Research on the Impact of Biodiversity in Tea Plantations on Tea Quality

Abstract

Tea plantation ecosystems, as typical human–natural integrated systems, rely on biodiversity to sustain yield, quality, and ecological sustainability. With the global popularization of ecological agriculture concepts, eco-oriented tea production has emerged as a core development direction for the tea industry. However, a systematic elucidation of the mechanisms by which tea plantation biodiversity modulates tea quality, alongside standardized assessment methodologies for this biodiversity, remains inadequate. This paper comprehensively synthesizes how genetic, species, and ecosystem diversity regulate the accumulation of tea polyphenols, amino acids, and aromatic compounds—key determinants of tea quality. It evaluates mainstream assessment frameworks and identifies DPSIR (Driving Forces-Pressure-State-Impact-Response) as the most comprehensive and practical option. This paper further dissects the impacts of genetic, ecosystem, and species diversity (the three core dimensions of tea garden biodiversity) on tea quality formation. Genetic diversity shapes metabolic traits; ecosystem diversity modulates secondary metabolism via microclimate and soil; and species diversity (plants, animals, microbes) exerts synergistic effects on nutrient cycling and pest control. All these collectively improve tea sensory quality, safety, and stability. Future research should focus on plant–microbe interactions, quantitative biodiversity–quality models, and precision ecological management, laying a theoretical foundation for sustainable, high-quality tea production.

1. Introduction

Tea (Camellia sinensis (L.) O. Kuntze), native to China, is a vital economic crop and cultural heritage widely cultivated in over 60 countries and is one of the most consumed beverages globally. Tea is highly favored for its unique flavor and aroma, while its diverse bioactive compounds confer various physiological benefits, including lipid reduction, weight management, blood sugar regulation, hepatoprotection, antioxidant activity, gut microbiota modulation, anti-inflammatory effects, cardiovascular and cerebrovascular protection, and anticancer potential [1]. Consequently, tea holds significant nutritional value in the food and health sectors.

The core chemical components of tea leaves include tea polyphenols, proteins and amino acids, alkaloids, volatile substances, and others [2,3]. Tea polyphenols, accounting for approximately 20–35% of the dry weight, are a general term for polyphenolic compounds, primarily comprising catechins, anthocyanins, phenolic acids, and flavonoids [4]. Catechins constitute 60–80% of total tea polyphenols [5]. Over 20 amino acids have been identified in tea leaves, with theanine—a unique free amino acid found almost exclusively in tea—comprising about 0.5–3% of the dry weight [6]. Alkaloids account for approximately 5% of the dry weight of tea leaves [7], mainly purine alkaloids, with caffeine being the most abundant (around 2–4%) [8]. The content and proportion of these components directly influence the sensory and physicochemical quality of tea: caffeine contributes primarily to bitterness, tea polyphenols affect infusion color and taste, theanine enhances umami and briskness, and the polyphenol-to-amino acid ratio is a key indicator for evaluating tea freshness and briskness.

The accumulation of chemical components in tea leaves is jointly regulated by genetic factors, environmental conditions (e.g., altitude, climate, and soil), and agricultural practices (e.g., fertilization, pruning, and harvesting). While traditional intensive tea plantation management methods (e.g., excessive chemical fertilizers and pesticides, monoculture) have temporarily increased yields, they have led to long-term consequences, including degradation of ecosystem services, increased pest outbreaks, and declining tea quality [9]. Diversified crops exhibit different adaptive abilities in response to climate change, pests and diseases, and other environmental pressures. Maintaining biodiversity can reduce the risk of significant losses to the entire agricultural system and improve the stability of the agricultural ecosystem. The root structure and growth patterns of various plants can improve soil structure, increase organic matter content, and promote microbial diversity, thus enhancing soil fertility and productivity. In addition, biodiversity provides abundant genetic resources, laying the foundation for crop breeding and improvement [10]. Recently, growing consumer awareness of food safety, quality, and environmental sustainability has prompted a shift in tea plantation management from intensive practices towards ecologically friendly approaches. In this context, tea plantation biodiversity, as a core indicator of ecosystem health and functional integrity, has become a key research focus for understanding its regulatory mechanisms on tea quality.

The biodiversity of tea gardens includes multiple dimensions, including genetic diversity, ecosystem diversity, and species diversity, providing basic ecosystem services such as pest control, soil fertility maintenance, and microclimate regulation. Its structure and function directly affect the efficiency of nutrient cycling, natural pest control ability, and the process of tea quality formation [11]. Therefore, this article systematically elaborates on the mechanism by which tea garden biodiversity affects the formation of tea quality, as well as the technical methods for investigating and evaluating this biodiversity, aiming to provide theoretical references for the construction of ecological tea gardens and the sustainable development of the tea industry.

2. Evaluation Methods for Biodiversity

A logical framework is an important foundation for building an indicator system and a basic guide for selecting practical indicators. By establishing a logical framework, one can organize and classify biodiversity indicators and clarify the interrelationships and mutual influences between socio-economic factors and biodiversity. There are currently three commonly used logical frameworks, namely the “pressure-state-responses” framework (PSR framework), the “driving forces-state-responses” framework (DSR framework), and the “driving forces-pressure-state-impact-responses” framework (DPSIR framework) [12].

2.1. PSR Framework

The PSR framework, also known as the “pressure-state-responses” framework, was initially proposed by Rapport et al. [13] as a model framework for analyzing and describing the interaction between ecological environment quality and socioeconomic development. The Organization for Economic Cooperation and Development and the United Nations Environment Programme further improved this framework, forming the PSR model framework [13,14].

In the PSR framework for biodiversity assessment, “pressure” refers to the direct or indirect impact of human production and consumption on biodiversity; “state” refers to the condition of ecosystems, species, and genetic levels, including the products and services provided by ecosystems, as well as the condition of nonbiological and inorganic environments; and “responses” refer to methods and measures used to change these pressures or states (Figure 1). The PSR model is suitable for ecological security assessment with small spatial scales and few influencing factors.

2.2. DSR Framework

Due to the simplification of causal relationships between indicators in the PSR framework, the complexity of the system, especially the driving factors of ecological security, is ignored [16]. To overcome this weakness, the United Nations Conference on Sustainable Development established the DSR framework in 1996, replacing “pressure” with “driving forces” to form the logical structure of “driving forces-state-responses” (Figure 2) in order to better incorporate new indicators related to social, economic, and institutional aspects [17].

2.3. DPSTR Framework

The DSR framework can better characterize the impact of driving force factors on ecosystem evolution, but the definitions of driving force factors and response factors in the model are unclear. To improve the applicability of the DSR model, the European Environment Agency officially adopted the DPSIR framework (“driving forces-pressure-state-impact-responses” framework) in 1999 [19]. “Driving forces” refer to potential influencing factors such as society, population, and economy; “pressure” refers to human activities that directly affect the environment, such as material emissions, resource utilization, and land use; “state” refers to the condition of nonbiological components such as soil, air, and water, as well as the biodiversity status of ecosystems/habitats, species/communities, and genetic levels; “impact” refers to the impact of environmental conditions on biodiversity, resource availability, human welfare, and ecosystem health; and “responses” refer to the strategies and measures for planning and addressing these issues [18]. The DPSIR framework combines the characteristics of the PSR and DSR frameworks and has the advantages of comprehensiveness, systematicity, and flexibility [20]. There is a direct causal relationship chain between these five components, and they interact with each other through feedback (Figure 3) [21]. In recent years, the application of the DPSIR framework in the assessment and evaluation of water ecological security in China has continuously expanded and deepened. Zhu et al. used the DPSIR model to evaluate the vulnerability of water resources [22]; Wang et al. used this model to evaluate the ecological security of Dianchi Lake Basin [23]; and Hou et al. used this model to evaluate the ecological security of plateau lakes in Yunnan Province [24].

Tea gardens are a complex scene that combines agricultural production attributes and ecosystem characteristics, involving biological communities such as tea trees, associated plants, insects, and microorganisms, as well as being influenced by human activities such as fertilizer and pesticide application, tea tree variety selection, and land use adjustment, requiring multidimensional integrated analysis. The DPSIR framework extends from “driving forces” (such as economic demand in the tea industry) to “pressures” (such as pesticide and fertilizer inputs), then to “states” (such as changes in species richness), to “impacts” (such as tea quality and pest control efficiency), and finally to “responses” (such as green prevention and control and compound planting), fully covering the evolution path of tea garden biodiversity. The evaluation of tea garden biodiversity is not only about ecological monitoring but also needs to provide a basis for production management and policy formulation. The “response” dimension of the DPSIR framework directly connects solutions and has strong practical value. Therefore, the DPSIR framework is the optimal choice for evaluating the biodiversity of tea gardens, which can effectively promote the ecological transformation and sustainable development of tea gardens.

3. The Synthetic Pathways of Chemical Components in Tea

Tea polyphenols (TPs) are a general term for polyphenolic substances in tea, which are abundant in the chemical components that make up tea. They mainly include catechins, phenolic acids, anthocyanins, flavonoids, and flavonols [4]. Epicatechins belong to flavanol monomers and are the most abundant monomers in tea. Epicatechin (EC), epigallocatechin (EGC), epicatechin gallate (ECG), and epigallocatechin gallate (EGCG) are the main components of catechin compounds, with chemical structural formulas shown in Figure 4 [25].

The phenolic acids in tea mainly include gallic acid and its derivatives. These compounds partially participate in oxidative polymerization reactions during tea fermentation, forming secondary metabolites such as theaflavins. Most phenolic acid derivatives in tea trees originate from the shikimic acid metabolic pathway [26,27].

Anthocyanins are a water-soluble pigment in tea, mainly present in the tender shoots and leaves of tea. Although their content is low, they have an important impact on the color and functional characteristics of tea. There are six very common anthocyanins, namely pelargonidin chloride, cyanidin chloride, delphinidin chloride, peonidin chloride, petunidin chloride, and malvidin chloride. The chemical structural formulas are shown in Figure 5 [28].

Flavonoids are mainly composed of flavonols, including quercetin, kaempferol, myricetin, etc. (see structural formula in Figure 6). These compounds are relatively low in content in tea but contribute significantly to its functionality and flavor [29].

The contribution of catechins to tea quality and health benefits is extremely high, and their metabolism, especially synthetic metabolism, has always been a top priority in secondary metabolism research of tea trees. Research has found that ester-type catechins are synthesized using non-ester-type EC and EGC as precursors, involving a two-step synthesis reaction. Gallic acid (GA) is first activated by galloyl-1-O-β-D-glucosyltransferase (UGGT) to form 1-O-galloyl-β-glucose (βG), which serves as the activated acyl donor (1-O-Glc esters). Then, under the action of 1-O-galloyl-β-D-glucose-O-galloyl-transferase (ECGT), the galloyl group is transferred to the C-ring 3 position of cis non-ester type catechins to form ester-type catechins ECG and EGCG. The schematic diagram of the biosynthesis pathway of catechins is shown in Figure 7 [30]. In addition, the research team also discovered high galloylated catechins hydrolase (GCH) activity, which may belong to the tannase class and can hydrolyze ester catechins such as gallic acid and non-ester catechins. Catechins can further polymerize to form proanthocyanidins (PAs), which accumulate in large quantities in tea roots and stems, while their content is very low in leaves [31].

There are more than 20 known amino acids in tea, including glycine, histidine, arginine, glutamic acid, theanine, and other substances. Most of them exist in a free state, while a few exist in the form of chemical bonds in large molecular compounds such as proteins, peptides, alkaloids, etc. They cannot be directly dissolved in water and need to be released as free amino acids through enzymatic hydrolysis or chemical hydrolysis. Among them, theanine is a unique free amino acid in tea, accounting for about 0.5–3% of the dry weight of tea [6]. Generally speaking, as the degree of fermentation of tea leaves increases, the content of theanine decreases. White tea and green tea have the highest content of theanine, followed by yellow tea. Pu’er tea, on the other hand, has the lowest content due to long-term fermentation [32]. The synthesis of theanine has significant spatiotemporal specificity and is involved in nitrogen storage and transport in multiple organs, subject to complex molecular regulation. It is also regulated by various biotic and abiotic factors, such as salt stress, exogenous application of ABA, NO, etc. [33]. In tea, theanine not only affects the taste and texture of tea (giving it a refreshing taste) but is also closely related to the health benefits of tea [34]. It not only endows tea with its unique relaxing effect but also forms a certain balance with caffeine, allowing tea to refresh oneself without causing excessive tension and anxiety [35]. Research has found that the effect of theanine is mainly related to its potential neuroprotective, cognitive-enhancing, and stress-reducing effects [36,37].

The specificity of theanine results in tea plants having different nitrogen metabolism pathways compared to other plants [38]. Konishi’s [39] study on the metabolic pathway of theanine showed that glutamate and ethylamine are direct precursors for the synthesis of theanine, and it is speculated that, due to the decarboxylation reaction that generates ethylamine, theanine is mainly synthesized in the roots. The alanine in the roots of tea trees is decarboxylated by alanine decarboxylase (AlaDC) to produce ethylamine, which is then catalyzed by theanine synthase (TS) to produce theanine from L-glutamic acid and ethylamine [40]. Research has found that theanine is mainly distributed in the surface phloem and central xylem of tea tree roots, indicating the possibility of tissue transport and redistribution [41,42]. In recent years, significant progress has been made in the research of theanine. The synthesis and metabolism of theanine are regulated by a series of enzymes (Figure 8) [43], including TS, AlaDC, glutamine synthetase (GS), glutamate synthetase (GOGAT), glutamate dehydrogenase (GDH), and arginine decarboxylase (ADC) [44]. Among them, AlaDC and ThYD are specific enzymes in tea trees.

The alkaloids in tea are mainly purine alkaloids, accounting for about 5% of the dry weight of tea [7]. It mainly includes caffeine, theobromine, theophylline, etc. Among them, caffeine has the highest content and is the key substance that gives tea a bitter taste, accounting for about 2–5% of the dry weight of tea [8]. Caffeine is mainly synthesized in young leaves and tea flowers, and the synthesis site may be in chloroplasts. The core pathway of caffeine biosynthesis in plants is xanthine nucleoside → 7-methylxanthine nucleoside → 7-methylxanthine → theobromine → caffeine, which includes three steps of demethylation catalyzed by N-methyltransferase and one step of nucleoside removal catalyzed by ribonucleoside hydrolase (Figure 9); the main degradation pathway of caffeine is caffeine → theanine → 3-methylxanthine → xanthine → uric acid → allantoin → uric acid → urea → NH₃ + CO₂ [46].

The volatile components in tea are complex and mainly include compounds such as alcohols, esters, aldehydes, and acids, accounting for only 0.01–0.05% of dry weight [47]. The content is not large, but the types are complex, mainly including β, γ-heptenol, and α, β-heptenoaldehyde [2,48].

Regarding tea plant growth and metabolism, increased plant diversity provides richer ecological niches. Some associated plants may secrete chemicals that promote tea plant growth and stress resistance. Diverse plant communities also offer richer food sources and habitats for beneficial insects and microorganisms, forming complex ecological relationships with tea plants that help promote metabolism and increase the content of quality components like catechins and amino acids, thereby enhancing tea taste and aroma.

4. The Impact of Biodiversity on Tea Quality

There is a certain correlation between the three levels of species diversity, ecosystem diversity, and genetic diversity. Species are the foundation of biodiversity and the fundamental building blocks of ecosystems and genetics; ecosystems and genetics reflect the overall structure and function of biodiversity. The influencing factors of biodiversity on tea are shown in

Table 1. The influencing factors of biodiversity on tea.

Biodiversity	Influencing Factors
genetic diversity	structural variations
	allelic variations
	differences in resistance genes
	artificial domestication and natural genetic
ecosystem diversity	microclimate of tea gardens
	soil environment
	regional habitat
species diversity	plant
	animal
	soil microorganisms

.

4.1. Genetic Diversity

Genetic diversity profoundly affects tea quality across core dimensions such as flavor level, processing adaptability, and quality stability by regulating the synthesis, morphological characteristics, and stress resistance of key metabolites in tea. Its essence is that genomic variation (structural variation, allele differences, and interspecies introgression) drives metabolic pathway differentiation, ultimately shaping differentiated quality characteristics.

Genomic structural variations (SVs) are key genetic factors that regulate the synthesis of tea flavor compounds, directly affecting the content of core components such as tea polyphenols and anthocyanins, and thus determining quality traits. Tao et al. [49] found, through constructing 22 pan genomes of tea trees and their wild relatives, that among the 1.37 million structural variations in the tea tree genome, 22% of the gene promoter regions contain PAVs, which can significantly regulate gene expression and are enriched in flavor-related pathways such as flavonoids and amino acid metabolism. Among them, the structural variation in the promoter region of the anthocyanin synthase gene (ANS3) is highly representative. Wild relatives carry a 192 bp insertion (haplotype Hap1), which can significantly enhance the promoter activity and increase the expression level of ANS3, and the anthocyanin content is much higher than that of cultivated species, resulting in purple shoots. The cultivated large-leaf species are of the deletion type (Hap2), while the small-leaf species carry 283 bp insertions (Hap3). During domestication, Hap1 is gradually lost, anthocyanin content decreases, and buds and leaves return to a green color. This variation is directly reflected in quality differences. As a germplasm carrying similar genetic characteristics, purple bud tea has significantly higher levels of tea polyphenols, catechins, and anthocyanins than ordinary green bud tea, with outstanding antioxidant properties. However, due to the strong astringency caused by anthocyanins, its unique flavor characteristics make it suitable for developing specialty tea products rather than traditional mainstream tea. Ning et al. [50] analyzed 52 germplasm samples of purple bud tea plants and found that their anthocyanin content was 1.70–14.95 mg/g, which was higher than that of Fuding Dabai tea. The deeper the purple color of the buds and leaves, the higher the anthocyanin content, and the phenol ammonia ratio was between 2.00 and 7.36. A total of 51 samples were suitable for making green tea, and only 1 sample could be used for both red and green tea, fully confirming the regulatory effect of genetic variation on processing adaptability.

The allelic variations brought about by genetic diversity can directly alter the synthesis efficiency of flavor substances such as tea polyphenols, amino acids, and caffeine, forming variety-specific quality characteristics and determining the processing adaptation direction of tea trees [51]. The allelic variation in the polyphenol oxidase (PPO) gene in different tea tree varieties is the key to distinguishing adaptability. Through the use of RFLP technology, it was found that the polymorphism of the PPO gene HpaII enzyme cleavage site (primer L7/L8 amplification region) is closely related to the adaptability of black tea varieties. The varieties suitable for black tea production are mostly AA genotypes, and this site can be completely cleaved by HpaII endonuclease. PPO activity is high, which is conducive to the oxidation of catechins to form theaflavins and thearubigins, meeting the requirements of black tea fermentation; the varieties suitable for producing green tea and oolong tea are mostly BB or AB type, which cannot be enzymatically cleaved or are incompletely cleaved. The PPO activity is low, which can reduce the oxidation of phenolic substances and preserve the fresh and refreshing flavor of green tea [52]. In addition, some Japanese green tea varieties have premature termination codons in the CsPPO1 gene, which can also lead to a decrease in PPO activity, a decrease in EGCG content, and an increase in the EGC/EGCG ratio, making it more suitable for steaming and greening processes [53]. In terms of amino acid metabolism, population species (sexual reproduction) retain high genetic diversity due to gene recombination, and their variation in free amino acid content is significantly greater than that of clonal varieties. Kong et al. [51] identified multiple alleles significantly associated with free amino acids and tea polyphenol content through metabolomic genomic association analysis (mGWAS). The variation in these genes directly determines the phenol–ammonia ratio of tea, which is a core indicator of tea freshness. The genetic background of high amino acids and low phenol–ammonia ratio is an important characteristic of high-quality green tea varieties.

The genetic diversity of tea plants brings about differences in resistance genes, which indirectly affect quality stability by affecting their ability to cope with stress during cultivation [54]. Research has found that wild relatives of tea trees have significantly weaker resistance to anthracnose fungus than cultivated varieties. The core reason is that there is a 159 bp insertion in the promoter region of the CtLRR1 gene in wild varieties, which leads to upregulation of the gene expression. LRR1, as a negative regulator of immune response, through its high expression, can reduce disease resistance. The cultivated species lose the insertion fragment during domestication, resulting in a decrease in LRR1 expression and an increase in disease resistance. At the same time, the functional differentiation of glycosyltransferase (UGT) genes also participates in stress resistance regulation. UGT H group genes can enhance tea tree disease resistance by regulating hormone levels, and their expression changes directly affect the area of disease spots after pathogen infection, thereby reducing metabolic disorders caused by stress [49]. Germplasm with strong disease resistance is less susceptible to pest and disease stress during cultivation and does not require extensive use of pesticides. This not only reduces the negative impact of pesticide residues on quality but also reduces metabolic disorders such as abnormal accumulation of tea polyphenols caused by stress (leading to increased astringency), resulting in lower processing losses and better quality stability [55]. In contrast, clonal varieties with a single genetic background, if lacking corresponding resistance genes, are susceptible to disease and pest invasion, have significant quality fluctuations, and require more cultivation and management measures to compensate.

Artificial domestication and natural genetic drift jointly shape the genetic diversity pattern of tea trees, thereby promoting the directional differentiation of quality characteristics. The genomic analysis of 1325 Camellia germplasm by Kong et al. [51] showed that compared to wild relatives, cultivated tea trees have fixed a large number of quality-related excellent alleles through artificial selection, and the core gene family tends to be stable (35.1% of the pan genome core gene family). At the same time, it is clear that Southwest China is the origin center of tea trees, revealing the genetic diversity and domestication status of ancient tea trees. During the domestication process, artificial selection focuses on quality and stress resistance, such as the functional differentiation of UGT G group genes under artificial selection. Among them, Group III genes enrich the aroma spectrum of tea due to differences in preference for aroma substrates and respond to low-temperature stress, which meets the requirements of the cultivation environment [56].

Genetic diversity is the core foundation for the diversification of tea quality and breeding innovation. At the level of resource conservation, it is necessary to focus on preserving the genetic diversity of wild relatives and population species and exploring specific germplasm with traits such as purple buds, high aroma, and low caffeine content. At the application level of breeding, molecular marker-assisted selection (MAS) can be used to aggregate excellent alleles such as high amino acids, suitable PPO activity, and strong disease resistance and to selectively cultivate new varieties with stable quality that are suitable for specific processes. At the production and processing level, the technology should be adjusted according to the genetic background of tea trees, such as adapting high tea polyphenol germplasm to black tea fermentation and high amino acid germplasm to green tea production, maximizing the quality advantages brought by genetic diversity.

4.2. Ecosystem Diversity

Ecosystem diversity indirectly affects the synthesis of metabolites in tea plants by regulating pathways such as microclimate, soil environment, and biological interactions, thereby shaping the flavor, nutrition, and safety quality of tea. The impact mechanism can be explored through three core dimensions: soil microbial diversity, vegetation structure diversity, and regional habitat diversity.

4.2.1. Microclimate of Tea Gardens

The tea garden microclimate (light, temperature, humidity) directly affects photosynthesis, respiration, and secondary metabolism in tea plants, thereby regulating the accumulation of quality components. Intercropping systems established by increasing plant diversity can effectively regulate light conditions, temperature, and humidity, creating a suitable growth environment for tea plants.

Light quality, intensity, and photoperiod are key environmental factors for tea plant growth, development, and quality formation. Intercropping systems improve photosynthetic physiological indicators and promote photosynthesis and dry matter accumulation in tea plants by modifying the garden canopy structure and adjusting light exposure duration, intensity, and the proportion of diffuse light. For instance, tea–pear intercropping increases shading, resulting in 25% and 36% higher transpiration rate and stomatal conductance, respectively, compared to monoculture [57]; after intercropping with fir trees, the net photosynthetic rate and water use efficiency of tea plants significantly improved [57]. Tea–legume intercropping can more efficiently capture solar radiation, improve chlorophyll photosynthetic parameters and photosynthetic activity, and promote the synthesis and metabolism of free amino acids [58]. Meanwhile, appropriate intercropping shade can reduce respiratory consumption in tea plants, balancing photosynthesis and respiration, which is significant for achieving high yield and quality [59]. Additionally, tea–pear intercropping can effectively regulate light distribution, significantly reduce high light stress at noon in summer, increase the diffuse light proportion, and create favorable conditions for tea plant growth and quality enhancement [60].

Increasing plant diversity in tea gardens enables precise control of temperature and humidity through canopy shading and plant transpiration. Multiple studies show that intercropping tea with giant juncao [61], chestnut [62], pear [59], camphor, and fir trees [63] can effectively mitigate the inhibitory effect of high summer temperatures on tea plant growth, increase the content of amino acids and tea polyphenols in leaves, reduce the polyphenol-to-amino acid ratio, optimize the composition of amino acids and catechins, and ultimately improve the taste and richness of green tea, enhancing its sensory quality.

In microclimate regulation, the uneven distribution of different plants forms a complex vegetation layer, enabling more precise light regulation. Tall trees can partially shade tea plants from intense summer sunlight, preventing damage from excessive radiation. Low-growing herbs help maintain soil moisture and reduce water evaporation. Plant transpiration also regulates garden temperature and humidity, creating a more suitable microclimate for tea growth, allowing leaves to accumulate more beneficial components.

4.2.2. Soil Environment

Soil pH, total phosphorus, total potassium, and organic matter content are important soil factors affecting tea quality, playing critical roles in tea growth, development, and yield formation. For instance, nitrogen, as a fundamental element constituting core quality components like tea polyphenols and theanine, is essential for the synthesis of compounds such as catechins, theanine, and flavonoids [64]. Soil nitrogen availability directly affects the accumulation of tea quality components. As an important agroforestry practice, intercropping involves cultivating two or more plant species in the same plot under shared environmental conditions [65]. Intercropping with leguminous plants, aromatic plants, and tree species can significantly improve soil nutrient levels, manifested as increased soil total nitrogen, available nitrogen, available phosphorus, available potassium, and organic matter content. The suitable soil pH range for tea plant growth is 4.5–6.5. Adjusting soil pH through intercropping can effectively alleviate excessive soil acidification in tea gardens, enhancing soil buffering capacity and stress resistance [66]. Related studies show that intercropping tea with legumes significantly increases soil organic matter and dissolved organic carbon content, with this improvement becoming more pronounced with soil depth; furthermore, the soil nutrient improvement effect is greater with multiple plant species compared to single-species intercropping [67]. Interplanting aromatic plants (e.g., sage, patchouli, basil, perilla) in tea gardens can lower soil pH, increase soil relative moisture content, and significantly elevate organic matter, total nitrogen, available nitrogen, available phosphorus, and available potassium levels [68,69]. Intercropping tea with osmanthus and mimosa increased soil total nitrogen, total potassium, available nitrogen, available phosphorus, and organic matter content by 16.4%, 10.5%, 14.2%, 26.7%, and 28.9%, respectively [70]. Intercropping with fruit trees such as loquat, bayberry, and citrus not only increases soil nutrients but also reduces heavy metal content and pH [71]. Notably, intercropping tea with clover can effectively reduce nitrogen and phosphorus nutrient loss under natural rainfall conditions, with total nitrogen and total phosphorus loss reduced by 59.28% and 51.82%, respectively, compared to monoculture tea gardens [72], providing an effective approach for non-point source pollution prevention and efficient nutrient utilization in tea gardens.

Soil enzyme activity is an important indicator for evaluating tea garden soil fertility and is significantly positively correlated with soil nutrient content. Intercropping can enhance soil enzyme activity by altering the soil microenvironment, thereby accelerating organic matter decomposition and nutrient release [73]. For example, intercropping tea with walnut forests increased soil sucrase activity [74]. Intercropping tea with clover [75], soybean [76], alfalfa [77], and fruit trees [78] significantly increased the activity of soil catalase, urease, and phosphatase. These enzymes play key roles in soil organic carbon decomposition, nitrogen transformation, and phosphorus release processes. Increased enzyme activity leads to higher soil carbon, nitrogen, and organic matter content, providing sufficient nutrients for tea plant growth.

Regarding the soil environment, diverse plant root systems distribute differently in the soil, absorbing nutrients from various layers and regions. Their residues also supplement the soil with abundant organic matter upon decomposition, further improving soil structure and aeration. This benefits root growth and development and enables more efficient nutrient absorption, laying the foundation for quality improvement.

4.2.3. Regional Habitat

The differences in climate, soil, terrain, and other habitat conditions in different regions have formed unique ecosystems, which directly lead to the differentiation of the content and proportion of quality components such as methylxanthine and phenolic compounds in tea, reflecting the ecological law of “one soil and water nurturing one tea”.

In Abdullah et al.’s [79] study, 129 tea samples from three different ecological zones in Bangladesh, namely Panchagar, Sylhet, and Chittagong, were analyzed, revealing the significant impact of regional habitat diversity on the active components of tea. The results showed that there were significant differences (p < 0.05) in the content of methylxanthine, phenolic compounds, and antioxidant activity of tea leaves in the three regions. The highest caffeine content (103.02 ± 5.55 mg/g dry extract) and significantly higher catechin content were found in the methanol extract of tea leaves from the Panchagar region compared to other regions; the caffeine content is the lowest in the Sylhet region, and the antioxidant activity is the best in the water extract. This difference originates from the inherent differences in habitat conditions, such as climate and soil fertility, in different regions, further confirming that regional ecosystem diversity is the core factor driving the geographical differentiation of tea quality, which is in line with the ecological law of “one soil nurtures one tea”.

4.3. Species Diversity

4.3.1. Plant

Increasing plant diversity in tea gardens creates suitable conditions for tea plant growth by improving the soil environment and microclimate, thereby affecting the growth and development of both aboveground and underground parts and regulating the accumulation of quality components.

Increased plant diversity provides favorable environmental conditions, positively regulating tea plant height, tenderness of new shoots, leaf color, and root growth. For example, intercropping tea with grapes enhances the tenderness of buds and leaves, resulting in tender green, brightly colored new shoots [80]. After intercropping with pine trees, growth indicators such as net photosynthetic rate, shoot length, hundred-bud weight, plant height, canopy width, and stem base diameter were significantly improved [81]. Furthermore, intercropping tea with white clover, goldenrod, chestnut, and plum trees can significantly increase the hundred-bud weight and bud density of tea leaves [62,82,83], laying the foundation for improving yield and quality.

As mentioned, increasing plant diversity can significantly improve soil nutrient status (e.g., increasing total nitrogen, alkaline hydrolyzable nitrogen, and available phosphorus). Enhanced soil nitrogen supply promotes amino acid uptake and synthesis while inhibiting tea polyphenol synthesis, thereby increasing free amino acid content, stabilizing or reducing tea polyphenol levels, and ultimately lowering the polyphenol-to-amino acid ratio and enhancing the umami taste of tea infusion [83,84,85]. From a molecular perspective, tea–pea intercropping promotes amino acid biosynthesis and enhances tea soup umami by regulating the expression of amino acid metabolism-related genes. Simultaneously, by regulating substrate competition upstream of flavonoid biosynthesis, it slows down the synthesis of flavonoids (especially catechins), reducing tea soup bitterness [86]. Tea–chestnut intercropping can enrich volatile compounds in tea leaves, imparting a unique exotic flavor [87,88]. Potassium nutrition can further affect tea quality by regulating the synthesis of epigallocatechin gallate through the expression of key genes [89]. Non-targeted metabolomics analysis showed that the expression of 100 differential metabolites was upregulated in tea leaves intercropped with chestnuts, among which elevated levels of free amino acids like glutamic acid and aspartic acid played a key role in improving green tea flavor [87].

However, increasing the diversity of tea garden plants does not always have a positive impact without challenges. In practical operation, it is necessary to choose suitable plant species for combination (such as loquat, pear tree, perilla) to avoid introducing harmful plants that may compete with tea trees for nutrients, water, or space. Furthermore, systematic research on the synergistic mechanisms of different plant combinations is currently lacking. Complex interactions may exist among chemicals secreted by different plants, requiring further experimentation and long-term monitoring to reveal how these interactions precisely affect tea plant growth and quality. Moreover, whether the same plant combination yields consistent effects under different climatic and soil conditions remains to be studied.

4.3.2. Animal

Animal diversity in tea gardens, particularly arthropods, is the core foundation for achieving ecological pest regulation. It establishes an ecological balance between natural enemies and pests, reduces chemical pesticide use, and avoids the negative impact of pesticide residues on tea safety and flavor. Meanwhile, moderate pest stress can induce defense responses in tea plants, promoting the accumulation of defensive secondary metabolites and further enhancing tea flavor and health value.

Compared to monoculture, diversified tea garden systems harbor richer natural enemy resources, lower pest abundance, and less crop damage [90]. Intercropping tea gardens significantly increases animal species richness by increasing plant species diversity and expanding habitats and activity spaces for insects, spiders, predatory mites, and other animals [91]. Research data indicate that after intercropping, the overall abundance, density, and richness of beneficial arthropods increased by 36%, 94%, and 27%, respectively, while the number and density of harmful arthropods decreased by 38% and 41%, respectively [92], effectively establishing an ecological balance between natural enemies and pests. Flowering plants (e.g., mung bean, purple clover, castor) can provide habitats and alternative food sources (pollen, nectar) for predatory or parasitic natural enemies like ladybugs, lacewings, and parasitic wasps, promoting population growth of dominant natural enemies (e.g., Chinese lacewing, ladybugs) [93]. Volatile compounds released by some intercropped plants can achieve targeted pest regulation; for example, placing branches of Asteraceae plants in South Indian tea gardens can trap tea branch borers [94]. Some plants contain volatile compounds such as β-pinene, β-caryophyllene, and cis-carvacrol, which can repel pests like tea leafhoppers [95]. This ecology-based regulation method using plant volatiles can further reduce chemical pesticide use and ensure tea quality and safety. Additionally, mild pest stress can induce tea plants to produce defense responses, synthesizing more defensive secondary metabolites, thereby enhancing tea flavor and health quality to some extent [96].

The artificial introduction of natural enemies (e.g., predatory insects, parasitic insects, pathogenic microorganisms) to control pest populations is an important biological control method in tea gardens. For instance, field release of Trichogramma wasps can achieve 60–70% control efficacy against tea geometrids, ensuring leaf safety [97]. Releasing predatory mites in tea gardens before the peak period of pest mite occurrence and during outbreaks of tea orange rust mites resulted in 76.13% and 81.76% control efficacy against tea tarsonemid mites after 20 days, respectively [98]. The predatory bugs Stethoconus japonicus and Orius similis exhibit strong predation capacity on 3rd instar nymphs of the tea lace bug and are important natural enemies [99]. They can be released according to pest occurrence dynamics for precise pest control.

After enriching the diversity of tea garden animals, birds, and insects, the pressure of pests and diseases on tea trees is reduced by predators. For example, some bird species feed on pests such as tea geometrids, reducing their gnawing and damage on tea leaves, allowing tea trees to grow healthily, thereby ensuring the yield and quality of tea. Moreover, some beneficial insects such as bees are active in tea gardens, which helps with the pollination process of tea trees, improves their fruiting rate and reproductive ability, and is beneficial to the stability of the tea garden ecosystem and the improvement of tea quality in the long run. Animal activities can also have an impact on the soil environment of tea gardens. Earthworms and other soil animals improve the soil’s aeration and permeability by stirring the soil and decomposing organic matter, promoting nutrient cycling and release in the soil. This provides a better growth environment for tea trees, which is conducive to the development of tea roots and the absorption of nutrients, thereby affecting the accumulation of chemical components in tea and the formation of tea quality.

However, increasing animal diversity may also pose potential problems. Some animals may directly damage tea plants, e.g., wild rabbits gnawing tender shoots and leaves. Introducing animals might disrupt existing ecological balances, leading to overpopulation of certain species and negatively impacting the tea garden ecosystem. Additionally, in-depth research on the quantitative relationship between animal diversity and tea quality is scarce, requiring more scientific experiments and long-term monitoring to clarify how different animal species, abundances, and their interactions precisely affect tea quality. In practical management, how to reasonably increase and regulate animal diversity to optimally improve tea quality is an urgent problem.

4.3.3. Soil Microorganisms

Soil microorganisms, as a vital component of tea garden ecosystems, directly impact soil fertility, tea quality formation, and yield stability through their community structure and function. Soil microorganisms enhance soil ecological functions and environmental stress resistance by participating in organic matter decomposition, nutrient cycling, and energy flow. They also regulate secondary metabolic pathways in tea plants, affecting the synthesis and accumulation of key quality components such as tea polyphenols, caffeine, and theanine.

The structure and function of soil microbial communities are significantly correlated with soil fertility, tea quality, and yield [100]. Rich microbial diversity and appropriate community structure can significantly improve soil ecological functions, enhance environmental stress resistance, and promote crop yield and quality. Microbial communities can induce systemic resistance in tea plants by altering the rhizosphere microenvironment, and their metabolites or cellular components (e.g., elicitors) can regulate secondary metabolic pathways, ultimately affecting the synthesis and accumulation of key tea quality components [101]. Research [102] showed significant differences in soil fungal community structure under different intercropping modes. Intercropping can significantly increase the abundance of beneficial microorganisms (e.g., Proteobacteria, Firmicutes, Ascomycota, Zygomycota) [74,103], while reducing the relative abundance of denitrifying bacteria (e.g., in tea–soybean intercropping) [104], further improving tea plant nutrient uptake and tea quality. Functional microbial communities in soil, such as nitrogen-fixing bacteria, phosphate-solubilizing bacteria, and potassium-solubilizing bacteria, can increase tea bud density, significantly elevate total phosphorus and free amino acid content in leaves, and reduce the polyphenol-to-amino acid ratio [105]. Among them, nitrogen-fixing bacteria convert atmospheric nitrogen into ammonium nitrogen, while phosphate-solubilizing bacteria release phosphatases to decompose organic phosphorus, significantly improving tea plant nitrogen and phosphorus absorption efficiency [106]. Additionally, mycorrhizal fungi like arbuscular mycorrhizal fungi can form symbiotic relationships with tea roots. Under different nitrogen application levels, they can increase the content of soluble sugars, soluble proteins, tea polyphenols, caffeine, amino acids, and water extract in tea leaves, reduce the polyphenol-to-amino acid ratio, and significantly improve quality [107].

Earthworms, nematodes, springtails, and other soil animals improve soil aggregate structure, increase porosity, and promote organic matter transformation through feeding and burrowing activities, indirectly affecting rhizosphere microbial activity and nutrient availability, thereby creating a favorable underground environment for healthy tea plant growth [108]. For example, intercropping tea with Platycodon grandiflorus can significantly increase soil animal diversity and density, promote the survival of beneficial animals like wolf spiders and earthworms, inhibit the reproduction of pests like locust larvae and beetle grubs, and indirectly improve tea quality by enhancing the soil ecological environment [109].

Tea garden microorganisms, including bacteria, fungi, and actinomycetes, play key roles in the soil ecosystem. Regarding soil fertility, microorganisms participate in nutrient cycling and transformation. Decomposer microorganisms break down complex organic matter into simple inorganic substances, releasing nutrients like phosphorus and potassium and improving soil fertility. Fertile soil provides ample nutrients for tea growth, aiding the synthesis of quality components like polyphenols and amino acids, thereby enhancing taste and aroma. For plant health, beneficial microorganisms can form biological barriers, inhibiting pathogen growth. Some Bacillus subtilis strains produce antibiotic substances antagonistic to tea pathogens, reducing disease incidence. Healthy tea plants maintain normal photosynthesis and metabolism, ensuring stable quality. Microorganisms can also induce systemic resistance in tea plants, enhancing adaptability to stresses like drought and high temperature, allowing plants to maintain good growth under adverse conditions and ensuring quality. Regarding flavor formation, microbial metabolites can directly or indirectly influence tea flavor. Some fungi produce special aroma substances during fermentation that may be retained during processing, imparting unique flavors. Interactions between microorganisms and tea roots may also affect secondary metabolite synthesis, further enriching flavor profiles.

However, higher microbial diversity is not always better. An imbalanced community structure may lead to the proliferation of harmful microorganisms, causing diseases and negatively impacting quality. Therefore, management should employ scientific measures to regulate microbial diversity, such as rational fertilization and microbial inoculant application, to achieve balance and stability in the tea garden microbial ecosystem and maximize its positive impact on quality. Currently, the understanding of the micro-ecological relationships among soil microorganisms, soil animals, and tea roots is insufficient. How soil microorganisms and animals affect nutrient uptake and utilization and quality component synthesis during organic matter decomposition and nutrient release requires advanced biotechnologies and analytical methods for investigation. Meanwhile, maintaining the long-term diversity and activity of soil microorganisms and animals through human intervention is also a future challenge.

5. The Influence of the Carbon Cycle and Nitrogen Cycle on Tea Quality

As a typical autotrophic green plant, tea trees mainly source carbon from two pathways, with the core source being carbon dioxide (CO₂) in the atmosphere and the secondary source being soil. Tea leaves use photosynthesis to absorb light energy through chloroplasts, synthesizing organic carbon compounds such as glucose from CO₂ in the atmosphere and water absorbed by the roots; soil quality is also one of the important factors determining the quality of tea. Soil carbon and nitrogen content are important indicators of soil quality, which is of great significance for soil productivity, sustainable utilization, and environmental protection [110].

Carbon is one of the core elements in the process of life, and the main carbon reservoir on the Earth’s surface is soil. Two-thirds of the carbon exists in the form of organic carbon, which is converted into CO₂ and released into the atmosphere through soil respiration. Therefore, soil organic carbon can serve as both a carbon source and a carbon sink [111]. Soil organic carbon is an important component of soil fertility, affecting the physicochemical and biological properties of soil, and its changes are regulated by soil quality and quantity. It plays an important role in maintaining and coordinating the supply and storage of soil nutrients, regulating the types and activities of soil enzymes, controlling primary and secondary production of plants, and cycling the flow of matter and energy in ecosystems [112,113]. The high vegetation coverage, large biomass, and vigorous photosynthesis of tea trees significantly enhance the carbon sequestration capacity of tea garden vegetation, achieving soil carbon accumulation by absorbing vegetation carbon [114]. Organic litter in tea gardens, such as dead leaves, branches, and pruning residues, is an important source of soil organic carbon. During the decomposition process, the relatively stable chemical components in these fallen leaves, together with the large amount of soluble organic carbon released from pruned leaves after decomposition, constitute an important component of soil organic matter [115,116]. As mentioned earlier in this article, it can be proven that, in addition to the cycling of tea trees themselves, increasing the species diversity of tea garden plants can also enhance the accumulation of soil organic carbon. For example, intercropping tea trees with leguminous plants (such as soybeans and white clover) can increase the content of soil organic matter and organic carbon, thereby improving the quality of tea.

Soil microorganisms participate in about 90% of soil reaction processes and are sensitive to changes in soil chemical properties. They play an important role in soil organic matter decomposition and mineral nutrient cycling [117]. Soil microorganisms have advantages such as nitrogen fixation, potassium and phosphorus solubilization, organic matter decomposition, and enhanced water retention. These effects not only promote tea bud growth, tea tree metabolism, and the generation of unique tea aroma substances but also effectively prevent and control diseases and pests, significantly improving tea yield and quality [118]. Enriching the diversity of tea gardens (such as intercropping) will increase the abundance of beneficial microorganisms in tea gardens, thereby improving the absorption of nutrients and tea quality.

Nitrogen is one of the essential nutrients for the growth and development of tea plants and is a constituent element of amino acids, chlorophyll, caffeine, and other substances in the tea plant. Nitrogen nutrition can affect the photosynthesis of tea plants, the growth of buds and leaves, and the germination of new shoots. It can also directly or indirectly affect the metabolism of tea quality components such as amino acids and tea polyphenols in tea plants [119,120,121]. Multiple studies have shown that intercropping aromatic plants such as basil and perilla can lower soil pH, increase soil relative moisture content, and increase organic matter, total nitrogen, available nitrogen, available phosphorus, and available potassium content [68,69,70]. This not only enriches the biodiversity of tea gardens but also improves the quality and yield of tea.

6. Summary and Future Research Directions

The excellent quality and functional attributes of tea stem from its complex chemical composition, and tea garden biodiversity influences quality formation through multidimensional regulation: soil microorganisms promote nutrient uptake and induce plant resistance, optimizing the accumulation of quality components; intercropping systems regulate metabolic processes by improving the microenvironment and nutrient supply; and natural enemy networks ensure safety and flavor through pest and disease control. Combined with the genetic diversity of tea plants, these factors collectively constitute an important ecological foundation for producing high-quality, safe tea.

Although current research has preliminarily revealed the regulatory role of biodiversity on tea quality, understanding of its impact on metabolic mechanisms remains largely correlative, lacking in-depth molecular mechanistic analysis. Future research should focus on the following directions: first, strengthening studies on the correlation between genetic diversity and environmental adaptability in tea plants, providing precise guidance for high-quality variety breeding and genetic resource conservation; second, integrating molecular biology, genetics, and multi-omics (transcriptomics, proteomics, metabolomics) technologies to clarify the molecular mechanisms by which specific microbial or intercropping plant signals are perceived and regulate key metabolic pathways in tea plants; third, establishing a correlation model of “biodiversity–ecological function–quality formation–economic output,” quantifying the contribution coefficient of species richness to yield and quality, and clarifying the economic value of biodiversity enhancement; fourth, exploring ecological management strategies based on specific quality targets, achieving “on-demand customization” of tea production, providing a scientific basis for management decisions, and promoting high-quality, sustainable development of the tea industry.

From an economic and practical standpoint, increasing biodiversity may increase costs and management difficulties. How to maximize economic benefits while ensuring quality improvement is a practical issue for the tea industry. Reasonable subsidy policies and management models need development to encourage tea farmers to actively participate in biodiversity enhancement practices. Only by comprehensively considering these factors can the role of biodiversity in improving tea quality and promoting sustainable industry development be fully leveraged.