Effect of Soil pH on the Uptake of Essential Elements by Tea Plant and Subsequent Impact on Growth and Leaf Quality

Abstract

Tea plant is an acidophilic plant, and soil pH has an important effect on the absorption and enrichment of elements, tea plant growth and quality. In this study, rhizosphere soils and leaves of tea plants from 30 tea plantations were collected to determine soil pH and multi-element content of soil and leaves of tea plants, to obtain and validate key elements that are enriched by pH affecting tea plants, and to analyze the effects of pH on the growth and quality of tea plants. The results showed that soil pH significantly affected the enrichment of 15 elements by tea plants, and the enrichment coefficients of 11 elements (C, Mg, Si, N, P, Mn, Sr, Cd, S, Ca and Sb) tended to increase significantly with the increase of soil pH, while the opposite was true for the other four elements (Cu, Rb, Ba and Al). TOPSIS analysis showed that soil pH had the greatest effect on tea plant enrichment of seven elements, namely N (100%), Mn (43.32%), C (39.22%), P (27.66%), Sr (15.30%), Mg (13.41%) and Ba (10.47%). Pot experiments with tea seedlings also verified that soil pH significantly affected the enrichment of tea leaves for seven key elements. Moreover, with the increase of soil pH, the growth indexes, photosynthesis indexes and quality indexes of tea seedlings showed a significant upward trend. Interaction analysis showed that the enhanced enrichment of N, Mn, C, P, Sr and Mg by tea plants was beneficial to increase the photosynthetic capacity of tea plants, promote the growth of tea plants and improve the quality of tea leaves. This study provides an important theoretical basis for the cultivation and management of tea plants.

1. Introduction

Soil is rich in elements and is the source of element absorption and accumulation in plants, which are absorbed into the soil through the root system and then transported to different parts of the plant for plant growth and development [1,2]. Plant uptake of soil elements is characterized by specificity, which comes from the selective nature of element uptake by plants themselves, on the one hand, and changes in element uptake by plants due to changes in environmental factors, on the other [3,4,5,6,7]. Soil pH has been described as a “major soil index” that affects biogeochemical cycling, and changes in soil pH can significantly affect elemental forms in the soil, which in turn affects elemental uptake by plants [8]. Appropriate soil pH facilitates the maximum uptake of soil elements by plants. Too low pH reduces the effectiveness of secondary macronutrients and increases the effective micronutrients in the soil. In addition, too low pH inhibits plant growth and not only reduces the uptake of macronutrients by the plant, but also hinders the utilization of micro-nutrients by the plant [9,10].

Tea plants are acidophilic plants; however, either too much acid or too much alkali can affect the growth of tea plants [11]. Plant growth is affected, to a large extent, due to the obstacles in the uptake and transportation of soil elements, which in turn triggers changes in the metabolism of the plant body, which is ultimately presented in the plant morphology [12]. Kang et al. [13] found that soil pH is a key factor driving changes in soil microbes, and that microbial changes result in changes in the effectiveness of soil nutrient elements. Xu et al. [14] found that soil pH can alter soil enzyme activity, which in turn affects soil nutrient element cycling and plant uptake of elements. Ye et al. [15] studied the effect of soil pH on the growth of tea plant from the perspective of metabolomics and found that, with the decrease of soil pH, the nutrient elements in the rhizosphere soil of tea plant are easily immobilized into the organic form, which in turn reduces the available nutrient content of the soil and lowers the nutrient uptake capacity of tea plants. Arafat et al. [16] analyzed the effect of soil pH on the transformation of rhizosphere nutrient elements of tea plant from a microbial perspective, and found that lowering soil pH reduces the activity of nutrient-transforming enzymes in the soil, the available nutrient content of the soil declines, and tea plant growth is hindered. Xie et al. [17] found that as soil pH decreases, nutrient elements in the soil are converted to the organic form, and the uptake and utilization efficiency of nutrient elements by the root system of tea plant declines. It can be seen that soil pH can significantly change the elemental form of the soil and thus affect the uptake and utilization of elements by plants. However, a large number of studies have focused on analyzing the effects of soil pH on soil N, P, K, and organic matter conversion and uptake by tea plants, while little has been reported on soil multi-elements analysis. Numerous elements are required for plant growth and different elements have significant effects on plant growth [18,19]. The rhizosphere is the location where the plant is in direct contact with the soil, and changes in rhizosphere soil pH have the most pronounced effect on the uptake of elements by the plant root system. The authors hypothesized that there may be significant changes in the uptake and enrichment of multi-elements in the soil by tea plants following changes in soil pH, and that such changes may affect the growth and quality of tea plants. Deep excavation of the key elements absorbed and enriched by tea plants due to soil pH changes is of great significance for tea plant cultivation management and exogenous regulation of tea plant growth.

Accordingly, in this study, the rhizosphere soil of tea plants and tea leaves from 30 tea plantations were collected, and inductively coupled plasma mass spectrometry (ICP-MS) was used to determine the elemental contents of the soil and tea leaves, to analyze the effect of soil pH on the multi-element enrichment of tea leaves and to obtain the key elements that were significantly affected by soil pH. On this basis, tea plants were planted in pots using soil culture to adjust soil pH and verify the effect of pH on the enrichment of key elements. In addition, the role and function of key elements on plant growth were analyzed and further verified, with a view to providing an important theoretical basis for the cultivation and management of tea plants.

2. Materials and Methods

2.1. Test Tea Plantation and Sample Collection

Anxi County, Quanzhou City, Fujian Province, is a major tea-producing area in China. In this study, 30 tea plantations in Anxi County, each with an area of more than 1 ha, were selected, and the tea plant varieties grown were all Tieguanyin. The specific distribution of the 30 tea plantations was shown in Figure 1, and detailed information on the location points was shown in Figure S1. During 2–17 May 2022, rhizosphere soils of tea plants and corresponding leaves were collected from 30 tea plantations on sunny days from 9 a.m. to 12 p.m. and from 2 p.m. to 5:30 p.m. to determine soil pH and elements. Five sampling sites were selected from each tea plantation, i.e., five replicates, with an interval of more than 15 m between each sampling site. Ten tea plants were selected from each sampling site, and rhizosphere soil and tea leaves were collected and thoroughly mixed separately, i.e., one replicate.

The sampling method of rhizosphere soil of tea plant was to remove the dead leaves and branches on the soil surface, shovel the soil layer-by-layer until 45–50 cm deep, dig out the tea plant, shake off the soil on the root system of the tea plant and the soil that was still attached to the root system of the tea plant was the rhizosphere soil. The soil sampled at each sampling point was approximately 100 g. The sampling method for tea plant leaves was to collect one bud and two leaves of the tea plant, with a sample size of about 200 g for each sample point.

2.2. Determination of Soil pH

Five replicates of each soil sample were performed using a pH meter (PB-10, Sartorius). The soil-water ratio was set at 1:2.5.

2.3. Multielement Determination of Soil and Tea Leaves

Fresh tea leaves were cleaned with deionized water to remove any embedded dust and impurities. The leaves were then dried at 80 °C until they reached a constant weight. This was followed by grinding the dried leaves into a fine powder and passing it through a 75 μm nylon mesh. A total of five independent replicates were made for each sample. For soil collection, each sample was air-dried at room temperature. The dried soil was then ground into a powder and also passed through a 75 μm nylon mesh. Five independent replicates of each sample were used for elemental determination.

Soil and tea samples were accurately weighed at 0.5 g and placed in a high-pressure digestion tank, with five replicates for each sample. 5 mL of HNO₃ (Sinopharm, Beijing, China) was then added to each treatment. The sample container was tightly screwed and placing into an oven for 4 h at 185 °C. After 4 h, the acid in the tank was removed for 1 h. The digestion tank was then washed with deionized water and transferred to 50-mL volumetric flasks. The sample was then diluted 10-fold with deionized water. The elemental content of the sample was then determined using inductively coupled plasma mass spectrometry (ICP-MS, Nexion 2000, Thermo Scientific, Boston, MA, USA). Three measurements were taken for each sample and the mean was calculated. The instrumental parameters of the ICP-MS were 1350 W of RF power, 0.94 L/min of carrier gas flow rate, 0.40 L/min of auxiliary gas flow rate, 4.5 mL/min of helium flow rate, 2 °C of the atomization chamber temperature, 0.3 r/s of sample lift rate, 7 mm of sampling depth and 50 ms of residence time, and the nebulizer was PFA, the sampling cone was nickel cone, the acquisition mode was peak hopping and the number of scans was six times.

The digestion of blank and standard solutions was performed using the same digestion method applied to test samples with a standard curve established. Six elements (Sc, Ge, In, Rh, Re and Bi) were utilized as internal standards to determine the limit of detection (LOD). This concentration was defined as the standard deviation of the blank sample multiplied by three. In addition, two standard control samples of tea (GBW10016, National Research Center for National Standard Substances (NRCNSS), Beijing, China) and soil (GBW07403, National Research Center for National Standard Substances (NRCNSS), Beijing, China) were digested using the same procedure as the samples to validate the analytical methods. Measurements were performed three times, with each value calculated as the average of the three measurements.

2.4. Tea Plant Seedling Planting and Sample Sampling

This study analyzed the effect of soil pH on the elemental enrichment of tea plant leaves in 30 tea plantations and found that changes in soil pH significantly affected the enrichment of seven elements (N, Mn, C, P, Sr, Mg and Ba) in tea leaves and should lead to changes in the growth and quality of tea plants. Therefore, this study further used soil potting tea seedling experiments and adjusted soil pH to verify the effect of tea plant leaves on the enrichment factors of the seven key elements after soil pH changed. At the same time, the growth indexes, photosynthetic indexes and quality indexes of tea seedlings were measured to analyze the effects of changes in the enrichment coefficients of key elements on the growth and quality of tea plants.

The soil sample was collected from a heavily acidified tea plantation and first air-dried to remove moisture. The soil was then sieved through a 40-mesh sieve for further analysis. The soil pH was 2.7 and the contents of N, Mn, C, P, Sr, Mg and Ba were 4132.16, 28.34, 156.48, 46.27, 2.46, 192.15 and 54.32 mg/kg, respectively. The soil was packed in pots of 10 kg each and lime (CaO, Sinopharm, Beijing, China) was used to mix and agitate with the soil, and then the soil pH was adjusted so that the soil pH was 3, 4 and 5, with three independent replicates for each treatment. One-year-old Tieguanyin tea seedlings (23 cm of plant height, 0.25 cm of diameter) with relatively uniform growth potential were selected and transplanted into pots, three plants per pot, and cultured for 60 days. During planting, potting tea seedlings were watered daily in the morning and evening using distilled water at a rate of 200 mL per pot per time. At the end of cultivation, the contents of seven key elements in the leaves of tea seedlings were determined, as well as growth indexes (leaf area and plant height), photosynthesis indexes (chlorophyll content and net photosynthetic rate) and quality indexes (content of tea polyphenols, theanine, caffeine and soluble sugar) of tea seedlings.

The plant height of the tea plant was measured directly using a straightedge, where the point of separation of the root and stock to the highest point of the tip was recorded. To determine the leaf area, five inverted second leaves of tea plants per pot were randomly selected, and their leaf length and leaf width were measured. Leaf area was calculated using the formula leaf area = leaf length × leaf width × 0.7, and the average value of these measurements was taken as one replicate. Chlorophyll content was determined using a chlorophyll analyzer (SPAD-502 PLUS, Tokyo, Japan). Finally, the Net photosynthetic rate was determined using the LI-6400XT Portable Photosynthesis System (Li-Cor, Lincoln, NE, USA). One bud and two leaves of tea seedlings were collected, and each replicate was about 15 g for the determination of tea polyphenols, theanine, caffeine and soluble sugars in tea leaves. The method of determination was described in “Technical specification for tea production, processing and safety testing” [20]. Briefly, tea polyphenol content was determined using a forintol colorimetric method, theanine content was determined by high-performance liquid chromatography, caffeine content was determined by ultraviolet spectrophotometry and soluble sugar content was determined by anthrone colorimetric method. The determination of the seven key elements in tea leaves was carried out using the methods described in Section 2.3.

2.5. Statistical Analyses

The raw data obtained were statistically and analytically analyzed for mean, standard deviation and coefficient of variation using Excel 2020. Rstudio software (version 4.2.3) was used for graphical production and model construction of the post-statistical data, in which box plots, heat maps, redundancy analysis and interaction network diagrams were produced using the R packages gghalves 0.1.4, pheatmap 1.0.12, vegan 2.6.4 and linkET 0.0.7.1, respectively.

3. Results and Discussion

3.1. Validation of Quality Control Methods

In this study, the standards of tea (GBW10016) and soil (GBW07403) were analyzed, and the results showed that 59 elements were detected in the tea standard, and the recoveries of each element ranged from 83.51% to 108.57%, and the detected concentrations were consistent with those of the standards (Table S1). The soil standards detected 64 elements, and the recoveries of each element ranged from 87.00% to 105.08%, and the detected concentrations were consistent with the standards (Table S2). It can be seen that the extraction method and test method of this study are reasonable, and the test results can be used for further analysis.

3.2. Analysis of Soil pH and Tea and Soil Elements

Tea plant is an acidophilic plant that is suitable for tea plant growth when the soil pH is between 4.5 and 5.5, and unsuitable for tea plant growth when the pH is less than 4.5 or greater than 5.5 [21,22,23]. In this study, rhizosphere soil pH of tea plants in 30 tea plantations was collected and determined, and the results showed (Table S3) that the distribution of soil pH was in the range of 3.29 to 5.58, with a mean value of 4.51 and a coefficient of variation of 17.29%. Among them, 43.33% of tea plantation soils had pH < 4.5, 50.00% had 4.5 ≤ pH < 5.5, and 6.67% had pH > 5.5. Thus, the rhizosphere soils of tea plants collected in this study basically cover the range of pH values that would have an impact on the growth of tea plants.

Elemental analysis of rhizosphere soil samples of tea plants from 30 tea plantations showed (Table S4) that a total of 71 elements were detected in the soil, and the distribution of the total elemental amount ranged from 17.83 to 21.76 g/kg, with a mean value of 19.77 g/kg and a coefficient of variation of 5.40%. A total of 61 elements were detected in leaves of tea plants from 30 tea plantations, and the distribution of total element amount ranged from 85.33 to 179.04 g/kg, with a mean value of 114.56 g/kg and a variation coefficient of 21.52% (Table S5). This result illustrates that although there is a small difference in the total amount of elements in different tea plantation soils, the difference in soil pH leads to a significant difference in the total amount of elements in the leaves of the tea plant. Accordingly, the study further analyzed the distribution of elements in soil and tea leaves at different pH values, and the results showed (Figure 2) that the same 61 elements were detected in tea leaves and soil and the distribution of their contents was very similar, with most elements in tea leaves being higher than in soil. In addition, 10 elements such as Br, Zr, Pd, In, Te, Hf, Os, Ir, Pt and Hg were detected in soil but not in tea samples. Soil is an important source of plant elements, and plants take elements from the soil through the root system and transport them to the leaves for accumulation, but there are some differences in the ability of plants to take up and transport different elements [24,25]. This result illustrates that tea leaves are somewhat exclusive and do not selectively accumulate some elements. In addition, although soil pH is different, the distribution of elements in tea leaves and soil still has some similarity, and only the content of elements in tea leaves has changed significantly. It can be seen that soil pH significantly affects the enrichment of different elements in tea leaves.

3.3. Effect of Soil pH on Elemental Enrichment Coefficients of Tea Plant Leaves

The high or low content of elements in plant leaves does not really reflect the enrichment capacity of plants for soil elements, which can be analyzed by enrichment coefficients [26]. When the enrichment coefficient is greater than 1, it indicates that the plant has an enrichment function for the element and is strong in enrichment and vice versa [27]. In order to analyze the effect of soil pH on the enrichment of tea plants for elements at a deeper level, this study further analyzed and found that for the elemental enrichment coefficients of tea leaves with different pH soils, the results showed (Table S6) that the enrichment coefficients of 44 elements in leaves of tea plants planted in soils with different pH were always less than 1, and there was the possibility of enrichment coefficients greater than 1 for only 17 elements. Accordingly, the study was further simulated to analyze the relationship between 17 elements with enrichment coefficients greater than 1 and soil pH, and the results showed (Figure 3) that the correlation between the enrichment coefficients of two elements and soil pH did not reach a significant level, namely B (p = 0.28) and W (p = 0.18), while the enrichment coefficients of the remaining 15 elements reached a significant level (p < 0.05) in relation to soil pH. Among the 15 elements mentioned above, the enrichment coefficients of 11 elements, namely C, Mg, Si, N, P, Mn, Sr, Cd, S, Ca and Sb, showed a significant increasing trend with the increase of soil pH, while the enrichment coefficients of four elements, namely Cu, Rb, Ba and Al, showed a significant decreasing trend. TOPSIS was further used to analyze the weights of soil pH effects on the enrichment coefficients of the 15 elements and to screen the key elements, and the results showed (Figure 4) that for the weighting of soil pH effects on the elemental enrichment coefficient, seven of the 15 elements were greater than 10%, namely N (100%), Mn (43.32%), C (39.22%), P (27.66%), Sr (15.30%), Mg (13.41%) and Ba (10.47%).

Accordingly, in this study, soil-cultivated tea plant seedlings were used and then soil pH was adjusted to verify the enrichment capacity of tea plant leaves for seven key elements at different pH values. The results showed (Figure 5) that the enrichment coefficients of N, Mn, C, P, Sr and Mg in tea leaves showed a significant increasing trend with increasing soil pH, as shown by their enrichment coefficients increased from 5.14 to 13.11, 2.23 to 8.20, 3.19 to 10.50, 2.00 to 5.22, 1.38 to 4.22 and 2.53 to 5.82, while Ba showed a significant decreasing trend, as shown by the decreases of 2.74 to 0.90. The results validated the conclusions of the above analysis and also indicated that pH did significantly affect the enrichment of tea plants for these seven key elements. It has been reported that N, C, P, Mn and Mg are essential elements for plant growth and are closely related to plant development and photosynthesis, and deficiencies in these elements are very likely to lead to a reduction in the photosynthetic capacity of the plant, slowing down its growth and lowering its quality [28,29,30,31,32]. Sr and Ba are not essential elements for plant growth. Sr has a significant effect on plant growth and photosynthesis [33]. Ba is toxic to plants and its accumulation in plant leaves reduces plant photosynthesis capacity and inhibits plant growth [34]. Therefore, the authors hypothesize that increasing soil pH will enhance the N, Mn, C, P, Sr and Mg enrichment capacity of tea leaves and decrease the Ba enrichment capacity, which in turn will enhance the photosynthesis capacity of tea plants, promote the growth of tea plants and improve tea quality.

3.4. Effect of Soil pH on the Growth, Photosynthetic Capacity and Quality of Tea Plant Seedlings

Based on the above roles of the seven key elements in the plant growth process, this study posits that changes in soil pH could potentially affect the enrichment of these seven elements. Consequently, this could affect the growth, photosynthesis capacity and quality of tea plants. To test this hypothesis, this study adjusted the pH of soil potting tea plant seedlings to three different levels (pH 3, 4 and 5). The effects on growth indexes, photosynthetic physiological indexes and quality indexes were then analyzed. The results showed (Figure 6) that the growth indexes (leaf area and plant height), photosynthesis physiological indexes (chlorophyll content and photosynthetic rate) and quality indexes (tea polyphenols, theanine, caffeine and soluble sugar) of tea plants showed a significant increase with the increase of soil pH. Specifically, leaf area and plant height increased from 27.90 to 35.66 cm² and 25.88 to 33.80 cm, respectively. Chlorophyll content and photosynthetic rate increased from 16.50 to 24.61 SPAD and 0.71 to 1.12 μmol /m²s, respectively. Tea polyphenols, theanine, caffeine and soluble sugar increased from 16.14 to 26.18 mg/kg, 2.10 to 3.72 mg/kg, 2.23 to 3.77 mg/kg and 3.12 to 6.36 mg/kg, respectively. These findings suggest that an increase in soil pH could potentially enhance the growth, photosynthesis capacity and quality of tea plants.

In addition, this study further analyzed the interactions between the seven key elements and tea plant growth indexes, photosynthesis indexes and quality indexes. Redundancy analysis showed (Figure 7A) that there were significant correlations between six key elements (N, Mn, C, P, Sr and Mg) and growth indexes, photosynthesis indexes and quality indexes of tea plants. Correlation network analysis showed (Figure 7B) that there was a significant positive correlation between the six key elements (N, Mn, C, P, Sr and Mg) and the growth indexes, photosynthesis indexes and quality indexes of the tea plant, while Ba showed a significant negative correlation. The results validated the above hypothesis, and also illustrated that, with the increase of soil pH, the enrichment capacity of N, Mn, C, P, Sr and Mg by tea leaves was enhanced, and that of Ba was reduced, which was conducive to the enhancement of photosynthesis capacity of tea plants, the promotion of their growth and the improvement of the quality of tea leaves.

4. Conclusions

The study explored the effect of different pH soils on elemental enrichment in tea leaves. It found that the same 61 elements were detected in soils and tea leaves, and the distribution of these elements in tea leaves was highly similar to soil. However, most of the elements were present in higher contents in tea leaves compared to soil. Soil pH significantly affected the enrichment of elements in tea leaves, and a significant correlation was identified between the enrichment coefficients of 15 elements and soil pH, where 11 elements showed a significant and positive correlation, and four elements showed a significant and negative correlation. TOPSIS analysis revealed that soil pH had the greatest effect on the enrichment coefficients of the seven elements. Soil cultivated tea plant seedlings were used and adjusted soil pH to verify the enrichment capacity of tea plant leaves for seven key elements at different pH values, and this result validated the above findings. Further analysis of the interactions between the seven key elements and the growth, photosynthesis capacity and quality indexes of tea plants revealed that there was a significant positive correlation between the six key elements (N, Mn, C, P, Sr, and Mg) and the growth indexes, photosynthesis indexes and quality indexes of tea plants, while Ba had a significant negative correlation with these indexes. It can be seen that with the increase of soil pH, the enrichment ability of tea leaves for N, Mn, C, P, Sr and Mg was enhanced, and that of Ba was reduced, which was conducive to enhancing the photosynthetic ability of tea plants, promoting the growth of tea plants and improving the quality of tea leaves. This study provides an important reference for regulating the cultivation and management of tea plants.