Study on the mechanism of modified biochar in reducing fluoride content in tea leaves and improving soil environment.

1. Introduction

Fluorine (F) is a ubiquitous environmental pollutant commonly found in soils and aquatic systems. In China, the background concentration of fluorine in soil is 478 mg/kg—nearly 2.4 times the global average of 200 mg/kg [1]. Fluorine in soil originates from two primary sources: natural geogenic inputs and anthropogenic activities. The dominant natural source is fluorine-rich parent materials—such as fluorite, apatite, and volcanic rocks—which release fluorine into soils and groundwater via geochemical processes including rock–water interaction and chemical weathering [2]. The second type comes from human activities; fluorine is widely used in industrial activities, including mining, coal combustion, and the production of fluoride-containing fertilizers and pesticides, with large amounts of fluorine being discharged into soil and water bodies [3].

Tea, derived from the leaves of *Camellia sinensis* plants, is a globally consumed non-alcoholic beverage. As such, tea cultivation serves as a vital driver of local economic development. Notably, tea plants exhibit a pronounced hyperaccumulation capacity for fluorine [4]. Research indicates that upon uptake from the soil, fluorine is predominantly sequestered in the leaves; concentrations in mature or senescent leaves can reach up to 2000 mg/kg—approximately three-to fourfold higher than those in young leaves [5]. Excessive dietary fluorine intake poses significant public health risks, including endemic fluorosis, which is associated with neurodevelopmental impairments (e.g., reduced IQ) and skeletal abnormalities (e.g., increased bone fragility) in children [6]. Tea plants mainly absorb water-soluble fluorine (such as F^–, fluorine-aluminum complexes, etc.) in the soil through their root systems, either actively or passively. This process is mediated by transport proteins such as CLC, FEX, and ABC. The fluorine is then transported upward along the xylem to accumulate in the leaves. It can also be transferred from old leaves to new shoots, but not to the underground parts [7]. Moreover, stringent regulatory limits on fluorine content in tea—imposed by trade agreements in numerous jurisdictions, including the European Union—have substantially constrained China’s tea export market. Consequently, mitigating fluorine accumulation in tea leaves has become an urgent priority, bearing direct implications for both public health and the sustainable development of the national tea industry. The majority of fluorine in tea originates from the soil, and its bioavailability—and thus plant uptake—is strongly governed by the concentration and speciation of soil fluorine, particularly water-soluble fluorine. Soil properties such as pH and organic matter content exert indirect but critical control over fluorine bioavailability by modulating fluorine speciation and solubility dynamics [8]. Accordingly, reducing and managing fluorine levels in tea garden soils represents a scientifically sound and practically effective strategy for lowering fluorine accumulation in harvested tea. To date, multiple soil remediation techniques—including soil washing, electrokinetic remediation, and chemical stabilization—have been empirically validated for their efficacy in reducing soil fluorine concentrations [[9], [10], [11]]. Among these, the application of exogenous amendments appears to offer superior long-term stability and sustainability in fluorine mitigation. For example, specific elements can be added, such as alkaline earth metal Ca, transition metal Fe, and chalcogen Se; alternatively, other materials with soil remediation and improvement functions can be used, including humates, soil conditioners, and functional fertilizers [12,13]. Biochar, a material widely used in soil remediation and environmental improvement, has also been extensively applied in fluorine reduction with remarkable results [14]. Owing to its highly porous structure and abundant surface functional groups, biochar effectively adsorbs fluorine species in soil. Furthermore, its inherent alkalinity enables pH regulation in acidic tea garden soils, thereby decreasing the solubility and bioavailability of fluorine [15]. Studies have shown that the application of biochar can increase the pH of tea garden soil by 0.5-1.5 units and significantly reduce fluoride ion activity [16].

From the perspective of biochar itself, its impact on soil is governed primarily by two interrelated factors: (i) the physicochemical properties of the biochar—including the type of feedstock biomass and the pyrolysis conditions used during production; and (ii) the application rate. Notably, the relationship between biochar dosage and soil fluorine reduction is nonlinear. At low application rates, biochar effectively decreases water-soluble fluorine concentrations; however, this beneficial effect reverses when the application rate exceeds 5%, leading to diminished or even counterproductive outcomes. Consequently, optimizing fluorine mitigation requires maintaining a low, yet effective, application rate while selecting biochar types with proven fluorine-removal capacity. Mineral constituents in biochar—particularly calcium, magnesium, and aluminum—can react with fluoride ions to form stable, insoluble precipitates (e.g., CaF₂ and MgF₂), thereby further immobilizing fluorine and reducing its bioactivity [17]. Moreover, biochars derived from different feedstocks exhibit distinct fluorine-reduction efficiencies. For instance, application of bamboo-derived biochar in tea gardens reduced fluorine concentration in harvested tea leaves by 36.69% [18]. This method has opened up a new way of using biochar to reduce fluorine. However, such biochar cannot yet be widely used universally, and the acquisition of its raw materials is rather difficult in the field of biomass. Another situation is one piece of biochar be made from pig manure and straw can adsorb fluoride, with an adsorption capacity of 73.66 mg/g [19]. This type of biochar can achieve a relatively high fluoride reduction effect, but it also has a relatively high content of heavy metals, which can easily lead to secondary pollution. Therefore, it is not a convenient method. From the biomass resources generated by the tea garden itself, Swapnila Roy et al. found that biochar prepared from tea waste could adsorb and remove fluorine from fluorine-containing wastewater at a rate of 52.5 mg/g [20]. In a study by Li et al., lanthanum-modified tea residue biochar was able to adsorb and remove fluorine from water, with a maximum theoretical adsorption capacity of 47.47 mg/g [21]. Although pig manure biochar has a higher adsorption capacity for fluorine, when considering the fact that biochars such as pig manure biochar and chicken manure biochar have excessively high heavy metal contents, their application to tea garden soils is likely to cause secondary pollution [22]. The comparison results of each method are presented in the Supplementary Materials. Among these studies, only the fluoride adsorption and reduction efficiency of biochar in aqueous environments and its underlying mechanisms have been investigated. Most of these common biochars reduce the free state fluorine in aqueous solutions through surface ion adsorption and electrostatic adsorption. The modified biochars lower the fluorine content in water by combining with fluorine ions through the co-precipitation of metal ions and fluorine ions. Meanwhile, the increase in pH caused by biochars also inhibits the substitution of fluorine ions for hydrogen in hydroxyl groups. However, there are still research gaps regarding the mechanism of biochars in reducing the absorption of fluorides in the soil-plant system, especially their impact on tea plants.

Based on the principles of circular agriculture, this study employs biochar derived from spent tea residue to ameliorate tea plantation soil and mitigate fluoride uptake and accumulation in tea plants. This approach simultaneously delivers multiple co-benefits, closing the loop within the tea plantation industrial chain, reducing carbon emissions, and promoting the sustainable and healthy development of the tea industry. The research specifically investigates the efficacy of this novel biochar material in lowering soil fluorine concentrations—with particular emphasis on water-soluble fluorine, the predominant bioavailable form absorbed by tea plants—and elucidates its migration behavior both in soil and within tea plant tissues. Furthermore, the study examines correlations between fluorine dynamics and changes in key soil physicochemical properties, as well as shifts in soil microbial community composition, thereby aiming to uncover the underlying mechanisms through which biochar improves soil health and suppresses fluorine bioavailability and plant uptake.

2. Materials and methods

3. Results and discussion

3.1. Biochar characteristics

3.1.1. Functional group and component analysis

The FTIR spectra of biochar, synthesized from varying raw materials and subjected to different protocols of K₂SO₄ impregnation pretreatment and CaO doping, are presented in Fig. 1. Fig. 1a illustrates the differences in functional groups among biochar samples without K₂SO₄ impregnation. A strong, broad absorption peak centered around 3450 cm^-1 is observed, which is primarily attributed to the stretching vibration of hydroxyl groups (O-H) [27]. All samples exhibit a distinct absorption peak at 2920 cm^-1, corresponding to the stretching vibration of alkyl groups (C-H) [28]. Notably, the peak intensity is highest in the C3 treatment group—a trend that is consistently reflected in Fig. 1b. For the TW series, the total peak area increases initially and then decreases, peaking at the C3 treatment; in contrast, the KSC series shows a continuous increase in total peak area. These observations indicate that K₂SO₄ is more favorable for the formation of long-chain alkanes. The peak at 1630 cm^-1 is mainly derived from the stretching vibration of carbonyl groups (C=O) or carbon-carbon double bonds (C=C). The C=O/C=C peak intensities in samples C3 and KSC3 are significantly higher than those in other samples, suggesting that this specific treatment may introduce additional functional groups containing carbonyl or carbon-carbon double bonds. After being converted into biochar, its pH value also increased, changing from an acidic feedstock to alkaline biochar (Table S1).This phenomenon is ascribed to the abundant alkaline functional groups (–COO^–, –O^–) on the BC surface [29].

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Fig. 1. Comparison of FTIR Spectra among Different Samples. (a) CaO-doped modified biochar. (b) CaO-doped modified biochar after K₂SO₄ salt washing pretreatment.

Ca can regulate plant growth by influencing the absorption of nutrients by plants [30]. Therefore, X-ray diffraction (XRD) was used to analyze the changes in the speciation and relative content of Ca in the samples. Fig. 2a focuses on biochars without pretreatment and raw materials. By comparison with standard cards, a weak diffraction peak appears at 20 = 14.7° in unmodified tea waste (corresponding to the (100) crystal plane of CaSO₄, with a standard peak position of 14.664°), indicating that the raw material contains a small amount of CaSO₄. Thus, it can be preliminarily determined that the raw material contains trace CaSO₄. However, with the continuous increase in CaO doping and its proportion, this peak disappears in modified biochars (e.g., the KSC series), suggesting that the form of calcium derived from CaSO₄ undergoes transformation during pyrolysis. Fig. 2b shows the biochars with pretreatment and raw materials. As the CaO doping ratio increases (from KSC1 to KSC3), a significantly intensified diffraction peak emerges at 20 = 29.4° (matching the (104) crystal plane of CaCO₃, standard peak position 29.398°). Concomitantly, the peak near 48.5° in both Fig. 3a and (b) (corresponding to the (116) crystal plane of CaCO₃, standard peak position 48.503°) is synchronously enhanced, further confirming the formation of CaCO₃. Notably, the secondary strong peak of CaSO₄ (e.g., the (110) crystal plane at 25.5°) is absent in modified biochars, verifying the completeness of its transformation. The amorphous structure peaks in XRD patterns of some samples at about 22° and 44° are attributed to the carbon gradually transforming into graphite after pyrolysis. No solid components of potassium were detected in all XRD data, thus it can be preliminarily inferred that potassium exists in biochar in forms such as ions. XRD data clearly reveal the transformation pathway of solid-phase components, CaSO₄ in the raw material is converted into CaCO₃ after pyrolysis, a process dominated by the amount of CaO doping. Pretreatment only affects surface properties without interfering with the evolution of crystal structures.

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Fig. 2. XRD analysis of all raw material and biochar. (a) CaO-doped modified biochar. (b) CaO-doped modified biochar after K₂SO₄ salt washing pretreatment.

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Fig. 3. Microscopic characterization of biochar surface structure and Ca/K element distribution. (a) and (c) are SEM images of samples C3 and KSC3 at a scale of 100 um, respectively. (b) and (d) correspond to the magnified images of the white-framed regions in (a) and (c), with a scale of 20 um. (e) and (g) represent the distribution of Ca element in samples C3 and KSC3, respectively. Similarly, (f) and (h) represent the distribution of K element.

After analyzing the surface functional groups and solid-phase components of the biochars, SEM and EDS analyses were performed on the two biochar groups that exhibited excellent fluoride removal efficiency in subsequent verification experiments. These analyses aimed to examine the specific morphology of the solid surface and the distribution of key elements (K and Ca). As presented in Fig. 3a and b, the surface of C3 featured a fine and complex pore structure. Both treatment groups displayed surface pore structures, with no pore collapse observed. As shown in Fig. 3c and d, the pore structure on the surface of KSC3 was not prominent; instead, the deposition of solid particles was clearly visible. In the EDS images (Fig. 3e and f), the deposited particles on the C3 surface were identified as the element Ca. Combined with the XRD patterns, these particles were confirmed to be solid CaCO₃. This phenomenon was more pronounced in KSC3 (Fig. 3g and h). This is consistent with the BET test results (Table S1), where the specific surface area increased without significant abrupt growth. Meanwhile, numerous tiny solid particles were observed adsorbed on the carbon surface. Regarding the distribution of K(Fig. 3f and h), it was uniformly distributed throughout the biochar. The K element labeling in the KSC3 treatment group (subjected to potassium sulfate washing pretreatment) showed no significant difference from that in the C3 treatment group, primarily due to the low concentration of potassium sulfate used (only 6 mmol L^-1). However, the added potassium, mainly in the form of potassium ions, facilitates direct uptake and utilization by plants.

3.1.2. Inorganic nutrient (N/P/K) contents in biochar

The content of nutrients (N/P/K) in biochar is a typical indicator for assessing the soil improvement potential of biochar, and there is also a close relationship between nutrient content and tea plant stress resistance. Marked differences were observed in the effects of treatments on N, P, and K contents. For total nitrogen (TN) content (Fig. 5-a), TW-KS exhibited the highest value (84.19 g/kg), followed by the biocharized TWBC (81.50 g/kg), with TW showing the lowest (50.21 g/kg). Since CO₂ can inhibit the volatilization reaction of nitrogen, reduce the migration of nitrogen to gaseous products, thereby reducing total nitrogen loss [31]. This indicates that potassium sulfate washing pretreatment enhances nitrogen retention in tea waste biomass. In contrast to this trend, TN content increased progressively with rising CaO doping ratios in the C series, peaking at 48.46 g/kg under 7% CaO modification. As shown in Fig. 4b, total phosphorus (TP) content followed a similar trend to TN. In the KSC series, both TN and TP decreased consistently with increasing CaO ratios. P can be retained through the formation of K-phosphates and Ca-phosphates; however, under KS treatment, it is more prone to dissolution and loss in the liquid phase [32].

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Fig. 4. Determination of Nutrient Elements in Feedstock Samples and Biochar. (a) Total nitrogen content in each sample. (b) Total phosphorus content in each sample. (c) Total potassium content in each sample. Different lowercase letters indicate significant differences between treatments at p < 0.05.

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Fig. 5. Water-soluble fluoride content. (a) Water-soluble fluoride content in tea under KSC treatments. (b) The FKSC treatment group series. (c)Variations in water-soluble fluoride content in soil among different treatment groups. Different lowercase letters indicate significant differences between treatments at p < 0.05.

For total potassium (TK) content (Fig. 4c), TW-KS displayed the highest value, significantly exceeding other baseline groups, which is attributed to the direct introduction of potassium ions via KS pretreatment. In contrast, TWBC exerted a limited effect on K enhancement. TK content fluctuated across treatments, exceeding that of the C series at the same ratios only when CaO was applied at 3% and 5%. This suggests that KS pretreatment and CaO can synergistically promote K retention under specific CaO ratios, with such synergy being ratio-dependent. TK showed weak correlations with TN and TP, which is attributed to differences in their sources (K being directly introduced by KS) and retention mechanisms (K relying more on physical adsorption, whereas N and P depend on biological adsorption or chemical binding), leading to distinct response pathways to modification conditions [33].

3.3. Changes in the soil environment

3.3.1. The pH of soil

As can be seen from the pH values of biochars mentioned earlier (Table S1), since biochars are alkaline, their application to soil results in an increase in soil pH. As illustrated in Table 1, the pH value of the CK treatment group was only 5.11, which was lower than that of the H₂O treatment group (5.35), mainly because the applied fluoride-containing nutrient solution was acidic. Under different treatment groups, the pH values of the soil were around 6.0. Meanwhile, at low concentrations, the pH value increased as the content of CaCO₃ in the biochar increased. However, at high concentrations, as in the case of a 7% doping ratio, the disruption of the precipitation-dissolution equilibrium of CaF₂ led to an increase in fluoride content, and the soil became even more acidic. Among them, the KSC3 treatment group, which exhibited a good fluoride reduction effect, had a slightly higher pH value, and this phenomenon was more pronounced in the FKSC series. The data analysis results indicate that under sufficient biochar application rates, the fluoride-containing environment did not weaken but rather may have significantly enhanced the biochar’s capacity to buffer soil acidity and its effect on increasing pH through certain mechanisms, such as the secretion of organic acids by soil microorganisms.

Table 1. pH of every group’s soil.

Empty Cell	pH	Empty Cell	pH
CK	5.11	H2O	5.35
KSC	5.34	FKSC1	5.78
KSC1	5.91	FKSC3	6.29
KSC3	6.05	FKSC5	6.15
KSC5	6.13	FKSC7	6.30
KSC7	5.97

3.3.2. Changes in soil nutrient elements

The absorption of N/P/K is one of the response mechanisms of plants to environmental stress [36]. Study had shown that potassium can regulate the translocation of fluoride in tea plants and reduce fluoride accumulation in young leaves [37]. Fig. 6a reflects the changes in soil TN content under different treatments. The CK group had the lowest TN content, significantly lower than that of other treatments, thus preliminarily indicating that fluoride treatment reduces soil nitrogen content. It can be observed in the KSC series that under low doping ratios, the total nitrogen content exhibited minimal variation. In the FKSC treatment groups, it was noted that the FKSC3 treatment group had the lowest nitrogen content, which was consistent with the performance of KSC3. The difference was that due to the doping of fulvic acid fertilizer, the FKSC treatment groups showed more significant differences in the variation of nitrogen content. The applied fulvic acid organic fertilizer may contain a higher proportion of ammonium nitrogen than biochar, leading to more absorption by tea plants and thus resulting in lower soil TN content. Fig. 6b shows the soil TP content, which generally exhibited a trend consistent with that of nitrogen. Interestingly, the TP content in the KSC3 treatment group was the lowest among the same series (0.273 g/kg), and the TP content in the FKSC3 treatment group was also the lowest in its series. Phosphate can strongly adsorb on the surface of the carrier through inner-layer complexation, competing directly with fluorine for active adsorption sites [38]. Therefore, the decrease in P concentration is beneficial for the adsorption and fixation of fluorine on the biochar, which is consistent with the results of tea tree fluorine content. The consistent variation trend in N/P contents is mainly attributed to the similarity in the absorption patterns of N and P. Their absorption depends on the transformation of N/P speciation by soil microorganisms, such as the nitrification process mediated by nitrogen-fixing bacteria, and the chelation of cations by organic acids secreted by microorganisms to release P [39].

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Fig. 6. Changes in contents of N/P/K nutrient elements in soil. (a) Total N. (b) Total P. (c) Total K. (d) Available K. Different lowercase letters indicate significant differences between treatments at p < 0.05.

Fig. 6c and d presents the results of soil TK and AK contents. The CK group had the lowest TK content (19.5 mg/kg), and the H₂O group (23.0 mg/kg) showed a slight increase, indicating that water has little effect on TK. In the KSC series, the KSC5 group (88.0 mg/kg) and KSC1 group (85.0 mg/kg) had the highest TK contents, while the KSC3 group (58.0 mg/kg) had a lower TK content, significantly different from other KSC treatments. In the FKSC series, the FKSC1 group (74.0 mg/kg) and FKSC5 group (69.0 mg/kg) had higher TK contents, while the FKSC3 group (39.5 mg/kg) and FKSC7 group (47.5 mg/kg) had lower TK contents, indicating that in co-application treatments, TK content is affected by the synergistic effect of fulvic acid fertilizer and biochar, and medium-low dosage co-application is more conducive to potassium retention [40]. The variation trend of K content differs from that of N and P, mainly because plants absorb K⁺ through ion channels, and this absorption process is regulated by the signal transduction of Ca²⁺ [41].

3.4. Analysis of soil microbial community

Changes in environmental factors exert a profound impact on microbial communities [42]. The impact of biochar on microbial communities, as focused on in this study, is the core and crucial part of the biochar-based fluoride reduction mechanism. Therefore, this study analyzes and investigates the changes in soil microbial communities. A total of 143 core OTUs were shared across all treatments, reflecting the fundamental stability of the soil bacterial community (Fig. S4a). As depicted in Fig. 7a, in KSC3, the abundance of Bradyrhizobium was significantly higher than that in FKSC groups. Given Bradyrhizobium‘s role in nitrogen fixation and close rhizosphere nutrient interactions with plants, nutrient gradients likely stimulated its enrichment [43]. Additionally, Pseudomonas showed marked abundance in KSC3, it has the properties of fluoride tolerance and fixation ability, and can alleviate plant fluoride stress [44]. Its abundance is significantly correlated with the environmental fluoride content. Among groups, the KSC3 exhibits the highest species abundance, which is consistent with its optimal fluoride reduction efficiency (Fig. S4b). Ascomycota is a type of fungus highly correlated with environmental fluoride content [45]. Among them, Ascobolus can be regarded as an indicator group for fluoride pollution. As can be seen from Fig. 7b, the Ascomycota proportion is relatively high in the KSC3 treatment group, which indicates that the fluoride content in the soil is relatively high. This is inversely proportional to the fluoride content in tea plants, which is in line with the experimental expectation. At the same time, according to the change in soil water-soluble fluoride content in Fig. 5c, the modified biochar has indeed fixed and transformed the fluoride into other bound states. This is consistent with the fluoride fixation function of Ascomycota [46]. In the KSC group, its abundance decreases, indicating that the increase in soil pH and organic matter content induced by pure biochar treatment has affected the growth of Ascomycota (Fig. S3). Soil organic matter significantly promotes the growth and abundance of ascomycetes, and ascomycetes may be involved in the decomposition and transformation of organic matter [47]. There is a positive correlation between them. In the KSC3 group, the abundance of Ascomycota increases significantly; the addition of Ca is the key differential factor here, suggesting that this mineral element promotes the growth and reproduction of Ascomycota. In the FKSC3 group, the abundance of Ascomycota decreases again. This indicates that the addition of fulvic acid fertilizer exerts an adverse effect on Ascomycota. The abundance of Frankia increased in FKSC3, potentially due to the anaerobic, organic matter-enriched microenvironment created by the treatment, which aligns with its nitrogen-fixing and decomposing traits. Pseudomonas showed a marked abundance in KSC3 (Fig. 7c), likely in response to changes in root exudates, thereby enhancing plant-microbe interactions. Gammaproteobacteria maintained stable abundance across multiple treatments, verifying its “broad adaptability”, as a “functional buffering taxon” in soil, it contributes to maintaining ecological resilience [48]. As observed in Fig. 7d, when the abundance of Ascomycota changes, the abundances of other taxa such as Basidiomycota and Mortierellomycota exhibit certain inverse or synergistic variation trends, which may indicate competitive or cooperative interactions between them. For instance, during the decomposition of plant residues, Ascomycota and Basidiomycota may compete for substrates [49]. Organic matter serves as a carbon source/nitrogen source, driving competition among microorganisms. Some microorganisms, such as Pseudomonadaceae, produce antagonistic metabolites (such as antibiotics) during the decomposition of organic matter, inhibiting the growth of competitors [50]. Calcium ions combine with organic acids to dissolve insoluble calcium-bound phosphorus in the soil, increasing the availability of phosphorus and alleviating the competition pressure for phosphorus resources among microorganisms, thereby indirectly promoting coexistence [51].

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Fig. 7. Community Composition Analysis. (a) Analysis of bacterial species present in different groups. (b) Fungal species. (c) Cluster analysis of bacterial community abundance similarity among different groups. (d) Fungal community.

The Principal Coordinate Analysis (PCoA) in Fig. 8a shows that the PC1 axis accounts for 43.46% of the variance, with the total explanatory power reaching 75.17%. Bray-Curtis dissimilarity analysis reveals a clear separation between the CK group and the KSC/FKSC groups along the PC1 axis, while the KSC and FKSC groups are differentiated along the PC2 axis—indicating that biochar and its composite treatments significantly restructure bacterial communities.

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Fig. 8. Soil microbial community diversity analysis. (a) PCoA of bacterial community composition in soil. (b) NMDS analysis of bacterial community aggregation-dispersion degree in soil. (c) PCoA of fungal community composition in soil. (d) NMDS analysis of fungal community aggregation-dispersion degree in soil. (e) Hierarchical Clustering Analysis of bacterial community in soil. (f) Fungal community in soil.

In contrast to bacterial communities, fungal communities exhibit distinct variation patterns [52]. NMDS based on Bray-Curtis distances yields a stress value < 0.2, confirming the reliability of the ordination (Fig. 8b). The KSC treatment groups exhibit tighter clustering, reflecting higher similarity in community composition; in contrast, the FKSC groups (compared to CK) show greater dispersion after fulvic acid addition, suggesting pronounced community differentiation and heightened bacterial sensitivity to fulvic acid fertilizer. The community composition of other groups as shows in Fig. S6. As depicted in Fig. 8c, the PC1 axis accounts for 74.15% of the total variance, and the PC2 axis explains 15.8%, with a cumulative explanatory rate of 89.95% for the two axes—indicating that the ordination effectively captures the key features of community structure. The ANOSIM test revealed no significant differences between groups (R = 0.1852, P = 0.700), suggesting that the fungal community structures across treatment groups did not undergo obvious separation. This lack of significant change in fungal communities may be attributed to the tolerance of fungal taxa to soil amendments [53]. In the NMDS analysis (Fig. 8d), sample points showed substantial overlap with no distinct clustering trends, reflecting a high degree of similarity among communities and thus further validating the PCoA results. The community composition of other groups as shows in Fig. S7. This phenomenon may arise because biochar did not significantly alter soil physicochemical properties—for instance, soil pH remained weakly acidic, and organic matter content showed only minor changes (Fig. S2). Additionally, studies have indicated that fungal communities tend to exhibit strong stability and are not easily perturbed by exogenous additives in the short term. Hierarchical clustering analysis (Fig. 8e) further illustrates that FKSC3 forms a distinct clade with a unique bacterial community structure, whereas the bacterial community under FKSC5 is more similar to that of the CK group. Notably, key ASVs exhibit significant abundance shifts, with potential enrichment in biochar-treated groups (KSC series)—a pattern that may be linked to oligotrophic carbon transformation or metal resistance processes [54]. More strikingly, KSC3 and KSC5 display minimal dissimilarity and highly similar compositional distributions, indicating that biochar exerts a relatively consistent regulatory effect on soil bacterial communities. Collectively, these observations suggest that biochar acts as the primary driver of community differentiation, with fulvic acid further amplifying such divergence (Fig. 8f).

4. Conclusions

In this study, ten distinct biochar types were synthesized and their fluoride reduction effects were verified through controlled pot experiments. Results demonstrate that the KSC3 treatment group reduced fluoride content in tea leaves by 48.6%, a value significantly greater than the 34.02% fluoride reduction rate achieved with bamboo charcoal and other materials [18]. Notably, KSC3 attained comparable fluoride mitigation efficacy within just 21 days of application—matching the performance of conventional biochar applied at the same dosage over a six-month period [16]. Mechanistically, modified biochar reduces free fluoride ions (F^–) in the soil through adsorption and precipitation on the one hand. On the other hand, it improves the soil environment for tea plant growth by enhancing the soil’s physicochemical properties such as organic matter content and N/P/K nutrient elements, thereby strengthening the stress resistance of tea plants. Furthermore, Furthermore, biochar promotes the differentiation of soil microbial communities and increases the abundance of microbial communities with the ability to fix fluoride [46,55], thereby hindering the absorption of fluoride by tea plants. During the short-term pot experiments, only the fluoride reduction effect of biochar could be verified. For the cultivation of perennial tea trees, the long-term effect of biochar still needs to be verified. Additionally, the universal effect of modified biochar on multiple tea tree varieties can also be an area for in-depth research in the future.

CRediT authorship contribution statement

Cheng Yi: Writing – original draft. Chengxuan Zhou: Methodology. Yan Mo: Software. Xianghua Peng: Investigation. Liangyang Chen: Conceptualization. Siyu Xin: Data curation. Yao Liu: Software. Yu Xie: Visualization, Validation. Zhi Zhou: Supervision. Wei Luo: Writing – review & editing.