This study was conducted on tea plantations in Huoshan County, Anhui Province, China (115°52′–116°32′E, 31°03′–31°33′N). The research area is characterized by yellow-brown soil and a north subtropical humid monsoon climate, providing abundant rainfall, light, water, and heat resources. The study sites are primarily located in low-altitude areas. The annual average temperature is approximately 15.3°C, and the annual average precipitation reaches up to 1366 mm.

The experiment commenced in June 2016. Soil samples from tea gardens were collected during seven days without rain. Sampling elevation ranges, and gradient divisions were determined based on the natural distribution of tea tree planting in the study area. The samples were taken from three elevation ranges: 100–500 m, 500–600 m, and above 600 m, with the maximum elevation not exceeding 700 m.

The elevation bands (100–500 m, 500–600 m, >600 m) were determined based on the topographic and ecological characteristics of the study area, ensuring each band represents a distinct environmental zone. Though the intervals are uneven, they were selected to reflect meaningful ecological transitions and minimize intra-band variability. Statistical adjustments were applied to account for area differences across bands.

A series of sampling points were selected within the study area ( Figure 1 ). These points are distributed across three elevation ranges: 100–500 m, 500–600 m and > 600 m, and the sampling numbers were 22, 14, 18, respectively. The average horizontal distance between sampling points were 100meters. All plots were positioned on similar slopes and slope directions to eliminate heterogeneity caused by terrain variations. Soil samples were collected from five randomly selected points within each plot at a depth of 0–20 cm. The samples were mixed and transported to the laboratory in an aluminum boxes, where they were air-dried and stones and debris were removed. The air-dried soil samples were sieved through a 2 mm sieve for available nutrient analysis and Mg, a 0.149 mm sieve for elemental determination including S and Cu. The soil samples were dried to a constant weight to measure soil moisture. Simultaneously, litter from the tea plantations was collected at each elevation sampling point during the soil sampling. Care was taken to avoid already decomposed litter. The collected litter was brought back to the laboratory and powdered for chemical property analysis. This comprehensive collection approach ensures that both soil and litter samples provide a thorough understanding of the nutrient dynamics within the tea plantations at different elevations.

Air-dried soil samples were passed through a 2 mm sieve for pH determination using a glass electrode; 1:2.5, soil/water ratio and available P analysis using a flow injection auto analyzer (Smartchem 200 Alliance Corp. France) (Xie et al., 2019). Samples passed through a 0.149 mm sieve were digested using a digester (FOSS. Tecator Digestor 20), and the total nitrogen content of soil (SN) and litter samples (TN) was measured with a flow injection auto analyzer. Soil organic C (SOC) and total organic C (TOC) of litter were determined using the potassium dichromic (H₂SO₄-K₂Cr₂O₇) method. The total phosphorus (TP) of the litter was measured after digestion with H₂SO₄. Available magnesium (Mg) was extracted using a 2 mol L⁻¹ KCl solution and determined by atomic absorption spectrometry. Available potassium (K) was extracted by a 0.2 mol L⁻¹ NaHCO₃ + 0.01 mol L⁻¹ EDTA + 0.01 mol L⁻¹ NH₄F and measured using atomic absorption spectrometry. Available S was extracted with 0.01 mol L⁻¹ Ca₃(PO₄)₂ and determined using a turbidimetric procedure (Schmidt et al., 2011). Available copper (Cu) was extracted with 0.005 mol L⁻¹ DTPA at pH 7.30 and determined by inductively coupled plasma atomic emission spectrometry (ICP-AES).

Tea plantation growth was assessed by measuring the height and width of the plants at different altitude gradients using a meter ruler. The ground diameter of the tea plants was measured using a diameter at breast height (DBH) ruler. Average values are calculated for each measurement.

A second-order polynomial function was used to examine the relationship between available S and pH, SOC, SN, available Mg, available P, and available K. This function was also used to determine the relationship between available S and the dominant soil nutrient gradient estimated by principal component analysis (PCA). SOC, SN, pH, and all measured soil available nutrients except available S were used for PCA to obtain the structure. The discriminant analysis examined whether plots from different elevation ranges could be separated based on SOC, SN, pH, and five available soil nutrients. Pairwise correlation analysis determined the variables contributing most to these separations. One-way analysis of variance (ANOVA) examined the effects of elevation on soil and litter chemical properties and on eight, crown width, and ground diameter. Post hoc Student’s t-tests were conducted for multiple comparisons. All statistical analyses were performed using JMP 9.0 (SAS Institute, Cary, NC, USA) and the significance was P<0.05.

The plant height and width of tea plantations showed a decreasing trend with increasing altitude ( Table 1 ). Compared to plants height at 100–500 m and 500–600 m, those at > 600 m exhibited 5.59% and 5.12% reduction, respectively. The height of tea plants at 100–500 m was similar to that at 500–600 m. Regarding width, plants at > 600 m were 6.00% narrower compared to those at 100–500 m and 5.62% narrower compared to those at 500–600 m. Again, the difference in width between 100–500 m and 500–600 m was insignificant. The diameter of tea plants also decreased with the increasing altitude, but this decrease was not significant.

The content of available P, K, and S increased with elevation. On the contrary, changes in other major nutrients with elevation were not consistent, with soil pH ranging between 4.12–4.31. The highest content of SN and available Mg were observed at 100–500 m, while the lowest content was at 500–600 m. There was no significant difference in the TOC, TN, litter TP, SOC, or available Cu across different altitudes ( Table 2 ). Canonical discriminant analysis suggested substantial separation of tea plantation by elevation gradient ( Figure 2 ); follow-up analysis showed that soil pH, available Mg, and S contributed to Canonical 1, with available S being the most influential. Soil pH and available Mg significantly contributed to Canonical 2, with available Mg contributing the most.

Using a second-order polynomial function, available S was found to be negatively correlated with SOC, SN, and available K but showed no significant difference from soil pH, available P, and available Mg ( Figure 3 ). The results of PCA indicated that soil SOC, SN, and available K had a higher degree of interpretation for PC1, while soil available Mg, pH, and available Cu had a higher degree of interpretation for the second principal component (PC2) ( Figure 4 ). According to the second-order polynomial function analysis, available S showed a significant correlation with PC1 ( Figures 4 , 5 ). In the subsequent pairwise correlation analysis between soil available nutrients, pH, SOC, SN, and PC1, SOC, SN, available K, available P, and available Cu were most relevant to PC1 ( Table 3 ).

Plant height, width, and diameter are important indices for evaluating plant growth. Elevation is a significant environmental factor that affects the distribution and growth of species. The impact of elevation on environmental conditions includes variations in temperature, water availability, light, and soil nutrients, affecting resource availability. As elevation changes, plants will show different adaptability in terms of growth and metabolism (Conlisk et al., 2017; Louthan et al., 2015). As elevations increase, temperatures decrease, precipitation and air humidity rise, and water availability generally improves, reducing water stress on plants (Soomro et al., 2024). In this study, tea plant height and width significantly decreased with the elevation increase, while the ground diameter showed a decreasing trend, albeit not significantly. This negative correlation between altitude and plant height and coarseness is consistent with previous findings (Pellissier et al., 2010), indicating that altitude has a strong effect on the growth of tea plantations. There are various hypotheses about the factors limiting tree growth at high altitudes, with lower temperatures being a common explanation. Research suggests that freezing at high altitudes can lead to poorer water conditions, thereby limiting tea growth (Fang et al., 2021).

Compared to other soil elements, available S is more sensitive to changes in the ecological environment. Despite its low content, S plays a vital role in plant growth (Shi et al., 2016). The decrease in S content with increasing altitude is likely due to a reduced soil temperature, which reduces soil microbial activity. Furthermore, the availability and decomposability of substrates releasing S also influence soil available S levels (Xiao et al., 2015). S deposition is a critical component of soil S cycling (Xiao et al., 2015), and changes in deposition levels with elevation gradients may alter available S levels. When comparing the dynamic changes of sulfur in different ecosystems, forest and farmland systems exhibit significant differences. The distribution of sulfur forms in forest soil is significantly affected by acid deposition, with soil acidification intensifying in high sulfur deposition areas. Sulfate leaching is accompanied by loss of base ions, resulting in nutrient deficiency. In contrast, soil acidification in farmland is mainly caused by intensive agricultural activities (such as fertilizer use), and its sulfur dynamics are more regulated by human management measures. Besides, the altitude difference affects the AS content of tea gardens through multiple mechanisms: in high-altitude areas (>600m), low temperatures (decreasing by 0.5 °C for every 100m increase) inhibit microbial activity, resulting in a significant decrease in organic sulfur mineralization rate. However, the enhanced adsorption of iron and aluminum oxides increases the proportion of fixed sulfur; The alternation of dry and wet conditions at mid altitude (100-500m) promotes the sulfur redox cycle; Heavy precipitation (>1200mm/yr) in low altitude areas leads to 60% higher sulfate leaching than in high altitude areas. In addition, microbial activity, ecosystem species composition, and productivity also affect available S distribution (Li et al., 2017).

The content of available P and K increases with elevation due to several factors. Low-altitude areas experience more human interference, leading to significant leaching losses of available P and K. In contrast, the lower temperature reduces the absorption and utilization of available P and K by tea plants. Tea plantations thrive in acidic and slightly acidic yellow and red soils. Soil acidification in tea gardens is affected by several factors, including the secretion of acidic substances by tea plant roots and the use of fertilizers, along with increased acid deposition. In our study, the soil pH of tea gardens ranged between 4.12–4.31, lower than the optimal pH range of 4.5 to 6.5 for tea growth. When soil pH falls below 4.0, tea plant growth and development are inhibited, affecting tea quality and yield. Therefore, addressing soil acidification in tea gardens is crucial.

Discriminant analysis showed significant changes in available S and Mg with altitude ( Figure 2 ; Table 4 ), revealing their importance in nutrient management and evaluation in tea plantations at different elevations (Nguyen and Goh, 1992). Amino acids, crucial for tea quality, are influenced by the application of S and Mg, highlighting the importance of magnesium sulphate for tea quality (Ruan et al., 1998).

The stoichiometry between available S and other elements indicates that other nutrients can influence available S (Shi et al., 2016; Tipping et al., 2016). In our study, available S showed a significant correlation with SOC, SN, and available K ( Figure 3 ), supporting the notion that S is an important component of soil organic matter (Biederbeck, 1978). Our findings further validate this view regarding effective nutrients. Soil S exists in organic and inorganic forms, cycling through oxidation, reduction, mineralization, and fixation processes (Wilhelm Scherer, 2009). Available S, which includes adsorbed S, soluble S, and some organic S, is the main source of plant S (Yang et al., 2007) and effectively reflects the soil-plant nutrient status.

Studies on the C: N: S ratios in Australian and global soils have shown that the stable C: N: S ratio in soil organic matter is consistent across different soils worldwide (Kirkby et al., 2011). This suggests a fundamental property of the organic material, with a stable relationship between C, N, and S. However, P is mainly derived from the soil parent material, leading to variability in different studies.

The significant correlation between PC1 and available S based on various soil nutrients, including pH, suggests that available S is closely related to soil environmental conditions and major elements ( Figures 4 , 5 ) (Chen et al., 2016; Fan et al., 2017). Available S has a strong sensitivity to the soil environment, making it a good indicator of the effect of soil nutrient changes. Pairwise correlations analysis showed that SOC, SN, and available K were most relevant to PC1 ( Table 4 ), indicating SOC strongly contributes to PC1 and potentially alters available S across elevation gradients. However, no significant correlation between pH and PC1 was observed ( Table 4 ), indicating environmental conditions of microbial communities in the tea gardens may not substantially change within the narrow elevation range investigated in this study. Future studies should consider a larger altitude gradient to examine pH changes and their contributions to nutrient availability.

Biogeochemical processes better predict the soil nutrient stoichiometry than individual biological or geochemical processes (Luo et al., 2015). In non-arid regions, soil available S content correlated positively with SOC and SN, but this correlation disappeared in the arid areas (Luo et al., 2015), indicating that climate is an important factor affecting soil biogeochemical processes. Future research should explore tea plantations in different climatic regions.

Considering the unevenness of sampling, it may exists difficulty in effectively separating altitude effects from local interference factors (Ma et al., 2022). Besides, the limitation of this study is that it did not systematically quantify the differences in tea tree varieties, changes in microbial community structure, and the interactive effects of long-term climate fluctuations on sulfur cycling under different altitude gradients, resulting in uncertainty in the mechanism explanation.