structure of the microflora in the wild orchid rhizosphere soil, total genomic DNA was carefully extracted from all the soil samples with a special soil DNA extraction kit to achieve high yield and good integrity of the microbial nucleic acids needed for subsequent molecular analysis. A standard procedure was followed to identify the bacterial community by amplifying the highly variable V3-V4 region of the 16S rRNA gene using universal primers. At the same time, the fungal community was assessed by amplifying the ITS1 region of the fungal rRNA gene using specific ITS primers, which are the main barcodes for identifying fungi due to high interspecies variation. Polymerase chain reaction (PCR) was carried out three times for each sample to reduce the randomness of the amplification results, and then the amplification products were combined and purified. High-throughput sequencing was performed on the Illumina sequencing platform, and paired-end sequencing by synthesis was used to obtain millions of reads, which was sufficient to cover the deep-sea bacteria in the complicated rhizosphere microbiome. After sequencing, a strict bioinformatics pipeline was employed. First, the raw reads were spliced together based on their overlapping areas; then, the bad-quality reads and chimera sequences were removed. The sorted high-quality sequences were then classified into Operational Taxonomic Units (OTUs) at a 97% similarity level, which is considered an appropriate way to roughly represent species-level resolution. Finally, taxonomic annotation was done systematically by aligning representative OTU sequences with authoritative databases: NCBI for bacteria and UNITE for fungi. The overall workflow resulted in accurate, detailed composition data of bacteria and fungi in all wild orchid rhizosphere soil samples, providing a necessary molecular basis for future studies on microbial interactions and nutrient cycling.
3.2.3 Statistical analysis of microbial community diversity
Statistical analysis of microbial community diversity is a necessary basis for revealing the ecological problems in the rhizosphere of wild orchid soil. In order to determine the level of diversity within a habitat accurately, the alpha diversity index was calculated from the rarified amplicon sequence variant (ASV) table. The Chao1 index is used to estimate the total species richness, and we have added the number of singleton and doubleton species to it to more accurately reflect the true situation of all rare species. The Shannon index was used to reflect the richness and evenness of the species, and the proportion of each ASV was multiplied by its natural logarithm to show uncertainty in species prediction. To clearly separate the distribution uniformity of ASV abundance from the richness effect, the Pielou evenness index is used to calculate how evenly microorganisms are distributed among taxa by dividing the Shannon index by the logarithm of the total number of observed ASVs. The above alpha indicators are needed for judging the ecological stability and niche characteristics of the orchid rhizosphere. A principal coordinate analysis (PCoA) was conducted to compare the differences in the microbial community structure among groups based on the Bray-Curtis dissimilarity matrix. PCoA can map high-dimensional species abundance data into a low-dimensional orthogonal space and display the relative locations of rhizosphere and non-rhizosphere soil samples to show their ecological differences. The robustness of community separation in the PCoA ordination was statistically verified by permutational multivariate analysis of variance (PERMANOVA). By permuting the Bray-Curtis distance matrix repeatedly, PERMANOVA can accurately determine whether there are significant differences in composition among different groups and then provide pseudo-F statistics and corresponding p-values. Non-parametric methods are naturally suitable for ecological data that do not meet the assumptions of normality and homoscedasticity. Alpha diversity assessment, PCoA ordination and PERMANOVA verification are used to comprehensively study whether there is a certain selection pressure in the wild orchid rhizosphere that has significantly changed the soil microflora compared to non-rhizosphere matrices.
3.3 Preliminary Analysis of Basic Characteristics of Tested Samples
3.3.1 Physicochemical property differences between rhizosphere and non-rhizosphere soil
Physicochemical analysis of rhizosphere and non-rhizosphere soils is the first work to understand the differences in nutrient supply caused by the activities of wild orchid roots. The values of total and available carbon, nitrogen, phosphorus, potassium, and soil pH were all used in the experiments; therefore, there is no difference in the results of all the paired samples. The data were analysed by one-way ANOVA to see whether there was a significant difference in the numbers of individuals in the rhizosphere and non-rhizosphere populations. This statistical verification is necessary to determine which changes in biogeochemical properties of the soil are caused by the orchid rhizosphere and which are due to other reasons related to soil conditions. According to the results of the analysis, it is known that many essential macronutrients are relatively high in the rhizosphere zone. Available carbon and nitrogen fractions were significantly higher in the rhizosphere soil than in the non-rhizosphere soil, probably due to the large amount of organic exudate from the rhizosphere and the quick turnover of fine orchid roots and mycorrhizal hyphae. At the same time, available phosphorus and potassium contents were significantly increased in the rhizosphere; therefore, a large number of specific mycorrhizal symbionts exist in the roots of orchids that can solve this problem. The soil in the rhizosphere has a lower pH value than the soil outside the rhizosphere; thus, it is reasonable to assume that organic acids and protons are released during the absorption of nutrients. Based on the above data, it can be hypothesised that there has been some variation in the physical and chemical properties of the sampling plots due to the way wild orchids have adapted to life in nutrient-poor environments. The three-way interaction among orchid roots, mycorrhizal fungi and the surrounding native microbes is responsible for creating a nutrient-rich area in the nearby soil, thereby bypassing the shortage of nutrients in the natural soil. To accurately investigate the changes in rhizosphere nutrient concentration, we conducted this quantitative analysis of the various nutrients in the soil.
3.3.2 Structural composition characteristics of wild orchid rhizosphere microflora
The structural composition of the wild orchid rhizosphere microflora has been shown to be an essential indicator for exploring the complex plant-microbe relationships that regulate the soil nutrient cycle. Based on the preliminary analysis of the samples, the structure of the microbial community in the rhizosphere and non-rhizosphere soils is quite different. The relative abundance distribution of dominant bacterial phyla shows that Proteobacteria and Actinobacteriota are the main groups in the rhizosphere, while Ascomycota and Basidiomycota are the main groups in fungi. This distribution shows that labile carbon substrates released by orchid roots are preferred metabolically, so copiotrophic bacteria will be abundant. At a lower level of taxonomic classification, the typical genera in the bacteria are Pseudomonas and Streptomyces, and in fungi they are Tulasnella and Serendipita; the latter is important as a mycorrhizal symbiont that can help orchids absorb nutrients from the soil for seed germination.
By analysing the difference in microbial diversity, we can learn about the characteristics of this microhabitat. Alpha diversity indices, such as species richness and evenness, are usually concentrated in the rhizosphere compared to the non-rhizosphere bulk soil and have a more specialized community structure. At the same time, the beta diversity index (Bray-Curtis dissimilarity) is high, indicating a large difference in community structure between the two compartments, and it is believed that this is due to the deterministic screening effect of orchid root exudates. LEfSe analysis and other statistics have shown that some microorganisms are more common in the rhizosphere habitat. These are not only passively present but also actively recruited according to the physiological needs of orchids.
In the end, the first round of the structural profiling can be used to identify the basic group of microorganisms that participate in nutrient transformation. Saprotrophic fungi and mycorrhizal fungi that have been enriched, along with copiotrophic bacteria, are believed to have played a crucial role in the biogeochemical cycle, such as dissolving difficult-to-decompose organic phosphorus and mineralising complex nitrogenous substances. It is necessary to set up a structural baseline first, as it will provide the necessary taxonomic information for the functional assays that follow, aiming to investigate how rhizosphere-driven nutrient cycles work.
3.4 Summary of the Chapter
Chapter 1 is to build a basic research framework and present some of the first empirical data that can explain why rhizosphere micro-organisms in wild orchids have high numbers and how they are connected in the nutrient cycle. Firstly, according to the ecological sampling regulations, a proper location will be chosen in the sampling area to collect the rhizosphere soil of wild orchids; thus, the soil is typical of the area and has not been disturbed by other activities. Accurate identification of the rhizosphere and bulk soil regions is necessary, as the rhizosphere is the place where root and micro-organisms communicate and exchange substances. Next, the experimental steps for soil physical and chemical testing are given. Parameters such as the availability of macro and micro nutrients in soil, the ratio of organic matter, pH and water content are determined by established analytical methods and serve as the abiotic basis for the metabolic processes of microbes. At the same time, the working principle of high-throughput sequencing of microbes will be introduced. Detailed information on DNA extraction, PCR amplification of the 16S rRNA and ITS gene regions, and bioinformatic pipelines for amplicon processing are provided to meet the taxonomic resolution and sequencing depth requirements for the analysis of microbial communities. According to the preliminary analysis results, there are considerable differences in soil physical and chemical properties between the rhizosphere and non-rhizosphere regions, and the structure of the microbial communities is also divided. Some specific types of microorganisms are enriched or depleted in the rhizosphere, and these are probably due to the selection pressure from root exudates in wild orchids. Based on these empirical observations, we can conclude that the rhizosphere of wild orchids is not a random matrix but a specialised environment. Based on the above-mentioned basic physicochemical gradients and preliminary characteristics of the microbial community structure, this chapter will provide some basic data. It will help to carry out the following multivariate analysis and network modelling to understand the complex interkingdom signalling pathways and functional gene expressions that regulate the rhizosphere microflora interaction and nutrient cycle coupling mechanism in the next chapter.
Chapter 4 Interaction Relationship Between Rhizosphere Microflora and Nutrient Cycle Regulation Mechanism
4.1 Construction of Wild Orchid Rhizosphere Microflora Interaction Network
4.1.1 Identification of keystone microbial taxa in rhizosphere microflora
Identify the keystone microbial taxa to solve the structure and function of the wild orchid rhizosphere microbial community. Using the relative abundance data of the microbial operational taxonomic units (OTUs), we will construct a strong co-occurrence network with the Spearman correlation coefficient. This coefficient shows that the various microbial species have a monotonic relationship, and the statistically significant correlations form the edges that connect the nodes in the network. To accurately identify keystone taxa, we first calculate the topological parameters node degree, betweenness centrality and closeness centrality. Node degree refers to the number of adjacent nodes, and betweenness centrality measures the influence of a node on the spread of information or other types of resources in the network. Closeness centrality is the reciprocal of the average shortest path length from a particular node to all other nodes, and it can reflect the transmission speed of the whole network. Therefore, the main microbial groups with relatively high values in all three indicators are considered to be keystone microbial groups, and they have high local connectivity and a key role in regulating the rhizosphere microflora. The above taxa are of great practical significance because they are the main forces for maintaining the stability of the community, promoting synergistic metabolic cascades and regulating nutrient circulation, and serve as ecological hubs for the holobiont. After dividing the topology, it is necessary to determine the taxonomic affiliation of the keystone taxa, that is, whether they mainly belong to bacteria or fungi, such as mycorrhizal partners and saprotrophic decomposers. Moreover, the analysis of the abundance distribution of different wild orchid rhizosphere samples reveals the host-specific enrichment patterns and ecological niche preferences of these plants. Based on the above method, we can identify the functional bottlenecks or catalysts of microbes in the rhizosphere and provide a mechanism to study how specific microbial communities promote nutrient cycling and absorption in wild orchids.
4.1.2 Correlation analysis between microbial symbiosis patterns and soil nutrient content
Correlation analysis of microbial symbiosis and soil nutrient distribution is required to understand the structure and function of the wild orchid rhizosphere microecosystem. Based on the topological indices of the constructed interaction network (modularity, average path length, and the proportion of cooperative correlation edges), together with the amount of soil carbon, nitrogen, phosphorus, and potassium in the soil. Modularity is the degree to which niches are partitioned in a community, and the average path length is how quickly a functional signal or material can spread through a network. At the same time, the proportion of cooperative edges is the relative amount of mutualistic or syntrophic associations, such as cross-feeding, that are important for resource sharing. Using strong correlation analysis, operational pathways can be used to map these topological features to the spatiotemporal distribution of soil nutrients. The results of the analysis show that there is a distinct correspondence between various types of microbial symbiosis and the nutrient level of the soil. Networks with high modularity often have dense clusters of cooperation, and such clusters tend to be positively correlated with high concentrations of labile organic carbon and available nitrogen, meaning that structured mutualism can accelerate the biological and chemical mineralization of difficult-to-decompose substances. Most importantly, the results of this correlation analysis show that there is a basic ecological principle in the wild orchid rhizosphere microhabitat; a more complicated network of symbiosis, with extended average path lengths and a high proportion of cooperative correlation edges, is closely related to the increase in available phosphorus and potassium in the soil. Therefore, under an operational system with high network complexity, the various parts will cooperate and support each other to address the problems of inorganic nutrients through joint enzymatic reactions. Therefore, it is necessary to explore the relationship between the two in order to understand how the fitness of orchids is maintained by the self-organizing ability of the rhizosphere microflora and provide a more solid theoretical basis for the study of plant-microbe mutualism under natural nutrient limitation conditions.
4.2 Quantitative Analysis of Microflora Driven Nutrient Cycle Processes
4.2.1 Measurement of extracellular enzyme activities related to carbon and nitrogen mineralization
Extracellular enzyme activity related to carbon and nitrogen mineralization is an important indicator to reflect the function of rhizosphere microflora in nutrient cycling. Extracellular enzymes, produced by soil microbes and plant roots, catalyse the decomposition of macromolecules into bioavailable monomers, and thus determine the rate-limiting steps in nutrient mineralisation. The four enzymes measured in this study are: β-glucosidase (involved in the last step of cellulose degradation and carbon mineralisation), urease (catalyses the hydrolysis of urea to ammonia, which is the first step in nitrogen transformation), acid phosphatase (mediates the hydrolysis of organic phosphorus esters) and arylsulfatase (promotes the mineralisation of organic sulfur esters). The quantification of the above enzymes is usually achieved by fluorometric or colorimetric microplate assays using specific synthetic substrates, and it is possible to detect the catalytic rate of the enzymes in a large number of samples accurately under controlled incubation conditions. In order to understand the effects of the rhizosphere, the difference in the soil's rhizosphere should be studied. Root exudate stimulation in the rhizosphere can lead to an increase in enzyme activity; at the same time, it promotes the growth and activity of microorganisms and the production of enzymes. Then, connect the changes in enzyme activity to the numbers of related functional microorganisms in the co-occurrence network and thus obtain the structure of nutrient cycling. For example, the positive correlation between β-glucosidase activity and the relative abundance of cellulolytic taxa, or between urease activity and ammonification-affiliated taxa, is directly related to community composition and functional output. This combination of enzymology and molecular ecology offers some reasons why certain microflora assemblages regulate the flow of elements, and a way to evaluate the functional stability and regulatory capacity of the rhizosphere micro-ecosystem of wild orchids based on this.
4.2.2 Metabolic functional profile prediction of microbial community involved in nutrient transformation
To learn more about how microbes affect the changes in nutrients in the environment, one can use Metabolic function prediction. By combining high-throughput amplicon sequencing data with bioinformatics inference tools, this method can go beyond the simple identification of taxa and predict the metabolic potential in the rhizosphere of wild orchids. Specifically, PICRUSt2 will be used to predict the functional gene abundance of the bacterial community, and sequence data will be systematically mapped to the KEGG orthology database to build pathways related to carbon, nitrogen and phosphorus cycles. At the same time, FUNGuild is used to sort fungal operational taxonomic units into different nutritional forms and ecological function groups, such as symbiotrophs, saprotrophs and pathotrophs. The purpose of this double tool is to address the functional differences in the rhizosphere and non-rhizosphere regions and find out how wild orchids selectively increase certain types of microorganisms that have certain metabolic capabilities to promote the availability of nutrients in the local environment.
A basic idea of this quantitive analysis is to compare the predicted function gene abundances of various parts of the soil in different places systematically. By comparing the differences in nutrient transformation gene copies in the rhizosphere and non-rhizosphere microflora, we can learn more about the specific metabolic changes caused by the orchid host. Some examples of increased genes for ammonia monooxygenase and alkaline phosphatase in the rhizosphere are direct indicators of enhanced microbial mediated nitrogen mineralization and phosphorus solubilisation.
In order to verify the reliability of these predicted functional genes by sequence comparison, the cross-referencing method should be employed. To confirm the consistency of the bioinformatic prediction results of hydrolytic and oxidative functional genes with the levels of soil extracellular enzymes determined by spectrophotometry or fluorimetry, the reliability of the nutrient circulation model was enhanced. To prove that the predicted functional profiles are consistent with the actual situation of nutrient mobilisation in the orchid rhizosphere through enzymes, it is necessary to know the theoretical metabolic potential and the actual biogeochemical functioning of the environment. Finally, a close mechanistic association between community structure and ecosystem functions was obtained from this quantitative analysis.
4.2.3 Path analysis of microflora regulating soil nutrient turnover rate
Path analysis of microflora regulating soil nutrient turnover rate is an important way to study the regulatory mechanism of wild orchid rhizosphere. Structural equation modelling (SEM) has been used to systematically analyse the direct and indirect pathways of microbial communities affecting soil nutrient dynamics and determine the reasons behind them. The constructed SEM model is based on the abundance of keystone microbial taxa, the degree of microflora network interaction and the activity of extracellular enzymes, etc. In concept, they are the structural basis of the rhizosphere microbiome, the connection in an ecosystem and the way the system works. In turn, the soil organic carbon mineralisation rate, nitrogen net mineralisation rate and phosphorus activation coefficient are selected as dependent variables to describe the basic nutrient transformation processes that support orchids.
The operational process of SEM path analysis is to establish a priori theoretical models according to certain biogeochemical rules and then modify them repeatedly to find a good model fit. The direct paths can measure the direct effects of certain microbial characteristics on nutrient cycling speed; that is to say, the direct impact of the activity of extracellular enzymes on the phosphorus activation coefficient is due to enzyme-induced mobilisation of the poorly soluble phosphorus pool. Indirect paths show that the regulation logic is that the abundance of keystone taxa increases the intensity of network interaction and thus promotes the production of extracellular enzymes, speeding up the mineralisation of organic carbon. By breaking down these effects, we can quantitatively analyse how different microflora interaction characteristics affect nutrient cycle rates. A strict quantification should be carried out for practice; that is, find the main organisms contributing to rhizosphere fertility, understand the relationship between the stability of the microbial network and the rate of nutrient turnover, and use it to develop a good plan to improve the soil microflora and meet the special nutrition needs of wild orchid ecosystems.
4.3 Coupling Response of Microflora Interaction to Nutrient Cycle Under Habitat Disturbance
4.3.1 Variation of interaction intensity under different vegetation coverage gradients
According to the data of the field survey, there are three different gradients of forest vegetation coverage in the sampling plots, namely high, medium and low; and the coupling response of rhizosphere microflora to the nutrient cycle under habitat disturbance is studied. Vegetation cover is one of the reasons for the difference in microclimates and the amount of organic carbon entering the soil, thus affecting the growth of rhizosphere microorganisms. Comparing the topological structure and interaction strength of the rhizosphere microflora network in different coverage gradient areas can help us understand this. In high-coverage areas, the microbial co-occurrence network has dense topological connections, and its nodes have high connectivity, short average path lengths and strong modularity. This complex structure will support many strong cooperative connections, and among them are the key microbial groups, such as mycorrhizal fungi and cellulolytic bacteria, that enable efficient enzyme reactions and stable nutrient mineralisation. When the vegetation cover is low, the habitat is disturbed due to the reduced accumulation of litter and increased soil environmental instability. As a result, the topology of the microflora network has been severely fractured. The connectivity of the interaction network has decreased significantly, and as a result, the degree of the network has dropped and the path length has increased; thus, the interdependence among species' metabolisms has weakened. In addition, at low coverage, the relative abundance and centrality of the keystone microbial taxa also decrease significantly, and the network loses its structural scaffolding and functional hub. The loss of topological integrity and the reduction in the number of keystone species will have a direct impact on the metabolic cross-feeding paths required for nutrient mobilisation. Finally, the reduced intensity of interaction will directly limit the nutrient supply in the wild orchid's rhizosphere microhabitat, lowering the bioavailability of nitrogen and phosphorus, the limiting factors, and increasing the vulnerability of wild orchid populations to continued environmental stress. Therefore, the aboveground vegetation's health needs to be maintained in order for the belowground microbes to cooperate effectively and the rhizosphere to receive enough nutrients.
4.3.2 Regulation path of microflora network stability on nutrient cycle continuity
The way in which microflora network stability can influence nutrient cycle continuity and how changes in nutrient levels are affected by disturbances at the top of the rhizosphere co-occurrence network will be explained in more detail below. The robustness index of the network for an ecosystem of wild orchid rhizosphere is employed to predict the resilience of this ecosystem. A relatively stable microflora interaction network can resist various small changes in the habitat and has a noticeable coupling reaction. Functional redundancy is an underlying reason why regulatory pathways are present in many highly connected microbial hubs. High stability of the network will keep the structure of the whole metabolic network relatively stable and prevent the occurrence of serious failures of the entire network system. Thus, the structural persistence will govern the continuity of the nutrient cycle. A strong network can maintain the activity of extracellular enzymes for the decomposition of carbon, nitrogen and phosphorus, and so on; therefore, they will not decrease suddenly when the environment changes. By stabilising the catalysis of enzymes, the interaction network can maintain the rate of nutrient conversion and ensure that the mineralisation and mobilisation processes do not experience significant, severe disruptions. This regulatory way is important for wild orchid preservation in practice.
The growth of wild orchids is easily affected by changes in the nutrients at the roots. 3 kinds.2 Kinds.1 A A A stable microbial network is thus a kind of ecological buffer that can convert topological resistance into functional continuity to provide a continuous and appropriate supply of nutrients even under less favourable environmental conditions. This is the case for the above reasons. Thus, the soil inoculum potential should be measured by network robustness, not species richness, to predict the feasibility of reintroducing species in the region.
Let me arrange the above in a more general way without any other numbers or symbols.
The regulatory path of the nutrient cycle under the action of the microflora network is to a certain extent determined by the topological strength of the rhizosphere co-occurrence network in the face of exogenous disturbances. Wild orchids grow in the rhizosphere of plants that have nutrient requirements and live in symbiosis with fungi and saprophytic bacteria to obtain energy; thus, the network's robustness index can be used to judge the ecosystem's ability to resist changes in the environment. When the wild orchid rhizosphere is disturbed, a more stable bacterial community can better resist changes in the environment. The regulatory pathway is mainly based on the functional redundancy of the numerous connected microbial centres in the food web. A high degree of stability of the network will not cause a serious collapse of the metabolic system. Therefore, the continuity of the nutrient cycle will be decided by this structure. A stable network can maintain the activity of extracellular enzymes in the soil to promote the circulation of carbon, nitrogen and phosphorus in the soil and prevent a large-scale decline in their amount due to changes in the environment. Stabilize the speed of enzymatic catalysis of the above three types of nutrients in the soil and prevent significant reductions in these nutrients due to environmental changes. It is important for the actual application of wild orchid conservation. Wild orchids have a special mycoheterotrophic or partially autotrophic mode of life and are very sensitive to changes in rhizosphere nutrients. A stable microbial network can provide ecological buffer capacity to maintain the functional continuity of the system under adverse conditions. Therefore, it is necessary to consider the robustness of the network when determining the inoculation potential of soil, not just the number of species, to predict whether the habitat is suitable for the reintroduction of orchids.
4.4 Protection Strategies for Wild Orchid Populations Based on Rhizosphere Microflora-Nutrient Synergy
In order to protect wild orchid populations in situ, we need to change our way of thinking from only considering the above-ground habitat parameters to actively manage the rhizosphere microflora and nutrients below-ground. At its base, this is to be aware of the close connection between the natural regeneration and long-term survival of wild orchids with the stability and functioning of their native rhizosphere microbial communities; thus, controlling the local nutrient cycling can help achieve this. First, it is essential to maintain a certain area of understory vegetation in wild orchid habitats. The soil moisture and temperature in the microclimate will be regulated by the form of the canopy, and the root exudates of the plants will provide nutrients to the heterotrophic microbial community for the formation of orchid mycorrhizal associations. At the same time, excessive human disturbance should be reduced. Human activities, such as trampling off-trail and unregulated extraction, will cause severe damage to the soil structure and destroy the delicate rhizosphere microflora network, leading to the loss of the important microbe-nutrient relationship for the formation of seedlings. In addition, adding specific carbon sources is another means of operation. Introducing certain organic substrates can promote the colonization of key symbiotic microorganisms, especially mycorrhizal fungi, and serve as the main channel for nutrient absorption. This way can compensate for the carbon deficiency in underground orchid protocorms and young plants during their achlorophyllous growth period. Finally, regulate the stoichiometric balance of nutrients in the rhizosphere soil. Change the ratios of important elements such as carbon, nitrogen and phosphorus to prevent competition from opportunistic, non-symbiotic microflora and ensure that the mobilisation of enzymes meets the exact stoichiometric requirements of the orchid-fungal symbiosis. With the help of these paths, conservation practices can use the synergistic effect of native rhizosphere microflora and nutrients to protect wild orchids. The comprehensive regulation system is designed to repair the functional integrity of the orchid holobiont and build a self-sustaining ecosystem, thus increasing the resilience and natural regeneration ability of endangered wild orchid populations in their natural environment.
4.5 Summary of the Chapter
The changes in the number of microflora in the rhizosphere of wild orchids and the processes of nutrient cycling have been sorted out, and the main results of the previous analysis have been summarized. Essentially, the rhizosphere is a very active micro-ecosystem where various co-occurrence and exclusion patterns among microorganisms form intricate interaction networks. These networks are not only structural aggregates but also the functional basis for the stability and resilience of the below-ground ecosystem. The influence of certain microbial communities on nutrient cycling is relatively small and has been proved by research in recent years. It has been shown that keystone taxa have a large impact on the activities of enzymes, thus regulating the turnover rates of essential elements and providing the necessary nutrients for the growth and development of orchids. In addition, the response patterns of microflora to nutrient cycling with different vegetation coverage changes are presented in the article, and they show strong ecological plasticity. With the change in vegetation coverage, the structure of the rhizosphere community will change regularly, and thus the path of nutrient cycling will be adjusted accordingly. High vegetation gradients often form synergistic microbial networks that improve nutrient retention and symbiosis; however, fragmented or low-coverage areas can break these topological relationships and result in nutrient cycling being separated from resources of mutual benefit. It is important to know how aboveground vegetation heterogeneity is related to below-ground microecological functions in practice. According to the above research results, a plan for the protection of wild orchid populations has been proposed. Instead of using generalised conservation frameworks, these interventions are aimed at the particular circumstances of the habitat; for instance, they adjust the structure of the overstory to maintain good vegetation gradients and add microbial inoculum to repair the damaged rhizosphere network in the degraded niche. According to the inherent regulatory mechanism of the rhizosphere microflora, the above method can help us maintain the viability of the wild orchid metapopulation and protect the overall function of its microecological habitat.
Chapter 5 Conclusion
The study has systematically revealed that wild orchid rhizosphere soil microflora and nutrient cycling are intertwined, and thus provides an important basis for the construction of the network of belowground biology that supports these environmentally important plants. In short, it can be said that the rhizosphere of wild orchids is not only the place where organisms live; rather, it is also a very dynamic micro-ecosystem with specific fungal communities, and mycorrhizal partners are the main source of nutrients for this area. The core idea of this study is that orchid nutrient acquisition is closely related to the metabolic activities of these microorganisms, which can break down large organic molecules through enzymes to release necessary nitrogen and phosphorus that the host plant cannot use directly [5]. The operational mode of this relationship is a close reciprocal exchange: fungal hyphae can enter the cortical cells of the root to transport mineralised nutrients to the orchid, and at the same time, the plant will provide photosynthates derived from its own photosynthesis to support the growth of the heterotrophic microbial biomass. To solve the problem that the dust-like orchid seeds lack endosperm and are difficult to germinate and grow into seedlings, a two-way synergy mechanism has been put forward. Therefore, these results will have a good guiding role in the work of conservation biology and horticultural application. Through the precise definition of the structural composition and functional thresholds of the orchid rhizosphere microbiome, this study has provided a basis for the development of targeted bioinoculation strategies that enable predictable and stable plant health maintenance via microbial homeostasis [10]. These measures can artificially repair the damaged rhizosphere network in degraded areas and thus increase the in situ survival rate of endangered orchid species. In conclusion, understanding the microbially mediated nutrient cycles will provide some reference parameters for the improvement of ex situ cultivation protocols and reduce the need for artificial chemical fertilisers by utilising the native microbial enzyme potential. In short, it has been shown that to protect wild orchid biodiversity, a complete ecosystem will be needed to maintain the soil microflora associated with it.
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