The Soil Ecological Stoichiometry Characteristics of the Highest Latitude Areas in the Main Tea-Producing Regions of China

Abstract

To investigate the contents of carbon, nitrogen, and phosphorus in tea plantation soils and their ecological stoichiometric characteristics, as well as their response to environmental factors in high-latitude regions of China, soil samples from 0 to 20 cm depth were collected from tea plantations at different altitudes and cultivation years in the main tea-producing areas of Shaanxi Province. These samples were used to determine the soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) contents and to calculate their stoichiometric ratios. The findings revealed the following: the average soil SOC and TN content in tea gardens were 13.15 and 1.30 g·kg⁻¹, respectively, exceeding the national soil average. These values met the Class I tea garden fertility standards. However, the average soil TP content, at 0.45 g·kg⁻¹, fell below the national soil average, meeting the Class II tea garden fertility standards. In tea gardens, the average ratios of carbon to nitrogen (C:N), carbon to phosphorus (C:P), and nitrogen to phosphorus (N:P) in the soil were 10.42, 30.98, and 3.32, respectively. These ratios were all lower than the national soil average, indicating relatively high phosphorus availability but nitrogen deficiency in tea garden soils. As altitude increased, there was a decline in soil SOC content, C N, and C P ratios, followed by a subsequent increase. No significant changes were seen in TN, TP, and N P ratio in the soil, but there was an increase in SOC content, TN content, and C P ratio during cultivation. The N-to-P ratio initially increased before decreasing, while the C-to-N ratio decreased before increasing. Soil TP content did not change significantly. The study recommends careful nitrogen fertilizer application in tea garden management to balance nitrogen and phosphorus.

1. Introduction

“South Tea North Migration” refers to a series of agricultural practices and research activities involving the transplantation of tea plants originally grown in the moist and rainy climate of the southern regions to drier and less rainy northern areas. This concept breaks the traditional belief that high-latitude regions are unsuitable for tea cultivation. Via a series of technological innovations and planting practices, the cultivation area for tea plants has successfully expanded northward, shifting China’s tea production areas more than 300 km northward. This phenomenon has created the remarkable miracle of tea cultivation in the Qinba Mountain area at latitude 33 degrees 44 min north.

Carbon (C) is a fundamental element of all life forms, while nitrogen (N) and phosphorus (P) are essential and limiting nutrients for plant growth [1,2]. Covariance and independence are observed in the interactions of the elements C, N, and P, which play crucial roles in the cycling and transformation of soil nutrients [3]. The study of ecological stoichiometry involves examining the balances and interactions of energy and chemical elements (mainly C, N, and P) within ecosystems. This scientific approach is based on analyzing the patterns of multiple elements’ fluctuations in physical, chemical, and biological processes [4]. It not only reflects the nutrient status of soils but also reveals nutrient availability and limiting factors, providing new perspectives and tools for studying soil-plant interactions and the mechanisms governing C, N, and P cycling and balance [5,6]. The applications of ecological stoichiometry have been extensive in different areas of research, encompassing the growth of individual plants, dynamics of populations, succession of communities, cycling of nutrients, identification of limiting elements, efficiency of nutrient utilization, and even global biogeochemical cycles of carbon, nitrogen, and phosphorus [7,8]. From a global perspective, the soil C:N:P ratios vary significantly across different countries: the United States (110:10:1.4), Scotland (113:10:1.3), India (144:10:1.9), and New Zealand (147:10:2.5) [9]. Within China, there are also significant differences in soil C:N:P ratios across different climatic zones, with tropical and subtropical soils having significantly higher C:N ratios than temperate soils. Globally, the molar ratio of C:N:P in the 0–10 cm soil layer remains relatively stable at 186:13:1. The relative stability of soil C:N is due to a good coupling relationship between soil C and N, which responds similarly to the same environmental factors [10]. When the C:N ratio is low, the decomposition rate of soil organic matter accelerates, promoting the release of soil carbon, which is not conducive to the accumulation of soil organic matter. Therefore, a decrease in the soil C:N ratio can reflect a decline in soil quality. The soil C:P ratio is an important indicator of the soil’s phosphorus mineralization capacity [11], and it also serves as an indicator of the potential for microbial mineralization and release of phosphorus from soil organic matter or the absorption and fixation of phosphorus from the environment [12].

Over the past few years, there have been productive research discoveries regarding the ecological stoichiometry properties of carbon, nitrogen, and phosphorus in soil, along with the factors that impact them. Zhang‘s study was carried out on the ecological stoichiometry features of soil in Chinese locust forests that were artificially planted in the loess hilly and gully region, with varying restoration ages [13]. According to their study, it was found that as the age of restoration increased, there was a significant increase in both the ratios of soil C P and N P, while the C N ratio remained constant. Moreover, the main factor that influences the content of soil C, N, and P, along with their ecological stoichiometric ratios, is the identified severe lack of soil moisture. The ecological stoichiometry characteristics of soil in different-aged Scots pine plantations in Wendaolin forest farm, Fushun County, Liaoning Province, were examined to determine the factors that influence them [14]. The study found that the age of the plantation and the depth of the soil had notable impacts on the ratios of soil C P and N P but did not have a significant influence on the soil C N ratio. The research conducted by Zhang’s study conducted in the Tianma National Nature Reserve in Jinzhai, Anhui, examined the impact of altitudinal gradients on the soil chemical stoichiometry characteristics of Chinese fir plantations [15]. The findings showed that the soil C N ratio initially increased and then decreased as the altitude increased. On the other hand, the ratios of C P and N P in the soil exhibited a pattern of initial decline followed by subsequent increases. Abrar conducted a study on the long-term fertilization effects on the black soil of Northeast China [16]. With increasing soil depth, the content of soil C, N, and P, as well as their chemical stoichiometric ratios, were observed to decline. Significantly correlated were only the ratios of C N, C P, and N P with the organic carbon component of the surface soil [17]. Research was conducted on the chemical stoichiometry features of fine roots and soil C, N, and P in various stages of vegetation recovery in the karst area of southwestern China. The results showed that the soil C, N, and P chemical stoichiometric ratios were significantly influenced by both the type of vegetation and the soil layer. In addition, there was a significant negative correlation between the C N and C P ratios of fine roots and the soil C N and C P ratios. Additionally, the N P ratios of fine roots were significantly correlated with the soil N P ratios. The contribution of these studies to enhancing our comprehension of the characteristics of ecosystem chemical stoichiometry has been significant.

Shaanxi Province is one of the key tea-producing regions in China, with the main tea-producing areas in Shaanxi known as the “Five Greens and One Red” (Gougunao, Wuyuan Green Tea, Lushan Yunwu, Fuliang Tea, and Ning Hong Tea), covering approximately 1.7 × 10⁴ hm² of tea gardens in 2020. This accounts for 10% of the total tea garden area in the entire province, which is approximately 1.7 × 10⁵ hm². Numerous academics have carried out extensive investigations into the characteristics of soil, including its physical and chemical properties, fertility, and the various factors that impact tea gardens [18,19,20,21,22]. Nevertheless, the comprehensive investigation of the ecological stoichiometry of carbon, nitrogen, and phosphorus in tea garden soils, along with their reactions to altitude and years of cultivation, lacks extensive scholarly resources. Research the contents and ecological stoichiometric characteristics of carbon (C), nitrogen (N), and phosphorus (P) in the soils of tea plantations at different altitudes and planting years in five major tea-producing areas in Shangluo City, Shaanxi Province (Chengguan Town, Shima Town, Xianghe Town, Qingshan Town, and Fushui Town). Explore how these soil properties respond to changes in altitude and planting years. Assess the impact of different planting years on the contents of carbon, nitrogen, and phosphorus in soil composition and their ecological stoichiometric characteristics. Provide an in-depth understanding of the nutrient balance and ecological stoichiometric characteristics of tea plantation soils in Shaanxi Province, offering a scientific basis for the management of soil fertility in tea plantations. We hypothesize that in the main tea-producing areas of Shangluo City, Shaanxi Province: (1) with increasing altitude, there will be significant changes in the levels of soil carbon, nitrogen, and phosphorus, indicating that the altitudinal gradient may affect soil nutrient levels. (2) with the extension of tea plantation cultivation years, the composition of C, N, and P in the soil may change, as well as their ecological stoichiometric characteristics. (3) different cultivation years may affect soil nutrient levels, thereby impacting the soil fertility and ecological balance of tea plantations. This study not only has scientific value in enhancing and supplementing data on soil nutrient balance and ecological stoichiometric characteristics of tea plantations in Shaanxi but also provides valuable insights for the management of soil fertility in tea plantations.

2. Materials and Methods

2.1. Summary of the Study Region

Shangnan County is situated in the southeastern part of Shaanxi Province, within the southern foothills of the eastern Qinling Mountains. The Danjiang River is located in the middle reaches of the Han River Basin, which is a component of the Yangtze River drainage system (Figure 1). The geographical coordinates of Shangnan County lie between approximately 110°24′ to 111°01′ E longitude and 33°06′ to 33°44′ N latitude. It shares its northern border with Lushi County in Henan Province and Mangling, while its southern boundary neighbors Yunxi County in Hubei Province. To the east, it borders Xixia County in Henan Province, and to the west, it connects with Leijiadong in Danfeng County. Shangnan County is situated in the area where the North Subtropical Zone and the Warm Temperate Zone meet, and its climate is classified as a continental monsoon climate.

Due to the natural barrier of the Qinling Mountains to the north, Shangnan County is shielded from the intrusion of cold air masses, resulting in a temperate climate with abundant rainfall, distinct seasons, mild winters, and moderate summers. The average yearly temperature hovers around 14 degrees Celsius, with the highest temperature ever recorded peaking at 40.5 degrees Celsius and the lowest temperature in history plummeting to −13.1 degrees Celsius. July is the warmest month, while January is the coldest. The annual average sunshine duration is approximately 1973.5 h. The county’s terrain is complex and varied, with a mix of hills and valleys, and the maximum relative elevation difference within the area reaches 1840.6 m. As a result, there are significant variations in temperature and moisture with increasing altitude, giving Shangnan County a distinct mountainous climate character. In Shangnan County, the duration of the period without frost is around 216.5 days, with a total of 4406.2 °C days of temperature above 10 °C. The annual average precipitation in the county is about 803.2 mm, with varying spatial distribution. The southern and northern mountainous areas receive higher rainfall, while the central areas, where tea cultivation is widespread, experience lower rainfall. Additionally, the eastern part of the county receives higher rainfall compared to the western part. Shangnan County has diverse soil types, including six soil classes, fourteen sub-classes, twenty-six soil genera, and seventy-two soil species. The six soil classes are yellow-brown soil, brown soil, paddy soil, new soil, rice soil, and purple soil. The county has a transitional location between the North Subtropical Zone and the Warm Temperate Zone, with an elevation averaging more than 1300 m. As a result, it is characterized by abundant deciduous broad-leaved forests and scattered natural Chinese pine and mixed forests. Chinese pine is the predominant tree species found throughout the area. Elevations ranging from 800 to 1300 m host various tree species such as Chinese pine, Chinese toon, Chinese fir, Masson pine, and oak. Below 800 m, the influence of the North Subtropical climate becomes more pronounced. The majority of tea plants in Shangnan County are located within an altitude range of 500 to 800 m above sea level. As of the end of 2022, the total area of tea gardens in the county exceeded 1.7 × 10⁴ hm², including 1.2 × 10⁴ hm² of harvestable area, 8000 hm² of high-yield tea gardens, 4000 hm² of clonal tea gardens, and 7300 hm² of pollution-free certified tea gardens. The annual tea production reached 65 million kilograms, with a total output value of 1 billion yuan. The county boasts over 100 tea enterprises and cooperatives, engaging more than 50,000 individuals involved in various aspects of the tea industry.

2.2. Experiment Design

From April to June 2023, soil sampling was conducted in five main tea-producing areas in Shangnan County, Shaanxi Province, including Chengguan Town, Shima Town, Xianghe Town, Qingshan Town, and Fushui Town. To ensure the randomness and representativeness of samples, sampling was conducted using an “S-shaped” point sampling method, with each 13.3 hm² tea plantation area as a sampling unit. This method covered various areas of the tea plantations to avoid sampling biases. Within each sampling unit, the research team dug 5 soil pits at different locations to collect soil samples at a depth of 0–20 cm, which is the most active layer for tea roots. Soil samples from the 5 pits were mixed to reduce local variability and combined into one representative soil sample. A total of 248 soil samples were collected from these five tea-producing areas. Each mixed sample was uniquely labeled, and specific coordinates, tea plantation age, altitude, and other information about the sampling points were recorded. The soil samples were properly stored in sealed containers under suitable temperature and humidity conditions to prevent sample deterioration during transportation and storage. They were safely transported to the laboratory for further chemical and physical property analysis. Throughout the sampling process, the research team meticulously documented all relevant environmental parameters and sampling conditions for subsequent analysis and review.

2.3. Analytical Parameters and Methods

In the laboratory, the soil samples that were collected underwent processing, and analyses were conducted to ascertain the content of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP). The specific analysis methods are outlined below: Soil SOC Analysis: The high-temperature external heat potassium dichromate method was used, employing an HH-S (Germany) constant-temperature oil bath device. This method typically involves heating the samples at high temperatures to decompose organic carbon, followed by measurement using potassium dichromate. Soil TN Analysis: The automated Kjeldahl nitrogen determination method was employed, utilizing a KQ 860 fully automatic Kjeldahl nitrogen analyzer (Germany). This method measures the total nitrogen content in soil through gas chromatography or chemical reactions. Soil TP Analysis: The HCIO₄-H₂SO₄ fusion-UV-visible spectrophotometry method was used, with a visible spectrophotometer (China). This method involves fusing soil samples and then measuring the total phosphorus content using spectrophotometric colorimetry. The specific procedures and conditions for these analytical methods may be adjusted and executed following the guidelines in the “Methods of Soil Agricultural Chemical Analysis” [23]. These analyses provide valuable information about the chemical properties and nutrient content of the soil, which is essential for agricultural production and soil management.

2.4. Data Compilation and Analysis

After organizing the measurement data from the sampling points and removing a few outliers, a total of 165 valid sampling points were obtained, distributed as follows: 34 in Chengguan Town, 41 in Shima Town, 25 in Xianghe Town, 32 in Qingshan Town, and 33 in Fushui Town. The processing and analysis of data were performed utilizing Microsoft Office Excel 2003 and DPS 7.05 software. In order to evaluate the importance of disparities, a one-way analysis of variance (one-way ANOVA) was utilized along with the least significant difference (LSD) test.

3. Results

3.1. Soil C, N, and P Content and Ecological Stoichiometry Characteristics in Tea Gardens from Different Main Producing Areas

The soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) content in the five primary production regions varied from 2.16 to 38.51 g·kg⁻¹, 0.14 to 3.47 g·kg⁻¹, and 0.12 to 1.42 g·kg⁻¹, respectively (Figure 2a). The average values for SOC, TN, and TP were 13.15 g·kg⁻¹, 1.30 g·kg⁻¹, and 0.45 g·kg⁻¹. There were significant differences in soil SOC content among the main producing areas, while soil TN and TP content did not exhibit significant differences. The soil SOC content in tea gardens located in Xianghe Town and Qingshan Town exhibited a significantly higher level compared to Chengguan Town and Fushui (p < 0.05). Additionally, the SOC content in tea gardens situated in Shima Town and Chengguan Town was significantly greater than that in Fushui (p < 0.05).

The soil C N, C P, and N P ratios in the five primary producing regions varied from 4.08 to 47.58, 8.74 to 84.49, and 1.06 to 9.69, respectively, with average values of 10.42, 30.98, and 3.32 (Figure 2b). Significant variations were observed in the soil C N and C P ratios among the primary producing regions, whereas no notable differences were found in the soil N P ratio. The soil nitrogen content in the tea gardens of Xianghe Town was notably higher compared to the other four primary producing regions, while the soil nitrogen content in the tea gardens of Qingshan Town was significantly higher than in Chengguan Town and Fushui. The levels of soil C P in the tea gardens of Xianghe Town and Shima Town were considerably greater compared to Chengguan Town and Fushui, while the soil C P in Qingshan Town tea gardens was notably higher than in Fushui.

3.2. Soil C, N, P Content, and Ecological Stoichiometry Characteristics at Different Altitudes

The sampling points in the five main producing areas ranged in altitude from 300 to 980.0 m. These altitudes were categorized into six levels for analysis: ≤400 m, 401–500 m, 501–600 m, 601–700 m, 701–800 m, and ≥800 m (Figure 3a). From Figure 3, it is evident that there are significant differences in soil SOC, TN, and TP content among tea gardens at different altitudes. In particular, tea gardens located at altitudes of 800 m or higher demonstrated notably greater soil SOC content in comparison to altitudes below. Tea gardens at altitudes ≤400 m had significantly higher soil SOC content compared to altitudes 401–500 m, 501–600 m, 601–700 m, and 701–800 m.

Tea gardens located at altitudes ≥800 m exhibited significantly elevated values in terms of soil TN content in comparison to other altitudes (Figure 3b). In the case of soil TP content, tea gardens at altitudes ≥800 m had significantly higher values compared to altitudes ≤400 m, 401–500 m, 601–700 m, and 701–800 m. Furthermore, the tea plantations situated at elevations ranging from 601 to 700 m exhibited notably greater soil TP concentration compared to those at elevations ranging from 401 to 500 m and 701 to 800 m. Similarly, tea gardens at elevations below or equal to 400 m and ranging from 501 to 600 m displayed significantly higher soil TP content than those at an elevation of 701 to 800 m. Significant variations were observed among tea gardens at different altitudes in terms of soil C N and C P ratios, whereas no significant differences were found in soil N P ratios. In particular, tea gardens situated at elevations of 701–800 m and above 800 m exhibited notably elevated soil C N ratios in comparison to those at elevations of 401–500 m. Furthermore, tea gardens situated at elevations of 701–800 m and above 800 m exhibited notably elevated soil C P ratios in comparison to those at elevations of 501–600 m. In general, as altitude increases, there is a pattern of initially decreasing and then increasing soil SOC content, C N, and C P ratios. However, the changes in soil TN, TP content, and N P ratio were not as noticeable.

3.3. Soil C, N, P Content, and Ecological Stoichiometry Characteristics at Different Planting Durations

The sampling points in the five main producing counties had tea gardens with planting durations ranging from 5 to 49 years. The planting durations were classified into five categories: less than or equal to 10 years, 11 to 20 years, 21 to 30 years, 31 to 40 years, and more than 40 years (Figure 4a). Significant variations in soil SOC and TN content among tea gardens with different planting durations can be observed in Figure 4a, whereas no significant differences are observed in soil TP content. In particular, tea gardens that were planted for more than 40 years had noticeably greater soil SOC content in comparison to those with planting durations of 10 years or less, 11 to 20 years, and 31 to 40 years.

Tea gardens with a planting duration of 21–30 years had significantly higher soil SOC content compared to those with a planting duration of ≤10 years. Tea gardens that have been planted for more than 40 years showed significantly higher soil TN content compared to those with planting durations of 10 years or less, 11–20 years, and 31–40 years. Tea gardens with a planting duration of 21–30 years had significantly higher soil TN content compared to those with planting durations <10 years. Significant variations were observed among tea gardens with varying planting durations in terms of soil C N, C P, and N P ratios. Tea gardens that were planted for more than 40 years exhibited considerably higher ratios of soil C N and C P in comparison to those with different planting durations. Tea gardens that were planted for 21–30 years exhibited considerably greater soil N P ratios in comparison to those with planting durations of ≤10 years, 11–20 years, and >40 years. In general, as the duration of planting increases, there is a rise in the levels of soil SOC content, TN content, and C P ratio, while soil C N initially decreases and then increases. The ratios of soil nitrogen to phosphorus initially rose and then fell as the duration of planting increased, although changes in soil total phosphorus content were less noticeable.

3.4. Effects of Attapulgite Application on the Contribution Rate of Organic Carbon, Inorganic Carbon, and Total Nitrogen in Aggregates

As shown in Figure 4, there are highly significant positive correlations (p < 0.01) between soil SOC, TN, and TP content. A strong coupling relationship between SOC content and TN content is indicated by the highest observed correlation coefficient of 0. 693. The ratio of carbon to nitrogen in the soil shows a strong positive correlation with the content of soil organic carbon (p < 0.01) and a strong negative correlation with the content of total nitrogen (p < 0.01). The ratio of C to P in soil exhibits a strong positive correlation with the content of SOC (p < 0.01) and a strong negative correlation with the content of TP (p < 0.01). The ratio of soil nitrogen to phosphorus shows a strong positive correlation with total nitrogen content (p < 0.01) and a strong negative correlation with total phosphorus content (p < 0.01). This suggests that the ratios of carbon to nitrogen, carbon to phosphorus, and nitrogen to phosphorus in the soil are strongly linked to the content of soil organic carbon, total nitrogen, and total phosphorus (Figure 5).

4. Discussion

4.1. Soil C, N, and P Content Characteristics

The presence of soil elements C, N, and P is vital for the healthy growth and progress of plants, as they play a critical part in the plant growth process [24]. Soil SOC, TN, and TP content are closely related to tea yield and quality [25]. This study examined the levels of soil SOC, TN, and TP in tea plantations located in Chengguan Town, Shima Town, Xianghe Town, Qingshan Town, and Fushui Town within Shaanxi Province. In these tea gardens, the mean soil SOC, TN, and TP concentration in the 0–20 cm soil depth was recorded as 13.15, 1.30, and 0.45 g·kg⁻¹, correspondingly. Among these, the mean soil SOC and TN levels exceeded the national averages for soil SOC (11.12 g·kg⁻¹) and TN (1.06 g·kg⁻¹), whereas the TP levels were below the national average for soil TP (0.65 g·kg⁻¹) [26]. The tea gardens in Shima Town, Xianghe Town, Qingshan Town, and Fushui Town exhibited elevated levels of soil SOC and TN content compared to the findings presented by Yang et al. based on samples collected and analyzed in 2009–2010 [27]. Moreover, based on the industry standard ‘Technical Conditions for Tea Production Areas’ (NY/T 853-2004), the soil fertility grading standards for tea gardens were satisfied by the average soil SOC and TN levels in the tea gardens of these five primary production regions in this investigation, indicating Grade I fertility [28]. However, the average soil TP levels only reached the criteria for Grade II fertility. Nevertheless, upon examining specific production regions, the soil SOC level in Fushui tea plantations fell below the national average yet still satisfied the requirements for Grade III fertility in tea gardens.

This study found that tea gardens at altitudes of ≤400 m had higher soil SOC, TN, and TP content compared to those at altitudes of 401–500 m. This is primarily because tea gardens at altitudes of ≤400 m are mostly located on flat terrain, while those at altitudes of 401–500 m tend to be hilly. Soil SOC content increased with the rise in altitude within a certain range (100–1000 m). This may be attributed to differences in temperature at different altitudes, where higher altitudes have lower temperatures, which are more favorable for the accumulation of organic carbon [29,30]. This finding is consistent with the hypothesis that altitude has a significant impact on soil nutrient levels. The soil SOC and TN contents showed an increasing trend with the increase in planting years, which is consistent with the hypothesis that an increase in planting years may affect the composition and ecological stoichiometric characteristics of carbon, nitrogen, and phosphorus in the soil. The findings of studies conducted by Wang et al. align with the observed increase in soil SOC and TN content as planting years increase [31,32]. One reason for this trend is the long-term fertilization in tea gardens. Additionally, with the extension of planting years, the litterfall from tea trees gradually increases, and soil C and N are replenished and accumulated mainly through long-term fertilization, the return of tea tree litterfall, and root exudates, among other factors [33]. Research by He et al. Research demonstrated that as the duration of tea cultivation grows, the stability of the soil’s organic carbon reservoir improves, leading to a notable enhancement in the carbon sequestration impact. Wang et al. According to [34], the rise in soil SOC and TN reserves prior to 17 years of tea cultivation primarily originated from particle aggregates larger than 2 mm, whereas after 17 years of tea cultivation, it predominantly originated from particle aggregates smaller than 0.25 mm. Additionally, studies suggest that as tea planting years increase, the microbial population in tea garden soil becomes more homogeneous. This homogeneity hinders the ability of microorganisms to break down organic matter in the soil [35]. In this study, soil TP content in tea gardens showed no significant pattern with respect to altitude or planting years. The reason for this is mainly due to the fact that the origin of soil TP remains relatively consistent, and its composition is primarily affected by the soil’s parent material, pedogenic processes, and agricultural methods [36]. Other sources, such as dry and wet deposition, animal and plant residues, and microbial activity, have relatively minor effects on TP [37]. Additionally, in this study, the soil SOC, TN, and TP contents in tea plantations exhibited a highly significant positive correlation with each other (p < 0.01). This indicates that the cycling processes of soil C, N, and P nutrients in the study area are interconnected and mutually influential. This finding is consistent with the hypothesis that the cycling and transformation processes of soil nutrient elements may be affected by environmental factors. It also aligns with the results of studies by Chomel and Camenzind [38,39].

4.2. Effect of Attapulgite Soil Application on Carbon and Nitrogen Content of Sandy Soil Aggregates

The ratios of carbon, nitrogen, and phosphorus in the soil, known as soil ecological stoichiometry ratios, play a crucial role in evaluating soil quality and maintaining the proper balance of these nutrients [40,41]. These ratios have a profound effect on the growth of plants. The ratio of carbon to nitrogen in soil indicates the state of decomposition of organic matter in the soil, the ability of the soil to release nitrogen, and the equilibrium between carbon and nitrogen nutrients in the soil [42]. The decomposition capacity of soil microorganisms is enhanced when the C N ratio is low, which leads to an increase in available soil N. On the other hand, a high ratio limits N availability, impacting soil microbial decomposition capacity and resulting in the accumulation of organic carbon [43]. In this research, the average soil C N ratio in tea gardens from the five primary production regions was 10.42, which is below the national average soil C N ratio of 11.90. The soil C N ratio in tea gardens from Xianghe Town exceeded the national average by 33.4%, whereas in Fushui, Chengguan Town, and Shima Town, the ratios were below the national average by 37.1%, 25.7%, and 6.6%, respectively, which is consistent with the hypothesis that the soil C:N ratio may vary across different regions. The carbon-to-nitrogen ratio in tea gardens located in Qingshan Town closely resembled the national mean. This implies that the nitrogen content in the soil of Xianghe Town is comparatively low, resulting in a weaker capacity for soil microbial decomposition. Consequently, the decomposition of organic matter and mineralization of nutrients occurs at a slower pace. On the other hand, the soil microbial decomposition capacity is stronger in Fushui, Chengguan Town, and Shima Town tea gardens, leading to faster mineralization of soil organic matter. The ratio of carbon to phosphorus in the soil can serve as an indicator of the capacity of soil microorganisms to decompose organic matter and release phosphorus or take up solid phosphorus from the surroundings. A smaller C:P ratio in the soil is more favorable for soil microorganisms to mineralize organic matter and release more P, replenishing the available soil P pool [44]. The study found that the tea gardens in all the major production areas had a soil C P ratio below the national average of 61, indicating that the soil microorganisms in these tea gardens possess a robust ability to decompose soil organic matter and liberate phosphorus, thus implying an ample supply of phosphorus in the soil. The ratio of nitrogen to phosphorus in the soil can be used as an indicator to determine if there is a lack of nitrogen or phosphorus in the environment, and it can offer valuable information about the availability of nutrients for plant development [45]. The soil nitrogen-to-phosphorus ratios in tea gardens from all the primary production regions were below the national average soil nitrogen-to-phosphorus ratio of 5.20, as observed in this research [46]. This indicates that the soil in these tea gardens is relatively rich in phosphorus, while nitrogen is relatively deficient, potentially acting as a limiting factor. Therefore, it is advisable to apply nitrogen fertilizer appropriately in tea garden management to maintain a balanced nitrogen/phosphorus ratio.

The soil C N ratio in tea gardens showed a pattern of decreasing at first and then increasing as elevation and planting years increased. This suggests that at mid-altitudes (100~299 m) and during the mid-term of planting (20~29 years), the decomposition rate of organic matter is relatively faster, which may not favor nutrient accumulation. The ratio of carbon to phosphorus in the soil exhibited a pattern of initially declining and subsequently rising as elevation increased, whereas it demonstrated an increase with the passage of time since planting. This is largely influenced by changes in soil SOC content with elevation and planting years. It also indicates that as planting years increase, the soil phosphorus availability decreases. There was no apparent pattern in the soil N P ratio with elevation. However, it did show an initial rise and subsequent decline as the planting years increased. The soil C, N, and P ecological stoichiometry in tea gardens can be affected by factors such as elevation and the number of years since planting. These results align with the discoveries made by Mao regarding the study of soil chemical stoichiometry characteristics in hilly tea gardens with different tea planting ages [47,48].

5. Conclusions

5.1. Summary

This study primarily investigates the characteristics of carbon (C), nitrogen (N), and phosphorus (P) content in the soil of major tea-producing areas in high-latitude regions of China and their ecological stoichiometric ratios in relation to soil quality and environmental factors. In terms of soil C, N, and P content characteristics, the mean values of SOC, TN, and TP in the study area’s tea plantation soils are 13.15, 1.30, and 0.45 g·kg⁻¹, respectively. The SOC and TN contents are above the national average, while the TP content is below the national average. According to the soil fertility grading standards for tea plantations in Shangluo City, the SOC and TN contents meet the first-grade fertility standards, whereas the TP content only meets the second-grade fertility standard. Regarding the impact of altitude and cultivation years on soil C, N, and P contents, tea plantations at low altitudes (≤400 m) have higher SOC, TN, and TP contents than those at medium altitudes (401–500 m). As altitude continues to increase, SOC content increases within a certain range, possibly related to temperature differences at different altitudes. With the increase in cultivation years, SOC, TN, and the C:P ratio gradually rise, which may be related to factors such as long-term fertilization, return of tea tree litter, and root exudates.

In terms of soil ecological stoichiometric ratios, the soil C:N ratio is lower than the national average, indicating a strong microbial decomposition capacity in the tea plantation soil, which is conducive to nitrogen release. The soil C:P ratio is lower than the national average, suggesting that soil microbes in tea plantations have a strong ability to decompose organic matter and release phosphorus. The soil N:P ratio is lower than the national average, indicating that tea plantation soil is relatively rich in phosphorus while being relatively deficient in nitrogen, which may become a limiting factor for plant growth. Regarding the trends in soil C, N, and P ratios, with the increase in altitude and cultivation years, the soil C:N ratio shows a trend of first decreasing and then increasing, which may be related to the decomposition rate of organic matter. The C:P ratio first decreases and then increases with altitude and increases with cultivation years, indicating that increased cultivation years may reduce soil phosphorus availability. The N:P ratio shows no obvious trend with altitude but first increases and then decreases with cultivation years.

Based on the findings of this study, we recommend the following management measures for local tea farmers: (1) Given that tea plantation soils are relatively deficient in nitrogen and rich in phosphorus, it is recommended to appropriately increase the use of nitrogen fertilizers in tea garden management. This will help balance the nitrogen-phosphorus ratio in the soil, promoting the balanced growth of tea plants. Additionally, it is suggested to develop and validate soil nutrient dynamic models to predict changes in soil nutrients and tea growth responses under different management practices. (2) Considering the microbial decomposition capacity and nutrient availability in the soil, it is advised to adopt soil management measures that enhance soil fertility. This includes studying the soil microbial community structure and its relationship with soil nutrient cycling, and how management practices can regulate microbial communities to improve soil fertility. Enhancing soil fertility can be achieved through the rational addition of organic matter and the promotion of microbial activity. (3) Since soil characteristics vary with different altitudes and cultivation years, it is recommended to further study the adaptability of tea plantation soils under various environmental conditions. Additionally, research should focus on the long-term impacts of climate change on soil characteristics and tea growth, predicting soil change patterns in response to different environmental conditions. These measures aim to optimize soil nutrient levels, improve tea yield and quality, and maintain the ecological balance of tea plantations. This will provide valuable insights and guidance for practical tea garden management in high-latitude regions.

5.2. Conclusions

As altitude increases, there are significant changes in soil organic carbon (SOC), total nitrogen (TN), the carbon-to-nitrogen ratio (C:N), and the carbon-to-phosphorus ratio (C:P) in the main tea-producing areas of Shangluo City. This indicates that the altitudinal gradient has a notable impact on soil nutrient levels, possibly affecting the accumulation of soil organic matter and nutrient cycling processes, and thereby influencing the tea-growing environment. With the extension of tea plantation cultivation years, the SOC, TN, and C:P ratios in the soil show a gradually increasing trend. This may suggest an enhanced capacity for soil to accumulate carbon and nitrogen over time. Such changes could be related to long-term fertilization practices and soil management measures, potentially impacting nutrient cycling and tea growth. Soils from tea plantations with different cultivation years exhibit varying nutrient levels and ecological stoichiometric characteristics. Longer cultivation periods might lead to the accumulation of certain nutrients in the soil, thus affecting soil fertility status and ecological balance. Proper nitrogen fertilizer application is crucial for maintaining the balance of nitrogen and phosphorus in the soil, which is important not only for tea yield and quality but also for the health and sustainability of the tea plantation ecosystem. Nutrient management and ecological stoichiometric characteristics of tea plantation soils are of great importance for the sustainable development of the tea industry. By utilizing fertilizers and soil management measures appropriately, soil nutrient levels can be optimized, enhancing both tea yield and quality while maintaining the balance of the tea plantation ecosystem. This study provides valuable references and suggestions for tea garden managers in high-latitude regions, aiding in guiding practical tea garden management practices.