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Article

Changes in Soil Properties and Enzyme Stoichiometry in Three Different Forest Types Changed to Tea Plantations

by
Ying Li
1,2,†,
Jinlin Zhang
1,2,3,†,
Qingyan Qiu
1,3,
Yan Zhou
4 and
Weibin You
1,2,3,*
1
College of Forestry, Fujian Agriculture and Forestry University, Fuzhou 350002, China
2
Research Centre of Southern Forest Resources and Environment Engineering Technology of Fujian Province, Fuzhou 350002, China
3
College of Juncao Science and Ecology, Fujian Agriculture and Forestry University, Fuzhou 350002, China
4
Wuyishan National Park Research and Monitoring Center, Wuyishan 354300, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Forests 2023, 14(10), 2043; https://doi.org/10.3390/f14102043
Submission received: 8 September 2023 / Revised: 1 October 2023 / Accepted: 7 October 2023 / Published: 12 October 2023
(This article belongs to the Section Forest Inventory, Modeling and Remote Sensing)
Browse Figures
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Abstract

:
Understanding the characteristics and driving factors of soil carbon, nitrogen, phosphorus, and enzyme stoichiometry during land use/cover change is of great significance for assessing microbial nutrient restriction and sustainable land development during the process. China, the world’s largest tea producer, is witnessing a significant expansion of tea plantations into previously forested areas. We performed field sampling in three forest types with the area partially converted to tea plantations in Wuyishan National Park. We examined the changes in soil carbon (TC), nitrogen (TN), phosphorus (TP), and three kinds of extracellular enzyme activities, β-glucosidase (BG), β-n-acetylglucosidase (NAG), and acid phosphatase (ACP). By analyzing the enzyme stoichiometric ratio, vector length (VL), and vector angle (VA), the relative nutrient limitations of soil microorganisms were explored. The results showed that soil TC and TN decreased significantly (p < 0.05), TP increased significantly, and soil carbon (C):nitrogen (N), carbon (C):phosphorus (P), and nitrogen (N):phosphorus (P) ratios decreased significantly after the conversion of forest land to tea plantation. Soil BG, NAG, and ACP contents decreased significantly (p < 0.05). There were no significant differences in enzyme carbon:nitrogen ratios (EC/N), enzyme carbon:phosphorus ratios (EC/P), enzyme nitrogen:phosphorus ratios (EN/P), VL, or VA (p > 0.05). Through the analysis of soil enzyme stoichiometry, it was found that forest soil was generally limited by P, which was, to some extent, relieved after the conversion to tea plantation. Redundancy analysis showed that TC, TN, and the C:N ratio were the main factors influencing enzyme activity and stoichiometry. These results indicated that land use/cover change had significant effects on soil nutrient status, enzyme activity, and stoichiometry. Soil enzyme activity is very sensitive to the changes in soil nutrients and can reflect the restriction of soil nutrients more accurately.

1. Introduction

The tea plant (Camellia sinensis (L.) O. Kuntze) is a well-known and important perennial evergreen woody economic crop, widely distributed in the tropical and subtropical regions of many countries, such as China, India, Kenya, and Sri Lanka. China is the country where tea cultivation and consumption started earliest in the world. Tea, a widely distributed and major economic crop, holds great importance in China. In the past half-century, the tea plantation area in China has expanded nearly tenfold, accounting for about one-half of the total tea plantation area in the world [1]. Thus, large proportions of tea plantations are expanding into areas originally occupied by forests [2]. The conversion of forests into tea plantations has become a significant land use/cover change process that influences the functioning sustainability of forest ecosystems in the southern mountainous and hilly areas of China.
Forest soil is an important component and nutrient reservoir of forest ecosystems. Soil carbon, nitrogen, and phosphorus elements are essential nutrients for plant growth. Their cycling regulates most ecological processes in terrestrial ecosystems, directly influencing the stability and productivity of soil ecosystems [3,4]. The ecological stoichiometry characteristics of soil carbon, nitrogen, and phosphorus can reveal the availability of soil nutrients, nutrient cycling, and balance mechanisms [5]. Land use changes can disrupt the material cycles and energy balance of the original ecosystem [6,7], resulting in significant impacts on soil carbon, nitrogen, phosphorus, and their stoichiometry by altering water thermal conditions, soil physicochemical properties, and biogeochemical processes over the long term [8]. On one hand, nutrient inputs and plant and animal residues differ under different land use types [9]. On the other hand, differences in human management practices, such as cultivation methods, can affect the mineralization, transportation, and utilization of soil nutrients, thus influencing the cycling of soil carbon, nitrogen, phosphorus, and other nutrients [10,11]. Research by Wang et al. showed that land use types, soil depth, and their interaction significantly affect the characteristics of soil carbon, nitrogen, phosphorus, and their stoichiometry [12]. Chen et al. found that when agricultural land is converted to grassland or forest, soil carbon increases, and the conversion to shrubland or natural grassland is more favorable for carbon sequestration than the conversion to plantation forests [13]. Tian et al. found that the C:N:P ratios of nine soil orders in China are influenced by vegetation types and human activities [14]. Previous studies on the impact of land use types on the ecological stoichiometry of soil carbon, nitrogen, and phosphorus mainly focus on different vegetation types [15,16], ecological restoration methods (such as converting farmland back to forests) [17], and vegetation succession [18]. However, there is still a lack of in-depth research on the effects of converting forests to tea plantations on the cycling of soil carbon, nitrogen, and phosphorus nutrients, as well as the nutrient demand limitations and changes in nutrient cycling within the forest soil ecosystem caused by tea plantation cultivation.
Soil enzymes are important biological indicators of soil fertility and nutrient cycling, and they have many ecological connections with soil carbon, nitrogen, and phosphorus [19,20]. Microorganisms secrete corresponding extracellular enzymes, such as β-glucosidase (BG) for carbon cycling, β-1,4-N-acetylglucosaminidase (NAG) for nitrogen cycling, and acid phosphatase (AP) for phosphorus cycling, in order to obtain the necessary nutrients such as carbon, nitrogen, and phosphorus for their own metabolic processes [21]. These extracellular enzymes participate in various biochemical processes in the soil and act as the main biocatalysts for organic matter decomposition, turnover, and mineralization. They are closely related to soil carbon, nitrogen, and phosphorus nutrient cycling and can reflect the metabolic levels of soil carbon, nitrogen, and phosphorus relatively rapidly [22,23,24,25]. Previous studies have shown that vegetation types, soil parent material, land use practices, and human activities significantly influence the activity of soil extracellular enzymes, suggesting that extracellular enzymes can sensitively reflect the direction and intensity of soil biochemical processes in response to the influence of natural conditions and human activities [11,26,27]. Therefore, studying soil extracellular enzymes is of great significance in indicating microbial nutritional requirements and changes in soil ecosystems.
With the emergence of the concept of ecophysiological stoichiometry, the concept of soil enzyme stoichiometry, which reflects the mutual relationship between microbes and soil enzyme activity, has been proposed [28,29]. Soil enzyme ecophysiological stoichiometry characteristics have attracted increasing attention from researchers. It pertains to the relative proportions of enzyme activities involved in carbon, nitrogen, and phosphorus cycling within soil ecosystems. This ratio can provide insights into the nutrient demands of soil microorganisms and the constraints imposed by soil nutrient resources [30,31]. Studies on soil enzyme ecophysiological stoichiometry in various ecosystems globally have shown that when the stoichiometric ratio of soil carbon, nitrogen, and phosphorus enzymes is approximately 1:1:1, the soil ecosystem environment is relatively stable [32]. However, due to changes in vegetation type/land cover, actual values deviate from 1:1:1, resulting in microbial activity being limited by one aspect (i.e., carbon, nitrogen, or phosphorus) [33]. Research by Sun et al. [11] found that the conversion of sloping farmland to woodland significantly improved soil nutrient content and enzyme activity, but exacerbated phosphorus limitation. Additionally, land use practices influence enzyme activity by controlling soil physical and chemical properties. Cui et al. [34] studied enzyme activity in different vegetation and soil types in the Loess Plateau area and found that, compared to soil properties, plant communities had a more significant impact on enzyme ecophysiological stoichiometry, while soil physical properties had a greater influence on enzyme activity than nutrient content. Xu et al. [35] investigated soil enzyme activity in nine forest ecosystems in China and found that temperate soil had higher enzyme activity than tropical and subtropical soil, and as pH increased, soil C:P and N:P ratios gradually decreased. Soil is the direct source of nutrient utilization for microorganisms. When land use types change, vegetation types, plant community composition, litter, and root exudates all change, which affects the microbial nutrient requirements in soil and thus leads to changes in soil biogeochemical processes. Therefore, in-depth research on the impact of changes in land use practices on soil enzyme activity and its stoichiometric ratios is crucial for accurately understanding soil nutrient cycling and microbial nutrient requirements.
Wuyi Mountain is located in the subtropical monsoon humid climate region and is a major production area for one of China’s top ten famous teas, “Wuyi Rock Tea”. Due to historical reasons, since the 1990s, with the rapid development of Wuyi Rock Tea, the conversion of forests into tea plantations has become a common land use change in the area [36]. These forests stand adjacent to existing tea plantations and have similar background conditions (parent rock, soil, and climate conditions), providing a good experimental site to explore the effects of the conversion of forests into tea plantations on soil carbon, nitrogen, phosphorus, and enzyme stoichiometry. Currently, there are few studies on changes in enzyme activity and its stoichiometric characteristics during the conversion of forests into tea plantations, which limits the in-depth understanding of nutrient cycling in soil ecosystems during land use change. In order to investigate the response of soil enzyme activity and its stoichiometric ratios to the conversion of forests into tea plantations, this study selected three forest types (Pinus massoniana forest, bamboo forest (Phyllostachys hterocycla forest), and mixed forest) within Wuyishan National Park and the tea plantations derived from them as research objects. Through the comparative analysis of soil carbon, nitrogen, phosphorus, enzyme activity, and their stoichiometric ratios, the study aims to address the following questions: (1) Does the conversion of different forest types in Wuyi Mountain into tea plantations significantly alters the stoichiometric ratios of soil carbon, nitrogen, phosphorus and their enzyme ratios, and which forest type is most sensitive to the conversion? (2) Does the alleviation of soil nutrient limitation after the conversion of different forest types into tea plantations shows differences in different nutrient elements? The results of this study can provide a theoretical basis for nutrient cycling in forest ecosystems and sustainable land resource management in the region.

2. Methods

2.1. Study Area

Wuyishan National Park (27°33′–27°54′ N, 117°27′–118°01′ E) is located in the northern part of Fujian Province, with a total area of 1001.41 km². It is situated in the central subtropical zone and has a humid monsoon climate. The park experiences four distinct seasons and has a superior natural environment with abundant fauna and flora resources. It is also home to numerous historical and cultural heritage sites, possessing outstanding and universal value. The forest coverage in Wuyishan National Park is as high as 87.86%. Since the Tang Dynasty, this area has been a major tea-producing region. Historically, there were few large-scale or concentrated tea plantations in the Wuyi Mountain area, and tea plantations were scattered [37]. Since the 1990s, with the thriving development of the tea industry in Wuyi Mountain driven by economic interests, there has been an occurrence of the rapid expansion and disorderly development of tea plantations, leading to a widespread change in land use/cover where forest land has been converted into tea plantations [38].

2.2. Site Selection and Sampling

From September to October 2020, based on the topographic map, soil type map, and forest stand map of Wuyishan National Park, and combined with field investigations in the study area, tea plantations (with a plantation age between 16–21 years) and three adjacent forest stand types (Masson pine forest, mixed forest, bamboo forest) that met the requirements of this research design were selected. For each type, 3 or more plots measuring 20 m × 20 m were selected. The adjacent grid method was used to measure the woody plants (with a minimum measured diameter of ≥2.5 cm) within the plots. Forest inventory factors, such as species composition, diameter at breast height, tree height, height under the lowest branch, and canopy density, were recorded. At the same time, the site factors of the plots, such as elevation, slope, and aspect, were recorded. Three small plots measuring 1 m × 1 m were set up at 7 m, 14 m, and 21 m along one diagonal of the standard plots. Two additional small plots were set up at 7 m and 21 m along the other diagonal as field replicates. After removing the litter, five-point composite samples were collected using a 7 cm diameter soil auger, with a sampling depth of 0–20 cm. The soil samples were collected and stored in self-sealing bags, sealed, and refrigerated before being transported to the laboratory. Upon arrival, the samples were carefully processed by removing roots, gravels, and any organic residues. Subsequently, the samples were sieved using a 2 mm mesh sieve. To conduct soil nutrient determination, one portion of the samples was allowed to air-dry naturally, while another portion was refrigerated at 4 °C to assess soil enzyme activity.
Tea plantation management history: Apply 150–200 kg/acre of compound fertilizer around October each year, around 80 kg/acre in March, and around 80 kg/acre in June. Use low-toxicity pesticides for disease prevention and pest control between late March, early April (15 days before tea picking), June, and July, and from 25 August to 10 September. The basic properties of the forest and tea plantation soils plots are shown in Table 1.

2.3. Soil Parameter Determination

Total carbon (TC) and total nitrogen (TN) were determined using an elemental analyzer (Vario isotope cube, Germany) [39]. Total phosphorus (TP) was measured using an inductively coupled plasma mass spectrometer (ICP-MS) [40]. Soil enzyme activity was determined using colorimetric methods. The principles of the measurements are as follows: β-glucosidase (BG) hydrolyzes para-nitrophenyl-β-D-pyranoglucoside to form para-nitrophenol, which has a maximum absorption peak at 400 nm; β-N-acetylglucosaminidase (NAG) hydrolyzes β-N-acetylglucosaminidase to form para-nitrophenol, which also has a maximum absorption peak at 400 nm; acid phosphatase (ACP) hydrolyzes casein to produce tyrosine, and under alkaline conditions, tyrosine reduces phosphomolybdate compounds to form tungsten blue, which has a characteristic absorption peak at 680 nm.

2.4. Calculation of Enzyme Stoichiometry Ratio

Enzyme stoichiometry refers to the ratio of C, N, and P in acquiring enzyme activity [41,42]. The calculation formulas are as follows:
C/N enzyme activity ratio (EC/N) = ln(BG)/ln(NAG)
C/P enzyme activity ratio (EC/P) = ln(BG)/ln(ACP)
N/P enzyme activity ratio (EN/P) = ln(NAG)/ln(ACP)
The calculation formulas for vector length (VL, dimensionless) and vector angle (VA, °) are as follows:
VL = [(ln(BG)/ln(NAG))2 + (ln(BG)/ln(ACP))2]1/2
VA = Degrees{ATAN2[(ln(BG)/ln(ACP)), (ln(BG)/ln(NAG))]}
The length of VL represents the degree of C limitation; VA < 45° indicates N limitation, while VA > 45° indicates P limitation.

2.5. Data Processing and Analysis

The ggpubr package 0.4.0 in R version 3.6.3 was used to compare the significant differences in soil carbon, nitrogen, phosphorus, enzyme activity, and their stoichiometric ratios between different forest types and tea plantations using paired t-tests (p < 0.05). One-way analysis of variance (ANOVA) was used to compare the significance of differences in carbon, nitrogen, phosphorus, and enzyme activity among different forest types (p < 0.05). The corrplot package 0.84 was used to analyze the Pearson correlation between soil enzyme stoichiometric ratios and soil physicochemical properties. The vegan package 2.5-7 was used for redundancy analysis (RDA) to analyze the relationship between soil enzyme activity and soil nutrients. The rdacca.hp package 1.0-8 was used to analyze the explanatory factors of soil nutrients on enzyme activity [43].

3. Results

3.1. Effects of Converting Three Forest Types into Tea Plantations on Soil Carbon, Nitrogen, Phosphorus, and Their Stoichiometry

From Figure 1, it can be observed that before conversion, among the three forest types, the background conditions of carbon, nitrogen, phosphorus, and their stoichiometry ratios were relatively consistent, except for the bamboo forest. Additionally, the TN content and TP content in the bamboo forest were significantly higher than those in the Masson pine forest and mixed forest. After the conversion of the forest to tea plantations, significant changes occurred in soil carbon, nitrogen, phosphorus, and their stoichiometry. Specifically, after conversion, the soil TC and TN in all three forest types showed a significant decrease (p < 0.05), while soil TP exhibited a significant increase. Furthermore, significant decreases were observed in all soil C:N, C:P, and N:P ratios, except for the C:N ratio in bamboo forests.
In terms of the magnitude of changes in each indicator, after converting Masson pine forests into tea plantations, the soil TC and TN content exhibited the greatest changes, significantly differing from the mixed forest and bamboo forest (Table 2). There was no significant difference in the magnitude of change in soil TP content among the three forest types (p > 0.05). The changes in soil C:N ratio were significantly different between Masson pine forest and mixed forest compared to the bamboo forest (p < 0.05). There were significant differences in the magnitude of change in soil C:P ratio among the three forest types (p < 0.05), with the mixed forest showing the greatest change. The magnitude of change in the soil N:P ratio in Masson pine forest and mixed forest was significantly higher than that in the bamboo forest (p < 0.05).

3.2. Impact of Converting Three Forest Types into Tea Plantations on Soil Enzyme Activity and Enzyme Metrics

3.2.1. Impact of Converting Three Forest Types into Tea Plantations on Soil Enzyme Activity

As shown in Figure 2, there were no significant differences in soil BG and ACP content among the three forest types. However, the NAG content in the soil of the bamboo forest was significantly higher compared to that in the Masson pine forest and mixed forest. After converting the forestland into tea plantations, the content of soil BG, NAG, and ACP decreased. Among them, the soil BG, NAG, and ACP content in the Masson pine forest showed the largest decrease after the conversion into a tea plantation.

3.2.2. Impact of Converting Three Forest Types into Tea Plantations on Soil Enzyme Metrics

As shown in Table 3, in terms of enzyme stoichiometry, there were no significant differences (p > 0.05) between the Masson pine forest and the tea plantation. The mixed forest soil was only significantly different from the tea plantation in terms of EC/P. The bamboo forest soil was significantly different from the tea plantation in terms of EC/P and EN/P. In terms of vector length, only the mixed forest was significantly difference from the tea plantation; both the Masson pine forest and the mixed forest showed an increase in VL after converting into a tea plantation. In terms of the vector angle, there were no significant differences (p > 0.05) between the three forest types and the tea plantation, and all VA values were greater than 45°. However, after converting the three forest types into tea plantations, there was a decrease in VA.

3.3. Relationship between Soil Enzyme Activity and Soil Nutrients

The correlation analysis between soil enzyme activity, enzymatic stoichiometry, and soil factors (Table 4) revealed the following relationships: Soil BG showed a significant positive correlation with TC, TN, C/P, and N/P (p < 0.05). NAG exhibited a highly significant positive correlation with TC and TN (p < 0.01), and a significant negative correlation with C/N. ACP displayed a significant negative correlation with TP. EC/N demonstrated a highly significant positive correlation with C/N. EC/P showed a highly significant positive correlation with TC and TN. EN/P exhibited a highly significant positive correlation with TC, TN, and TP, and a highly significant negative correlation with C/N.
The results of the redundancy analysis (RDA) for forest soil enzyme activity and the explained variance by environmental factors are shown in Figure 3a and Figure 4a. Soil physicochemical factors explained 61.84% of the disparity in enzyme activity and enzymatic stoichiometry. The first ordination axis explained 54.08% of the relationship between enzyme activity and enzymatic stoichiometry, while the second ordination axis explained 7.76% of the relationship between enzyme activity and soil characteristics. Among them, TC (p = 0.001) and TN (p = 0.02) were found to be the main factors influencing soil enzyme activity and enzymatic stoichiometry, explaining 27.95% and 15.72% of the variance, respectively.
The results of the redundancy analysis (RDA) for tea plantation soil enzyme activity and the explained variance by environmental factors are shown in Figure 3b and Figure 4b. Soil physicochemical factors explained 37.82% of the disparity in enzyme activity and enzymatic stoichiometry. The first ordination axis explained 26.04% of the relationship between enzyme activity and enzymatic stoichiometry, while the second ordination axis explained 11.78% of the relationship between enzyme activity and soil characteristics. Among them, C/N (p = 0.007) and TC (p = 0.049) were found to be the main factors influencing soil enzyme activity and enzymatic stoichiometry, explaining 14.37% and 7.81% of the variance, respectively.

4. Discussion

4.1. The Impact of Forest-to-Tea Conversion on Soil Carbon, Nitrogen, Phosphorus, and Their Stoichiometry

The conversion of land use practices can alter plant species composition, litter, fine root biomass, and microbial community structure, leading to various changes in soil TC, TN, and TP content [44,45]. This study found significant effects of converting forests to tea plantation on soil TC, TN, and TP content. After the conversion, soil TC and TN content decreased significantly. On the one hand, due to the complex vegetation composition and structure of Masson pine forests, bamboo forests, and mixed forests compared to tea plantation, the coverage of herbaceous plants in the forest understory is also greater than in tea plantation. The composition, quantity, and quality of leaf litter in forests are several times higher than in the tea plantation. The enhanced activity of soil animals and microorganisms accelerates the decomposition of leaf litter, leading to increased organic matter input, and as a result, TC content and TN content in forests are higher than in tea plantation [18]. On the other hand, forests have developed root systems that can secrete nutrients into the soil, promoting the accumulation of TC and TN nutrient content in the soil [46]. Soil TP is a sedimentary element primarily influenced by parent material and climate, and soil phosphorus is relatively stable and not easily transported [44]. Therefore, the increase in soil TP content after the conversion from forests to tea plantation is related to the artificial application of phosphorus fertilizer during tea cultivation. Zhou et al. [47] found a substantial increase in the available phosphorus content in tea plantation soils in the Wuyi tea area from 2008 to 2015, with an increase of over 40 times, which indicates the widespread occurrence of excessive fertilizer application in this region’s tea plantation.
Soil carbon, nitrogen, and phosphorus ecological stoichiometry are important indicators for evaluating soil quality and the health status of soil ecosystems [48]. The average C:N ratios (15.69, 13.30) of the three forest types and tea plantation in the research area are higher than the average value reported nationally China (11.9) [14] and the global average (13.3) [49]. This indicates that the organic matter decomposition rate in the soil of the study area is relatively slow, which is beneficial for maintaining soil fertility. It was found in the study that the C:N ratios of the three forest types were higher than that of tea plantation, which is consistent with the results of Yang et al. [9]. This suggests that forest soil with less human interference and less fertilizer use has higher C:N, C:P, and N:P ratios, while agricultural soil with more frequent human activities and larger fertilizer application has lower C:N, C:P, and N:P ratios. The average C:P ratio of forest soil in this study is 753.05, which is higher than the global average (186) [50], the national average for China (136) [14], and the soil of evergreen broad-leaved forests in southern Fujian Province (39.9) [16], indicating the low phosphorus availability in the forest in this region and intense competition between soil microorganisms and plants for phosphorus. The average C:P ratio of tea plantation soil is 70.24, significantly lower than the global average (186) [50] and the national average for China (136) [14], indicating the relative abundance of phosphorus in tea plantation soil in this region. The C:P ratio of soil in the three forest types significantly decreased after the establishment of tea plantation, possibly due to greater disturbance from agricultural cultivation activities in the tea plantation, resulting in a higher carbon loss rate compared to nitrogen and phosphorus (see Figure 1 and Table 1), leading to lower C:P ratios in tea plantation soil compared to forests. The average N:P ratio of forest soil in this research is 45.33, higher than the national average for China (5.2) [14] and for global forest soil (5.9) [50], indicating sufficient nitrogen supply and limited phosphorus content in the forest soils of Wuyishan National Park. This suggests that forest growth in the study area may be constrained by soil phosphorus. The average N:P ratio of tea plantation soil is 5.22, similar to the national average for China (5.2) [14]. Research has demonstrated that when the N:P ratio in the soil is lower than 14, it indicates that vegetation growth in the area is limited by nitrogen [51]. After the establishment of tea plantation, the N:P values of soil in all three forest types significantly decreased, which may be due to soil tillage during tea cultivation, which disrupts soil aggregation and leads to nitrogen loss from the soil. In addition, the application of phosphorus fertilizer during tea cultivation increases the phosphorus content in the soil, resulting in a significant decrease in the N:P ratio.

4.2. The Effects of Converting Forest Land to Tea Plantation on Soil Enzyme Activity and Its Stoichiometric Ratios

Soil enzyme activity is mainly influenced by various environmental factors, such as microorganisms, soil nutrients, plant root systems and their secretions, litter, and human activities [52]. The most direct and rapid way for humans to interfere with soil quality is through changes in land use types, which can alter soil moisture, thermal conditions, nutrient content, soil structure, and microbial activity, thereby affecting the biogeochemical cycles within an ecosystem and resulting in changes in soil quality [11,53,54]. In this study, the soil contents of β-glucosidase (BG), N-acetylglucosaminidase (NAG), and acid phosphatase (AP) in the three forest types decreased significantly after the conversion of forest into tea plantation, which is consistent with the results of Tian et al. [55] and Ning et al. [56]. Forest soil tends to have higher enzyme activities compared to cultivated land, and the conversion from forest land to tea plantation leads to a decrease in soil enzyme activity. Previous research has found variations in root exudation rates for different vegetation types and soil nutrient conditions [57,58], which inevitably affect soil enzyme activity and its stoichiometric ratios. This study found that the conversion of vegetation types significantly influenced soil enzyme activity. The conversion from forest land to tea plantation changes the aboveground vegetation cover, litter types, thickness, as well as belowground root morphology and root exudates, resulting in differences in the quantity, type, and chemical properties of organic matter input in the litter layer and soil input layer [59,60,61]. Consequently, soil physicochemical properties are altered (decreased TC and TN, in-creased TP content), the conversion from forest land to tea plantation leads to a decrease in soil enzyme activity. The activity of soil enzymes is influenced by the content of active soil organic matter. Forests, with their plentiful litter on the forest floor, contribute a substantial amount of organic matter to the soil through decomposition. Additionally, trees in the forest ecosystem are tall and have dense foliage, enabling them to perform more photosynthesis. The well-developed root systems of trees can efficiently absorb and input fixed nutrients into the soil, thereby enhancing the accumulation of active soil organic matter, stimulating microbial activity, increasing microbial population, and promoting the secretion of microbial extracellular enzymes [62,63]. Therefore, forest soil tends to have higher enzyme activity compared to tea plantation.
Soil ecological enzyme stoichiometry can effectively measure the nutrient requirements of soil microorganisms, such as carbon, nitrogen, phosphorus, etc. [64]. The findings of this study indicate that the soil EC/P and EN/P ratios increased after converting forest land to tea plantation. Among them, the soil EC/P and EN/P ratios of mixed forests were significantly different from tea plantation, and the soil EC/P and EN/P ratios of bamboo forests were also significantly different from tea plantations. This indicates that the limitation of soil phosphorus has been alleviated after converting forest land to tea plantation, but the limitation of soil nitrogen has been aggravated. In this study, the soil EC/N ratios of forest land and tea plantation ranged from 0.97 to 1.09, which is below the global average of 1.41. The soil EC/P and EN/P ratios ranged from 0.82 to 0.87 and from 0.79 to 0.98, respectively, which are significantly higher than the global average of 0.62 and 0.44 [33,65]. This indicates that the soil in the study area has high enzyme activity of nitrogen and phosphorus, which suggests that the decomposition of soil organic matter is hindered by a phosphorus limitation and nitrogen deficiency. According to the resource allocation theory, soil microorganisms tend to secrete more hydrolytic enzymes related to the nutrient deficiency to meet their own needs [66]. This study found that the soil VA of the three forest types and tea plantation were all greater than 45°, suggesting that the phosphorus limitation of soil microorganisms in forests and tea plantation in this region is prominent. This may be due to the acidity of the soil in this region, where phosphorus is easily complexed into forms that are not easily utilized by plants and microorganisms, leading to limited phosphorus in the soil [67]. After conversion to tea plantation, the VA showed varying degrees of decline, indicating that the conversion to tea plantation has alleviated the phosphorus limitation to some extent. This may be attributed to the application of phosphorus fertilizer during tea cultivation, which reduces the activity of acid phosphatase. It has been shown that the activity of acid phosphatase is negatively correlated with the availability of phosphorus in the environment [66].

4.3. Driving Factors of Changes in Soil Enzyme Activity during the Conversion from Forest Land to Tea Plantation

Prior research has indicated that both biotic and abiotic elements within the soil can impact soil enzyme activity and its stoichiometric ratios by regulating microbial metabolism [35,68,69]. In this study, the enzyme activity and stoichiometric ratios of forest soil were mainly influenced by total carbon (TC) and total nitrogen (TN), with TC being the key influencing factor (27.95%). However, after the conversion to tea plantation, the soil enzyme activity and stoichiometric ratios were mainly influenced by the C:N ratio and TC, and the key influencing factor changed from TC to C:N ratio. This is consistent with the findings of Peng et al. [61], indicating that soil enzyme activity and stoichiometric ratios are greatly influenced by soil nutrients and their stoichiometry. Soil TC and TN are important elements in the soil that can affect soil structure and microbial activity, thereby influencing soil enzyme activity [55]. During the conversion from forest land to tea plantation, soil TC, TN, BG, NAG, and ACP contents all decreased, and there was a significant positive correlation between TC, TN, BG, NAG, and ACP. This is because soil carbon content directly affects the cycling of materials and the growth and reproductive capacity of microorganisms, providing a suitable environment for enzyme activity, while nitrogen is a source of plant-available nitrogen that can provide energy for microbial enzyme secretion. Therefore, higher carbon and nitrogen content is beneficial for the synthesis of soil enzymes [70,71]. The impact of the soil C:N ratio on soil enzymes may be due to the fact that soil enzymes mainly come from the decomposition of litter, microbial production, and root secretion products. Changes in nutrient inputs from litter and other sources can alter soil nutrient stoichiometry and microbial biomass, thus affecting soil enzyme activity and stoichiometry [11].

5. Conclusions

The expansion of the Wuyishan tea plantation has led to significant changes in surface soil nutrient content and ecological enzyme activity, which have particularly affected soil nutrient content. After conversion of the three forest types into tea plantations, the soil TC and TN decreased significantly, while TP increased significantly. The stoichiometric ratio of carbon, nitrogen, and phosphorus also showed a significant decrease. Among them, the changes in carbon, nitrogen, and phosphorus and their stoichiometric ratios in mixed forests were the most significant, with the C/P element response being the most pronounced. After the conversion from three types of forests to tea plantation, there was a significant decrease in soil BG, NAG, and ACP content, and the change in soil enzyme activity was most significant in Masson pine forests. Through the analysis of soil enzyme stoichiometry, it was found that forest soils in this area were generally limited by phosphorus (P), and the conversion to tea plantation partially alleviated the soil P limitation but still had some nitrogen (N) limitation. Soil TC, TN, and C/N were the key driving factors for enzyme activity and its stoichiometric ratios, and the determining factor for the impact on enzyme activity and its stoichiometric ratios shifted from TC to C/N after the conversion from forests to tea plantation.

Author Contributions

Conceptualization, Y.L., J.Z. and W.Y.; methodology, Y.L.; software, Y.L.; validation, W.Y. and Q.Q.; formal analysis, Y.L. and J.Z.; investigation, J.Z. and Y.Z.; resources, Y.Z.; data curation, Y.L.; writing—original draft preparation, Y.L., J.Z. and W.Y.; writing—review and editing, W.Y. and Q.Q. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Discipline Innovation Team Project of Fujian Agriculture and Forestry University (72202200205) and the Study on Environmental Impact Assessment of Tea Plantation in Wuyishan National Park (118/KH190360A).

Data Availability Statement

Due to the need for further research and exploration of the relevant data in this article, the original data of this article is not currently available.

Conflicts of Interest

The authors declare no conflict of interest.

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Forests 14 02043 g001
Figure 1. Soil carbon, nitrogen, and phosphorus and their stoichiometric ratios in forest and tea plantations. (a) Soil carbon content, (b) Soil nitrogen content, (c) Soil phosphorus content, (d) Soil carbon:nitrogen ratio, (e) Soil carbon:phosphorus ratio, (f) Soil nitrogen:phosphorus ratio. Note: Different lowercase letters indicate significant differences between forest land and tea plantations; different capital letters signify notable differences among various forest types.
Figure 1. Soil carbon, nitrogen, and phosphorus and their stoichiometric ratios in forest and tea plantations. (a) Soil carbon content, (b) Soil nitrogen content, (c) Soil phosphorus content, (d) Soil carbon:nitrogen ratio, (e) Soil carbon:phosphorus ratio, (f) Soil nitrogen:phosphorus ratio. Note: Different lowercase letters indicate significant differences between forest land and tea plantations; different capital letters signify notable differences among various forest types.
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Figure 2. Characteristics of soil enzyme activity in forest and tea plantation. (a) C-acquiring enzyme (BG, b-1, 4-glucosidase), (b) N-acquiring enzymes (NAG, β-n-acetylglucosidase), (c) Organic P-acquiring enzyme (ACP, acid phosphatase). Note: Different lowercase letters indicate significant differences between forest land and tea plantations; Distinct capital letters indicate significant differences among various forest types.
Figure 2. Characteristics of soil enzyme activity in forest and tea plantation. (a) C-acquiring enzyme (BG, b-1, 4-glucosidase), (b) N-acquiring enzymes (NAG, β-n-acetylglucosidase), (c) Organic P-acquiring enzyme (ACP, acid phosphatase). Note: Different lowercase letters indicate significant differences between forest land and tea plantations; Distinct capital letters indicate significant differences among various forest types.
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Figure 3. Redundancy analysis (RDA) of soil enzyme activities and ecoenzymatic stoichiometry ratios in forest (a) and tea plantation (b).
Figure 3. Redundancy analysis (RDA) of soil enzyme activities and ecoenzymatic stoichiometry ratios in forest (a) and tea plantation (b).
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Figure 4. The significance of soil nutrients in relation to soil enzyme activities and their stoichiometric ratios. * significant correlation at 5% probability level; ** significant correlation at 1% probability level; *** significant correlation at 0.1% probability level.
Figure 4. The significance of soil nutrients in relation to soil enzyme activities and their stoichiometric ratios. * significant correlation at 5% probability level; ** significant correlation at 1% probability level; *** significant correlation at 0.1% probability level.
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Table 1. Basic properties of forest and tea plantation soils.
Table 1. Basic properties of forest and tea plantation soils.
TypepHSWC (%)Soil TypeSoil Temperature (℃)
PM4.85 ± 0.6223.54 ± 0.07Red soil22.4 ± 1.23
PM tea plantation4.54 ± 0.2621.45 ± 0.06Red soil21.05 ± 0.99
MF4.65 ± 0.1529.94 ± 0.05Red soil22 ± 0.24
MF tea plantation4.04 ± 0.1227.97 ± 0.03Red soil21.5 ± 0.37
BA5.14 ± 0.2236.09 ± 0.06Red soil20.5 ± 0.69
BA tea plantation4.24 ± 0.2428.45 ± 0.01Red soil19.6 ± 0.45
Note: PM: Pinus massoniana forest; PM tea plantation: Pinus massoniana forest converted into tea plantation; MF: mixed forest; MF tea plantation: mixed forest converted into tea plantation; BA: bamboo forest (Phyllostachys hterocycla forest); BA tea plantation: bamboo forest converted into tea plantation.
Table 2. Variation range of soil carbon, nitrogen, and phosphorus and its stoichiometry after forest land conversion to tea plantation.
Table 2. Variation range of soil carbon, nitrogen, and phosphorus and its stoichiometry after forest land conversion to tea plantation.
TypeΔTC (%)ΔTN (%)ΔTP (%)ΔC/N (%)ΔC/P (%)ΔN/P (%)
PM-Tea plantation−138.9 ± 19.51A−128.98 ± 16.86A65.9 ± 27.85A−22.23 ± 4.17A−1224.47 ± 238.41B−1130.35 ± 320.62A
MF-Tea plantation−90.61 ± 19.33AB−63.37 ± 22.55B81.88 ± 16.30A−25.33 ± 6.51A−1892.32 ± 347.86A−1159.5 ± 306.60A
BA-Tea plantation−67.64 ± 19.17B−58.02 ± 10.97B70.11 ± 20.47A−4.30 ± 0.98B−622.16 ± 154.43C−513.83 ± 100.28B
Note: Range of change = tea plantation index value—Forest land index value/Forest land index value. Different capital letters indicate significant differences in the magnitude of changes in indicators after the transformation from forest to tea plantations.
Table 3. Stoichiometric ratio of soil enzymes in forest and tea plantation.
Table 3. Stoichiometric ratio of soil enzymes in forest and tea plantation.
TypeEC/NEC/PEN/PVector LengthVector Angle
PM1.06 ± 0.04a0.85 ± 0.09a0.80 ± 0.07a1.36 ± 0.09a51.29 ± 2.54a
PM_tea plantation1.06 ± 0.09a0.86 ± 0.08a0.81 ± 0.03a1.37 ± 0.13a50.91 ± 0.91a
MF1.04 ± 0.15a0.83 ± 0.05b0.79 ± 0.06a1.33 ± 0.07b51.58 ± 1.92a
MF_tea plantation1.09 ± 0.09a0.87 ± 0.01a0.80 ± 0.06a1.40 ± 0.07a51.49 ± 2.27a
BA0.98 ± 0.05a0.85 ± 0.03b0.87 ± 0.02b1.30 ± 0.06a48.94 ± 0.73a
BA_tea plantation0.97 ± 0.08a0.87 ± 0.04a0.89 ± 0.04a1.30 ± 0.09a48.23 ± 1.10a
Note: Different lowercase letters indicate significant differences between forest land and tea plantations.
Table 4. Correlation coefficient between soil enzymes, enzymatic stoichiometry, and soil properties.
Table 4. Correlation coefficient between soil enzymes, enzymatic stoichiometry, and soil properties.
Soil PropertyTCTNTPC/NC/PN/P
Enzyme
BG0.784 **0.640 **−0.0770.0250.378 *0.406 **
NAG0.805 **0.949 **0.088−0.368 *0.1080.172
ACP0.416 **0.320 *−0.391 *0.3010.2660.289
EC/N0.171−0.117−0.1020.426 **0.2120.148
EC/P0.515 **0.427 **0.246−0.1220.1350.114
EN/P0.427 **0.597 **0.404 **−0.536 **−0.051−0.017
Note: * significant correlation at 5% probability level; ** significant correlation at 1% probability level.
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Li, Y.; Zhang, J.; Qiu, Q.; Zhou, Y.; You, W. Changes in Soil Properties and Enzyme Stoichiometry in Three Different Forest Types Changed to Tea Plantations. Forests 2023, 14, 2043. https://doi.org/10.3390/f14102043

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Li Y, Zhang J, Qiu Q, Zhou Y, You W. Changes in Soil Properties and Enzyme Stoichiometry in Three Different Forest Types Changed to Tea Plantations. Forests. 2023; 14(10):2043. https://doi.org/10.3390/f14102043

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Li, Ying, Jinlin Zhang, Qingyan Qiu, Yan Zhou, and Weibin You. 2023. "Changes in Soil Properties and Enzyme Stoichiometry in Three Different Forest Types Changed to Tea Plantations" Forests 14, no. 10: 2043. https://doi.org/10.3390/f14102043

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