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Article

Chemical Stoichiometry and Enzyme Activity Changes during Mixed Decomposition of Camellia sinensis Pruning Residues and Companion Tree Species Litter

by
Hongjiu Zhao
1,
Rui Yang
1,*,
Congjun Yuan
2,
Shaqian Liu
1,
Chunlan Hou
1 and
Haodong Wang
1,2
1
College of Forestry, Guizhou University, Guiyang 550025, China
2
Guizhou Academy of Forestry, Guiyang 550005, China
*
Author to whom correspondence should be addressed.
Agronomy 2023, 13(7), 1717; https://doi.org/10.3390/agronomy13071717
Submission received: 3 June 2023 / Revised: 22 June 2023 / Accepted: 24 June 2023 / Published: 27 June 2023
(This article belongs to the Special Issue Environmental Ecological Remediation and Farming Sustainability—Volume II)
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Abstract

:
(1) Background: In managing ecological tea gardens, litter composed of pruned and fallen tea leaves from companion tree species is an important component of tea garden soil. The decomposition of litter plays a crucial role in regulating nutrient cycling in tea garden ecosystems. (2) Methods: This study employed the litterbag method to investigate chemical stoichiometry characteristics and enzyme activity changes during the decomposition process of pruned and fallen Camellia sinensis leaves from companion tree species in an ecological tea garden located in central Guizhou Province. (3) Results: With decomposition duration, the general trend of changes in the C/N and C/P ratios showed a decrease in the activity of UE (urease), AP (acid phosphatase), and PPO (polyphenol oxidase) followed by an increase, while CAT (catalase) and CEL (cellulase) activity decreased, then increased, and then decreased again. On the other hand, the N/P and the activity of SC (sucrase) first increased and then decreased. The C/N and the activities of UE, PPO, and AP generally reached their maximum values during the late decomposition stage (366–428 d), while the N/P and the CAT activity peaked during the mid-decomposition stage (305 d). In contrast, the activity of SC and CEL reached its maximum value during the early decomposition stage (123 d). The N/P ratios were significantly higher than those of the CS (C. sinensis) litter in the mixed treatment, while C/N and C/P ratios were significantly lower than those in the CS during decomposition for 184–366 days. The UE, CAT, AP, and SC activities of CBL (C. sinensis + B. luminifera) litter were significantly higher than those of the CS litter during decomposition. During the experiment, antagonistic effects were observed in the C/N and C/P ratios of the different litter types. Most mixed litter exhibited additive effects on enzyme activity, while a few showed nonadditive effects. For the nonadditive effects, most were antagonistic effects, mainly in the CPM (C. sinensis + C. glanduliferum) litter. A small portion, mainly observed in the CBL and CCG (C. sinensis + C. glanduliferum) litter, showed synergistic effects. (4) Conclusions: Selecting B. luminifera and C. glanduliferum to be part of the tree species composition in ecological tea gardens can produce positive mixed effects on enzyme activity during litter decomposition, increase nutrient return capacity, maintain tea garden fertility, and achieve the ecological development of tea gardens.

1. Introduction

Litter refers to all organic matter produced by plants and returned to the soil in a forest ecosystem. Litter decomposition is a complex process and a major link in material cycling and energy flow in forest ecosystems [1]. Under natural conditions, litter generally exists in the form of mixtures. Mixed litter decomposition creates different microenvironments compared to those created by the decomposition of single litter types, which affects microbial communities and thus impacts litter decomposition. Studies have shown that compared to single litter decomposition, mixed litter decomposition can better improve soil nutrients and significantly increase soil enzyme activity [2]. The decomposition of litter is closely related to environmental conditions, litter properties, and soil microbial communities [3]. Microbes can directly affect the process of litter decomposition [4]. The enzymes secreted by microbes play an important role in this process. Litter decomposition can be seen as an enzymatic hydrolysis process based on the decomposer cell level [5]. Enzyme activity can quickly respond to changes in litter decomposition conditions and reflect to some extent the speed of decomposition and the release level of nutrients such as carbon, nitrogen, and phosphorus from the litter [6]. Therefore, enzyme activity during litter decomposition is considered an important indicator of litter decomposition [7]. It plays an important role in material cycling and energy flow in forest ecosystems. The magnitude of enzyme activity varies depending on litter composition and environmental conditions [8,9]. Ecological stoichiometry is a theory that involves studying the balance of interactions between multiple chemical elements in ecological processes and the coupling relationships between chemical elements. It can provide insights into the interactions between elements and the limiting role of elements in ecosystem productivity, nutrient cycling, and nutrient allocation [10]. Studying the changes in ecological stoichiometry and enzyme activity during mixed litter decomposition can help in understanding the internal material cycling mechanisms in species composition configuration, which is important for maintaining soil fertility in forested areas and sustainable management.
Camellia sinensis is one of the characteristic tree species in Guizhou. With the improvement of its consumption levels, tea has become one of the world’s three major nonalcoholic beverages. Due to the long-term cultivation of a single type of tea plant during the management and operation of tea production, tea gardens commonly experience soil depletion, erosion, and an increased prevalence of pests and diseases, severely affecting both tea yield and quality. As a result, the establishment of ecological tea gardens, which prioritize tea plants as the main species and are guided by both ecological and economic principles, has received increasing attention. Mixing different tree species with tea plantings can enrich the diversity of species in the ecosystem and improve the chemical composition of litter [11], which more effectively enhance and improve the soil fertility, structure, and function of the ecosystem. Research has found that mixed litter decomposition can effectively increase the rate of litter decomposition [12], element release [13], and enzyme activity [14]. Research has also shown that there is no significant interaction between the nutrient release rate and decomposition rate during mixed litter decomposition [15]. The diversity of species composition and the unknown chemical properties of different species’ litter accumulation, in addition to regional environmental differences, contribute to the complexity of variations in litter decomposition among different tree species. Mixing tea-pruning residues with the litter of accompanying tree species may effectively enhance the enzymatic activity and promote decomposition. The main challenge in managing ecological tea gardens lies in how to select and combine tree species to fully utilize the litter from different species in tea garden management. In this study, litter from pruned C. sinensis materials and associated tree species such as Cinnamomum glanduliferum, Betula luminifera, Cunninghamia lanceolata, and Pinus massoniana were selected as research objects. Using the decomposition mesh bag method to simulate natural litter decomposition and analyzing the dynamic changes in chemical stoichiometry and enzyme activity during the decomposition process of these litter types in ecological tea gardens provides a theoretical basis for the appropriate recycling of resources in ecological tea gardens, the long-term maintenance of soil fertility, and the full realization of the production potential and natural maintenance functions of plants under natural conditions.

2. Materials and Methods

2.1. Study Area

The research area is located in Jiu’an township, Huaxi District, Guiyang city, Guizhou Province (26°31′8″~26°31′12″ N, 106°36′47″~106°36′50″ E). The average annual temperature is 13.6 °C, with warm winters and cool summers. The average temperature in the coldest month is 3–4 °C, while the hottest month has an average temperature of 22 °C. There are 260 frost-free days per year, and the average annual rainfall ranges from 1000 to 1150 mm. The elevation ranges from 1100 to 1446 metres, and the area has a subtropical plateau monsoon climate that is environmentally friendly. The dominant soil type in the area is Orthic Acrisols [16].

2.2. Experimental Methods

Experimental design: The study utilized the litter bag method to examine the chemical stoichiometry characteristics and enzyme activity changes of pruned single tea material and litter from four common companion tree species in the garden. Four different mixed litter types were analyzed, revealing their combined decomposition effects and correlations.
Sample collection and processing: C. sinensis (CS) pruned materials (including branches and leaves) and leaf litter from P. massoniana (PM), C. lanceolata (CL), C. glanduliferum (CG), and B. luminifera (BL) were collected in July 2019 from the Jiu’an Ecological Tea Garden in Huaxi, Guizhou. The litter was brought back to the laboratory and dried to a constant weight. The dried litter was placed into litter bags with dimensions of 35 cm x 25 cm and a pore size of 1 mm. Each bag contained 40 g of litter. The study included one single treatment consisting of C. sinensis pruned materials (CS) and four mixed treatments (1:1 ratio) consisting of C. sinensis + C. glanduliferum (CCG), C. sinensis + P. massoniana (CPM), C. sinensis + C. lanceolata (CCL), and C. sinensis + B. luminifera (CBL). There were a total of five treatments with three replicates for each sample. Three bags were set aside for each sample before burial to analyze the initial nutrient content, as shown in Table 1.
Experimental arrangement: A flat area was selected in the experimental zone, and a 25 m × 20 m decomposition field was established. Surface litter was removed, and litter bags were spread out and buried in the soil layer between tea tree rows in the ecological tea garden. The same treatment was applied to each row, buried underground to a depth of 15–20 cm, and the bags were laid out parallel to each other with a distance of 2–5 cm between them without overlapping to mimic their natural state.
Sample recovery: From July 2019 to September 2020, samples were collected every two months (sampling was restricted in March 2020 due to epidemic control). Therefore, the fourth decomposition stage lasted for four months (184–305 days). Six bags were randomly selected from each treatment, and after removing impurities, they were brought back to the laboratory. Three of the bags were dried to a constant weight at 65 °C in an oven, and dry weight data were recorded. The litter was then crushed according to the measurement requirements to determine its chemical properties, such as total carbon (TC), total nitrogen (TN), total phosphorus (TP), total potassium (TK), lignin, and cellulose content. An additional three bags of fresh samples were stored at 4 °C for enzyme activity measurements.
Soil sample collection: According to the five-point sampling method, soil samples were collected from a depth of 15–20 cm, and visible plant roots and residues were removed from the soil. The soil was air-dried, sieved through 2 mm and 0.25 mm meshes, and made available for testing soil chemical properties, such as the contents of soil organic carbon (SOC), soil total nitrogen (STN), soil total phosphorus (STP), soil total potassium (STK), soil hydrolyzed nitrogen (SAN), soil available phosphorus (SAP), and soil available potassium (SAK). Table 2 shows the basic physical and chemical properties of the soil.
Test method: The carbon (C) contents were determined using the concentrated sulfuric acid–potassium dichromate method, the semimicro Kjeldahl method was used to determine the nitrogen (N) contents, the molybdenum–antimony resistance colorimetric method was used to determine the phosphorus (P) contents, and flame photometry was used to determine the potassium (K) contents. The contents of lignin and cellulose and the activities of cellulase (CEL), sucrase (SC), polyphenol oxidase (PPO), catalase (CAT), urease (UE), and acid phosphatase (AP) were determined by a biochemical kit from Beijing Solarbio Science & Technology Co., Ltd. (Beijing, China).

2.3. Data Analysis and Processing

Chemical stoichiometry characteristics and calculation of the expected enzyme activity period:
kw = (1/2) k1 + (1/2) k2
where kw represents the expected chemical stoichiometry characteristics and expected enzyme activity, k1 represents the chemical stoichiometry characteristics and enzyme activity of treatment 1, k2 represents the chemical stoichiometry characteristics and enzyme activity of treatment 2, and 1/2 represents the weight ratio of the two species in the mixed litter.
The data were statistically analyzed using SPSS 22.0 software (version number: 22.0.0.0.202), and the graphs were drawn using Origin 2018 (version number: 9.5). The significance of the litter decomposition chemical stoichiometry characteristics and enzyme activity was tested by one-way ANOVA. The significant differences between the measured and expected values for the mass decomposition rate and nutrient release rate of the mixed plant residue were tested by independent t tests. If there was no significant difference between the measured value and the expected value (p > 0.05), then it was an additive effect. If there was a significant difference between the expected value and the measured value, then it was a nonadditive effect [17]. If the measured value was significantly higher than the expected value (p < 0.05), then it was considered a synergistic effect; if the measured value was significantly lower than the expected value (p < 0.05), then it was considered an antagonistic effect [18].

3. Results

3.1. Stoichiometric Characteristics of Different Litter Types

3.1.1. Initial Nutrient Content of Different Mixed Litter Types

The initial nutrient contents of the five types of litter are shown in Table 3. There were certain differences in the initial nutrient contents among the different types of litter, among which, except for the lower contents of TC and TK in the CS treatment, the initial nutrient contents of the other nutrients in the CS treatment were higher than those in the four mixed treatments (p < 0.05).

3.1.2. Dynamic Changes in Stoichiometric Characteristics of Different Single Litter Types

The changes in stoichiometric characteristics during the decomposition of individual litter types are shown in Figure 1. The trends in the C/N ratio were generally similar, exhibiting an “N” shape, i.e., increasing–decreasing–increasing. The C/P ratio showed three different trends: PM and CG showed a decreasing trend followed by an increasing trend, BL and CL exhibited an “N” shape, and CS showed a decreasing–increasing–decreasing trend. In terms of N/P, CS, CL, PM, and CG, all showed a decreasing–increasing–decreasing trend, while BL followed an “M” pattern, i.e., increasing–decreasing–increasing–decreasing. After 428 days of decomposition, significant differences (p < 0.05) were observed in the C/N, C/P, and N/P ratios among the various litter types compared to their initial values.

3.1.3. Dynamic Changes in Stoichiometric Characteristics of Different Mixed Litter Types

Chemical stoichiometric characteristics change during mixed litter decomposition, as shown in Figure 2. The trend in C/N changes was generally consistent, with a decrease followed by an increase. C/P was divided into two different trends, where CCG, CPM, and CBL exhibited a “V” shape and CCL exhibited an “N” shape. In terms of N/P, there were no significant changes in CCG and CPM during the early–middle stages of decomposition (0–184 days), but they showed an increasing then decreasing trend during the late-middle stages (184–428 days); on the other hand, CCL and CBL showed a similar trend, characterized by a decrease–increase–decrease–increase–decrease. After 428 days of decomposition, there were significant differences (p < 0.05) in the C/N, C/P, and N/P ratios of all litter types compared to the undecomposed state.

3.1.4. Mixing Effects of Stoichiometric Characteristics in the Decomposition Process of Different Mixed Litter Types

The expected and measured stoichiometric characteristics of mixed litter, as shown in Figure 3, Figure 4, Figure 5 and Figure 6. During the decomposition process, the mixture effects of the four types of mixed litter with different stoichiometric characteristics were observed. The measured C/N ratios of the CCG, CCL, and CBL litter after 62 days of decomposition were significantly lower than the expected values (p < 0.05), indicating antagonistic effects. In contrast, the measured C/N ratio of the CPM litter was significantly higher than the expected value after 428 days of decomposition (p < 0.05), indicating synergistic effects; however, for the remainder of the time, there was no significant difference between the expected and measured values (p > 0.05), indicating additive effects. Analysis of the N/P ratio revealed that the CCG litter exhibited synergistic effects during 123 and 428 days of decomposition and additive effects for the remaining time. The CPM litter exhibited synergistic effects during 123–184 days of decomposition and antagonistic effects after 428 days. The CCL litter exhibited antagonistic effects during the later stages of decomposition (305–428 days), whereas the CBL litter showed antagonistic effects during 62, 184, and 428 days of decomposition and synergistic effects during 123 days of decomposition. Furthermore, the C/P ratios of all four types of litter were significantly lower than the expected values (p < 0.05) during the middle and later stages of decomposition (184–366 days), indicating antagonistic effects.

3.2. Changes in Enzyme Activities of Different Litter Types

3.2.1. Dynamic Changes in the Enzyme Activities of Different Single Litter Decomposers

The enzymatic activity changed during the decomposition of individual litter types, as shown in Figure 7. The UE activity exhibited three different trends, with the CL and PM litter showing an initial increase followed by a decrease, while the changes in the UE activity of the CG and BL litter followed an “N” pattern. In contrast, the UE activity of the CS litter decreased initially before increasing over time. The SC activity of all five litter types exhibited a similar trend, with an “M”-shaped pattern, i.e., an initial increase followed by a decrease, then followed by another increase and a final decrease. The dynamic changes in the AP, CEL, and PPO activities during the decomposition process were similar for all five litter types, with a decrease in enzyme activity during the early stages of decomposition, an increase during the middle stages, and a subsequent decrease. Notably, the PM and CL litter showed a sharp increase in PPO activity during the later stages of decomposition. The changes in CAT activity were also similar for all five litter types, remaining relatively unchanged during the 62–184 days of decomposition, followed by an increase and subsequent decrease. Only the PM litter showed a significant increase in CAT activity during the later stages of decomposition. After 428 days of decomposition, both the SC and CEL activities were lower than those during the early stages of decomposition, while AP and CAT activities were higher (p < 0.05).

3.2.2. Dynamic Changes in the Enzyme Activities of Different Mixed Litter Decomposers

The dynamic changes in enzyme activity during the decomposition of the mixed litter types are shown in Figure 8. The UE activity of CS, CCG, and CBL litter displayed a consistent trend of initial decrease followed by an increase, while the UE activity of CPM litter followed a “N” shape trend; the UE activity of CCL litter exhibited an inverted “V” shape trend. The SC activity of all five litter types showed a similar pattern, with an initial increase followed by a subsequent decrease. The dynamic changes in the AP and PPO activities were similar for all five litter types, displaying decreased enzymatic activity during the early stages of decomposition, followed by an increase during the middle stages. The dynamic changes in CEL and CAT activities were also similar, with an “N” shaped trend of initial decrease followed by an increase and a subsequent decrease. After 428 days of decomposition, the UE, AP, and CAT activities of all five litter types were significantly higher than those during the early stages (62 days) of decomposition, whereas the SC and CEL activities were both significantly lower compared to those during the early stages of decomposition (p < 0.05).

3.2.3. Mixed Effects of Different Mixed Litter Decomposition Enzyme Activities

The expected and measured enzyme activity characteristics of mixed litter are shown in Figure 9, Figure 10, Figure 11 and Figure 12. All four mixed litters showed different mixture effects during the decomposition process. The actual value of the UE activity for the CCG litter was significantly lower than expected (p < 0.05) before 184 days of decomposition, indicating an antagonistic effect, while it became significantly higher than expected (p < 0.05) afterward, indicating a synergistic effect. This trend was opposite to that of the SC activity. The PPO activity showed an additive effect before 184 days of decomposition and a synergistic effect afterward. The expected and actual values for the AP and CAT activity did not significantly differ (p > 0.05) throughout the entire decomposition process, indicating an additive effect. However, the actual value of the CEL activity was significantly lower than expected throughout the entire decomposition process, indicating an antagonistic effect. The expected values for the UE, CEL and CAT activities of the CCL litter were significantly higher than the measured values during the middle stages of decomposition, indicating antagonistic effects. The SC activity showed a synergistic effect during the early stages of decomposition, while the AP activity demonstrated a synergistic effect during the later stages. The PPO activity exhibited an additive effect throughout the entire process of decomposition. The measured values for the UE, AP, and CAT activities of the CBL litter were significantly higher than the expected values after 184 days of decomposition, indicating a synergistic effect. The SC activity showed a synergistic effect before 184 days of decomposition, while it exhibited an antagonistic effect after 184 days. Throughout the entire process of decomposition, the CEL activity demonstrated an additive effect, while the PPO activity exhibited an antagonistic effect.

3.3. Correlation between the Stoichiometric Characteristics of Litter and Enzyme Activity

Table 4 provides details on the correlation between litter stoichiometry and enzyme activity. The TC content of the four types of litter showed a negative correlation with CAT activity, with the CCG litter exhibiting a significant negative correlation (p ˂ 0.05) and the CCL litter showing an extremely significant negative correlation (p ˂ 0.01). The TN content and UE activity of the four types of litter showed a negative correlation. The CCG and CPM litter exhibited an extremely significant negative correlation, while the CBL litter showed a significant negative correlation. Additionally, the TP content showed an extremely significant positive correlation with the PPO activity (p ˂ 0.01). Furthermore, cellulose content showed an extremely significant negative correlation with the CEL activity (p ˂ 0.01). The lignin content of the CPM litter showed a significant negative correlation with the PPO activity (p ˂ 0.05). At the same time, the lignin content of the CCL litter showed an extremely significant negative correlation with the PPO activity (p ˂ 0.01). Additionally, the UE activity of the CCG, CPM and CBL litter exhibited an extremely significant positive correlation with the C/N ratio (p < 0.01) and a significant negative correlation with the N/P ratio (p < 0.05). There was no significant correlation (p > 0.05) found between the SC, AP, and CAT activities of CPM litter and several chemical stoichiometry characteristics. The SC activity of CCL litter also showed no significant correlation with several chemical stoichiometry characteristics.

4. Discussion

The C/N and C/P ratios of litter can reflect their decomposition rate and the efficiency of plant uptake and utilization of nitrogen and phosphorus elements. When the C/N and C/P are lower, the litter is more easily decomposed [19]. In this study, the C/N and C/P ratios of the CBL litter were lower than those of the other litter types, indicating that the CBL litter was more easily decomposed. The N/P ratio of litter is also an important indicator for determining whether N or P limits litter decomposition rates [20]. When the N/P ratio of litter is high, especially when it exceeds 25 and the P content is less than 0.22 g/kg, litter decomposition is mainly limited by P [21]. Based on previous research results [22] and the findings in this study, all N/P ratios of the different litter types in our study area were found to be lower than 25, and their P content was greater than 0.22 g/kg, indicating that N is the main limiting element for litter decomposition in the study area. The nutrient content changes during litter decomposition can be generally divided into three stages: a rapid decomposition stage, a slow decomposition stage, and a stabilization stage [23]. In the litter decomposition processes investigated in this study, the C/N ratio generally showed a decreasing trend followed by an increasing trend. The reason for these trends may have been that in the rapid and slow decomposition stages, the easily decomposable organic matter in the litter was broken down and released carbon dioxide, leading to a decline in total carbon content. However, as decomposition progressed, the resistant organic matter in the litter gradually decomposed, and the amount of carbon dioxide released decreased, resulting in a gradual decrease in the rate of decline in the total carbon content. At the stabilization stage, the recalcitrant organic matter was transformed into stable organic matter, and compared with the slow decomposition stage, the carbon element content in the litter increased. Studies have shown that there are three processes involved in the variation in nitrogen content during litter decomposition, namely, leaching (N loss), immobilization (N absorption), and mineralization (N release). The leaching stage involves the rapid release of unstable nitrogen in litter [24]. In this study, the N/P dynamic changes during the decomposition of the CBL and CCL litter followed this trend, while the N/P ratio of the CS, CCG, and CPM litter showed an initial increase followed by a decrease, skipping the leaching stage. This may have occurred because these four types of litter had already undergone leaching during their preparation indoors. Many studies have also confirmed that the nitrogen content of litter tends to increase during the early stages of decomposition [25,26]. The trends of C/P and C/N variation in this study were generally similar because the dynamic changes in TP content during litter decomposition exhibited a similar trend to that of TN content, which is consistent with previous research findings [27].
Litter decomposition can regulate material cycling in forests, with enzymes playing a crucial regulatory role. Based on substrate composition, litter-decomposing enzymes can be classified as protein-degrading enzymes, phosphatases, cellulases, and lignin-degrading enzymes [28]. In this study, UE belonged to the protein-degrading enzyme class, AP belonged to the phosphatase class, CEL belonged to the cellulase class, PPO and CAT belonged to the lignin-degrading enzyme class, and SC participated in the further decomposition of soluble sugars and cellulose degradation products. The results of this study indicate that overall, the changes in UE, AP, and PPO activities followed a pattern of initial decrease followed by an increase over time, while CAT activity decreased initially and then increased before declining again. CEL activity first decreased and then increased before decreasing again, whereas SC activity first increased before ultimately decreasing. During the early stages of decomposition, when the content of soluble sugars was high, microorganisms maintained their growth and reproduction by preferentially utilizing easily decomposable substances [29], which led to an increase in SC secretion and enhanced activity due to the release of soluble substrates from litter. The protein and soluble sugar substrate contents were highest during the initial stage of decomposition, resulting in an increase in protease and SC activity. In the middle stage of decomposition, the relative content of soluble sugars decreased, while insoluble polysaccharides such as cellulose increased. This finding corresponded to the increase in CEL activity associated with the decomposition of such substances. In the later stage of decomposition, the proportion of recalcitrant components such as lignin gradually increased with the progress of decomposition [30], leading to a gradual decrease in SC and CEL activity but noticeable increases in CAT and PPO activity. These findings indicate that different enzymes participated in various stages of litter decomposition. UE is a neutral enzyme that can hydrolyze organic matter and plays a crucial role in the transformation of organic nitrogen [31]. Its activity level is indicative of the nitrogen content in soil [32]. From a correlation perspective, the UE activity of the various litter types in this study showed a significant negative correlation with their TN content. This finding occurred because during the decomposition process, as N is released from the litter, the soil nitrogen content continues to increase, and UE activity also increases accordingly. Zhao Linsen’s research on mixed-species plantations showed that UE activity was positively correlated with soil organic matter content, total nitrogen, and alkali-hydrolysis nitrogen [33], and these results are consistent with those of this study. Phosphate is an inorganic phosphorus that can be directly absorbed by plants. During the initial stages of decomposition, the level of phosphorus in litter is relatively low. As such, microorganisms must synthesize a large amount of phosphatase to obtain the necessary phosphorus to sustain degradation [34]. Therefore, the activity of phosphatase significantly increases during the early stages of decomposition. In addition, the mass fractions of total nitrogen and ammonium nitrogen also have certain influences on the activity of phosphatase [35], with nitrogen being its main constituting element. Introducing nitrogen can enhance the activity of phosphatase [36]. Consequently, the trend in the time-based changes in AP activity is generally consistent with that of UE activity. The decomposition of cellulose can convert organic matter in litter into soil organic matter, which has significant implications for the carbon cycle of forest ecosystems. However, its ultimate degradation relies on the joint participation of various cellulase components [37]. The dynamic change trend of CEL activity in this study showed a pattern of decrease–increase–decrease, presenting an “N” shape. During the early stage of decomposition, CEL demonstrated relatively high activity levels due to the participation of fungi as the main microorganisms involved in cellulose degradation in litter. The sufficient nutrients in the litter during the early stage promoted the rapid growth of fungi, which consumed soluble sugars and nonwoody cellulose materials in the litter and converted them into components that could be directly absorbed and utilized [38]. Later, there was a slight increase, which was attributed to the difficulty in directly degrading lignocellulose by cellulase. PPO effectively delignified lignocellulose, and its product of delignified cellulose served as the substrate for CEL degradation [39]. At the later stage of decomposition, the increase in PPO activity led to an increase in the content of delignified cellulose, resulting in an increase in CEL activity. For the correlation, CEL activity showed a significant negative correlation with the cellulose content in the litter. This finding could be attributed to the fact that as the decomposition of litter proceeded, the microbial quantity and diversity gradually increased, resulting in an elevation of secretion and activity of CEL. PPO and CAT are the key enzymes participating in the degradation of recalcitrant components within plant litter [40,41], and they also play a crucial role in maintaining the carbon cycle of ecosystems [42]. During the initial stages of decomposition, the total carbon content gradually decreased in the five types of litter. As a result, CAT and PPO activities also decreased gradually during the early decomposition phase. In the later stages of decomposition, an increase in lignin content was observed in all five types of litter, which led to significant increases in CAT and PPO activities. However, as decomposition progressed, the quality of the litter matrix declined gradually, resulting in decreased soil animal activity. Moreover, low-nutrient levels during the later stages of decomposition suppressed microbial CAT secretion and reduced its activity [43]. This suggests that while enzyme activity affects nutrient release, the nutrient content of litter also influences enzyme activity [44]. In terms of correlation, the PPO activity and lignin content of the CCL and CPM litter were significantly negatively correlated, while the other two types of litter did not show a clear correlation. This finding may be related to the initial lignin content of the litter, as the initial lignin content of the CCL and CPM litter was significantly higher than that of the CCG and CBL litter. This indicates that the mixture of needle and broadleaf litter can significantly increase PPO activity, which is consistent with the findings of Song et al. [45]. The CAT activity of the four types of litter did not show a significant correlation with lignin content, indicating that enzymes acting on the same substance are coordinated in the decomposition process.
The decomposition of litter is typically influenced by multiple factors, including environmental factors, litter properties, and biological factors [46]. In the past, some studies have suggested that climate factors are the primary drivers of litter decomposition [47], but recent research indicates that plant litter traits may play a dominant role [48]. These findings primarily depend on the scale of measurement, where climatic factors have a significant impact on litter decomposition at a larger scale, while under similar environmental conditions, the chemical properties of litter are the main drivers of litter decomposition [49]. The nonadditive effects represent a critical criterion for assessing interspecies coordination, whereby the promotion of mass loss and nutrient release from litter mixtures becomes more pronounced, indicating higher degrees of coordination among different tree species, thereby rendering mixed forests more suitable for practical applications [50]. The results of this study demonstrate that the C/N and C/P ratios for the CCG, CCL, and CBL litter during the 184–366 day decomposition period were significantly lower than those of the single litter material, the CS litter. In terms of N/P ratios, all four mixed litter types showed significantly higher values than the single litter, CS, while remaining stable at approximately 12, indicating a relatively balanced nitrogen and phosphorus content. Under these conditions, microbes can more effectively utilize the nutrients contained within litter, thereby facilitating decomposition and cycling processes. Throughout the entire decomposition process, the UE activity of the four mixed litter types was significantly greater than that of the single litter, CS. Furthermore, with respect to AP activity, during the early decomposition period, there was no significant difference among the five litter types, while the activities of the CCL and CBL litter were significantly greater than those of the CPM and CCG litter in the mid-decomposition period. In the late decomposition period, the PPO activity of the CBL and CCG litter was significantly lower than that of the single litter, CS. The mixture effect analysis indicates that during the decomposition process, the C/N ratios of the CCG, CCL, and CBL litter, as well as the C/P ratios of the four mixed litter types, exhibited antagonistic effects, where lower C/N and C/P ratios promoted litter decomposition. The majority of the enzyme activities of the mixed litter displayed additive effects, while only a few exhibited nonadditive effects. In terms of the nonadditive effects, most exhibited antagonistic effects, with mixed litter containing pine needles as the primary contributor. Nutrients within broadleaf litter can facilitate microbial growth. The UE, SC, AP, and CAT activities of the CBL litter all displayed significant synergistic effects during the decomposition process. This result primarily relates to the inherent properties of the litter material itself. In mixed litter, individual components that comprise the mixture are typically referred to as component litter. Litter that possesses higher N content, lower C/N ratios and lower lignin content is generally considered to be of higher quality, while those with the opposite characteristics are considered to be of lower quality [51]. B. luminifera, with its high initial nitrogen content and low-lignin content, is considered a high-quality litter. Microbial activity is not limited by nitrogen, which promotes enzyme secretion by microbes and contributes to increased enzymatic activity. In this study, the UE, CAT, and AP enzyme activities of the CBL litter throughout the entire decomposition process were significantly greater than those of the CS litter. The leaves of C. lanceolata generally fall in the form of small twigs, and the C/N ratio of the leaf litter is relatively high [12]. The needle-like leaves of P. massoniana exhibit a high-lignin content and a small leaf surface area [52], resulting in the slow mineralization of organic materials and thereby limiting microbial growth and proliferation. Additionally, as decomposition time increases, P. massoniana exhibits high-lignin and resin contents, which are difficult to decompose and lead to the release of acidic substances, inhibiting microbial predation and reproduction [11]. From the perspective of correlation analysis, the activities of SC, AP, CAT in the leaf litter CPM showed no significant correlations with various chemical stoichiometries. In contrast, significant correlations were observed between the enzymatic activities in the leaf litter of CBL and several chemical stoichiometries. This phenomenon may be attributed to the fact that the materials that comprise the CBL leaf litter are more easily decomposed by microorganisms, leading to the production of soluble organic matter and elements that can provide microorganisms with the necessary energy and nutrients for growth and provide a suitable environment for enzyme degradation. Therefore, these organic resources and elements may act as catalysts for enzymes during the decomposition process and facilitate their pronounced effects. The leaf litter of C. glanduliferum is thick and wide, with a leathery surface that makes it difficult for soil animals to destroy it and microorganisms to degrade it [53]. However, as leaching and soil animal destruction take place, the availability of nutrient substances contained within C. glanduliferum litter gradually increases. CCG litter generates significant synergistic effects during the decomposition process. The chemical and physical properties of litter materials collectively impact the enzymatic activity during the decomposition of mixed litter.

5. Conclusions

Litter serves as a crucial “link” connecting forest plants to soil, with the litter decomposition process being a critical component of nutrient cycling. The diversity of litter materials provides multifaceted microenvironments for litter decomposition, and the interaction between high-quality and low-quality litter materials has a positive impact on decomposition rates and enzymatic activity. This study revealed that during the decomposition of mixed litter, the CBL and CCG litter exhibited antagonistic effects in terms of C/N and C/P ratios, while their enzymatic activities displayed synergistic effects that promoted decomposition. Therefore, in the management of ecological tea gardens, to promote sustainable operations of modern ecological tea gardens, manage agroforestry plantations, and address existing ecological issues of tea gardens, the selection of B. luminifera and C. glanduliferum as tree species in the configuration of tea gardens can result in a positive synergistic effect during litter decomposition, increasing nutrient return capacity and maintaining soil fertility, thus achieving the ecological development of tea gardens. However, the optimal mixing ratio of these species still needs further study.

Author Contributions

Conceptualization, H.Z.; methodology, R.Y.; software, S.L.; investigation, C.H.; resources, R.Y. and C.Y.; data curation, H.W.; writing—original draft preparation, H.Z.; writing—review and editing, H.Z. and R.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Science and Technology Program of Guizhou Province, Integration and Demonstration of Key Technologies of Species Configuration in Ecological Tea Gardens in Mountainous Areas of Guizhou, grant number Qian Ke He support [2020]1Y011; the Guizhou Science and Technology Conditions and Service Capacity Construction Project, Construction of research and innovation capacity of under-forest economy in Guizhou Province, grant number Qian Ke He Fu Qi [2020] 4010, and the Forestry Scientific Research Project of Guizhou Province, Study on Dynamic Mechanism of Plant Residue Decomposition in Ecological Compound Management, grant number Qian Lin Ke He [2020]22.

Data Availability Statement

Not applicable.

Acknowledgments

The authors gratefully acknowledge the Forestry College of Guizhou University for the technical support, especially appreciate Rui Yang for the valuable advice.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Changes in stoichiometric characteristics during the decomposition process of single litter from C. sinensis pruning and accompanying tree species litter. Note: a, b, c, d, e—differences between decomposition stages of the same treatment (p < 0.05); A, B, C, D, E—differences between treatments in the same decomposition period (p < 0.05); (a)—C/N; (b)—C/P; (c)—N/P.
Figure 1. Changes in stoichiometric characteristics during the decomposition process of single litter from C. sinensis pruning and accompanying tree species litter. Note: a, b, c, d, e—differences between decomposition stages of the same treatment (p < 0.05); A, B, C, D, E—differences between treatments in the same decomposition period (p < 0.05); (a)—C/N; (b)—C/P; (c)—N/P.
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Figure 2. Changes in stoichiometric characteristics during the decomposition process of mixed pruned C. sinensis and accompanying tree species litter. Note: a, b, c, d, e—differences between decomposition stages of the same treatment (p < 0.05); A, B, C, D—differences between treatments in the same decomposition period (p < 0.05); (a)—C/N; (b)—C/P; (c)—N/P.
Figure 2. Changes in stoichiometric characteristics during the decomposition process of mixed pruned C. sinensis and accompanying tree species litter. Note: a, b, c, d, e—differences between decomposition stages of the same treatment (p < 0.05); A, B, C, D—differences between treatments in the same decomposition period (p < 0.05); (a)—C/N; (b)—C/P; (c)—N/P.
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Figure 3. Expected and measured stoichiometric characteristics of the CCG litter decomposition process. Note: a, b—differences between expected value and measured value in the same decomposition period (p < 0.05).
Figure 3. Expected and measured stoichiometric characteristics of the CCG litter decomposition process. Note: a, b—differences between expected value and measured value in the same decomposition period (p < 0.05).
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Figure 4. Expected and measured stoichiometric characteristics of the CPM litter decomposition process. Note: a, b—differences between expected value and measured value in the same decomposition period (p < 0.05).
Figure 4. Expected and measured stoichiometric characteristics of the CPM litter decomposition process. Note: a, b—differences between expected value and measured value in the same decomposition period (p < 0.05).
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Figure 5. Expected and measured stoichiometric characteristics of the CCL litter decomposition process. Note: a, b—differences between expected value and measured value in the same decomposition period (p < 0.05).
Figure 5. Expected and measured stoichiometric characteristics of the CCL litter decomposition process. Note: a, b—differences between expected value and measured value in the same decomposition period (p < 0.05).
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Figure 6. Expected and measured stoichiometric characteristics of the CBL litter decomposition process. Note: a, b—differences between expected value and measured value in the same decomposition period (p < 0.05).
Figure 6. Expected and measured stoichiometric characteristics of the CBL litter decomposition process. Note: a, b—differences between expected value and measured value in the same decomposition period (p < 0.05).
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Figure 7. Changes in enzyme activities during the decomposition litter of C. sinensis and associated tree species. Note: a, b, c—differences between decomposition stages of the same treatment (p < 0.05); A, B, C—differences between treatments in the same decomposition period (p < 0.05); (a)—UE activity (U·g−1); (b)—SC activity (U·g−1); (c)—AP activity (U·g−1); (d)—CEL activity (U·g−1); (e)—CAT activity (U·g−1); (f)—PPO activity (U·g−1).
Figure 7. Changes in enzyme activities during the decomposition litter of C. sinensis and associated tree species. Note: a, b, c—differences between decomposition stages of the same treatment (p < 0.05); A, B, C—differences between treatments in the same decomposition period (p < 0.05); (a)—UE activity (U·g−1); (b)—SC activity (U·g−1); (c)—AP activity (U·g−1); (d)—CEL activity (U·g−1); (e)—CAT activity (U·g−1); (f)—PPO activity (U·g−1).
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Figure 8. Changes in enzyme activities during the decomposition of the mixed litter of C. sinensis and associated tree species. Note: a, b, c—differences between decomposition stages of the same treatment (p < 0.05); A, B, C—differences between treatments in the same decomposition period (p < 0.05); (a)—UE activity (U·g−1); (b)—SC activity (U·g−1); (c)—AP activity (U·g−1); (d)—CEL activity (U·g−1); (e)—CAT activity (U·g−1); (f)—PPO activity (U·g−1).
Figure 8. Changes in enzyme activities during the decomposition of the mixed litter of C. sinensis and associated tree species. Note: a, b, c—differences between decomposition stages of the same treatment (p < 0.05); A, B, C—differences between treatments in the same decomposition period (p < 0.05); (a)—UE activity (U·g−1); (b)—SC activity (U·g−1); (c)—AP activity (U·g−1); (d)—CEL activity (U·g−1); (e)—CAT activity (U·g−1); (f)—PPO activity (U·g−1).
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Figure 9. Expected and measured enzyme activity characteristics of the CCG litter decomposition process. Note: a, b—differences between expected value and measured value in the same decomposition period (p < 0.05).
Figure 9. Expected and measured enzyme activity characteristics of the CCG litter decomposition process. Note: a, b—differences between expected value and measured value in the same decomposition period (p < 0.05).
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Figure 10. Expected and measured enzyme activity characteristics of the CPM litter decomposition process. Note: a, b—differences between expected value and measured value in the same decomposition period (p < 0.05).
Figure 10. Expected and measured enzyme activity characteristics of the CPM litter decomposition process. Note: a, b—differences between expected value and measured value in the same decomposition period (p < 0.05).
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Figure 11. Expected and measured enzyme activity characteristics of the CCL litter decomposition process. Note: a, b—differences between expected value and measured value in the same decomposition period (p < 0.05).
Figure 11. Expected and measured enzyme activity characteristics of the CCL litter decomposition process. Note: a, b—differences between expected value and measured value in the same decomposition period (p < 0.05).
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Figure 12. Expected and measured enzyme activity characteristics of the CBL litter decomposition process. Note: a, b—differences between expected value and measured value in the same decomposition period (p < 0.05).
Figure 12. Expected and measured enzyme activity characteristics of the CBL litter decomposition process. Note: a, b—differences between expected value and measured value in the same decomposition period (p < 0.05).
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Table
Table 1. Basic properties of nutrients in the litter for the test.
Table 1. Basic properties of nutrients in the litter for the test.
Litter TypeTC
g·kg−1		TN
g·kg−1		TP
g·kg−1		TK
g·kg−1		Lignin
mg·g−1		Cellulose
mg·g−1
C. sinensis (CS)450.01 ± 11.11 b17.85 ± 0.81 ab1.88 ± 0.10 a1.26 ± 0.04 c185.96 ± 10.04 a16.37 ± 0.54 a
C. lanceolata (CL)452.12 ± 14.05 b15.28 ± 1.07 c1.27 ± 0.09 b0.72 ± 0.01 d184.74 ± 11.31 a12.64 ± 0.95 c
P. massoniana (PM)530.34 ± 16.94 a11.78 ± 1.62 d0.87 ± 0.05 c0.26 ± 0.01 e195.06 ± 25.34 a14.79 ± 0.49 b
C. glanduliferum (CG)509.17 ± 8.5 a16.38 ± 0.74 bc0.91 ± 0.03 c2.14 ± 0.01 a117.20 ± 15.57 b16.51 ± 0.55 a
B. luminifera (BL)432.22 ± 13.73 b19.37 ± 0.70 a1.82 ± 0.12 a1.49 ± 0.01 b159.05 ± 25.38 ab10.31 ± 0.14 d
Note: Different letters in the same column indicate significant differences in the data (p < 0.05). The significance level gradually decreases from a to d.
Table
Table 2. Basic physical and chemical properties of soil.
Table 2. Basic physical and chemical properties of soil.
Soil TypeSoil-
Texture		PHSOC
g·kg−1		STN
g·kg−1		STP
g·kg−1		STK
g·kg−1		SAN
mg·kg−1		SAP
mg·kg−1		SAK
mg·kg−1
Orthic Acrisolsclay4.596.53 ± 4.561.87 ± 0.160.72 ± 0.045.99 ± 0.07198.33 ± 7.2913.09 ± 1.50207.16 ± 6.90
Table
Table 3. Initial nutrient content of the litter.
Table 3. Initial nutrient content of the litter.
Litter TypeTC
g·kg−1		TN
g·kg−1		TP
g·kg−1		TK
g·kg−1		Lignin
mg·g−1		Cellulose
mg·g−1
C. sinensis
(CS)		450.01 ± 11.11 b17.85 ± 0.81 a1.88 ± 0.10 a1.26 ± 0.04 c185.96 ± 10.40 a16.37 ± 0.31 a
C. sinensis + C. glanduliferum(CCG)479.59 ± 7.99 a17.12 ± 0.73 a1.39 ± 0.05 c1.70 ± 0.2 a144.30 ± 12.77 b16.44 ± 0.53 a
C. sinensis + P. massoniana
(CPM)		490.17 ± 2.79 a14.82 ± 0.62 b1.37 ± 0.07 c0.76 ± 0.02 e183.23 ± 7.47 a15.58 ± 0.22 b
C. sinensis + C. lanceolata
(CCL)		451.06 ± 9.57 b16.57 ± 0.71 ab1.57 ± 0.08 b0.99 ± 0.02 d178.07 ± 13.20 ab14.51 ± 0.13 c
C. sinensis + B. luminifera
(CBL)		441.12 ± 8.41 b18.61 ± 0.35 a1.85 ± 0.03 a1.37 ± 0.03 b165.22 ± 13.99 ab13.34 ± 0.18 d
Note: Different letters in the same column indicate significant differences in the data (p < 0.05). The significance level gradually decreases from a to e.
Table
Table 4. Correlation between the stoichiometric characteristics of litter and enzyme activity.
Table 4. Correlation between the stoichiometric characteristics of litter and enzyme activity.
Litter TypeFactorTC ContentTN ContentTP ContentTK ContentLignin ContentCellulose ContentC/NC/PN/P
(mg·g−1)(mg·g−1)(mg·g−1)(mg·g−1)(mg·g−1)(mg·g−1)
CCGS-UE−0.014−0.907 **−0.1990.817 **−0.511 *0.3140.708 **0.115−0.698 **
S-SC0.1950.710 **0.221−0.814 **0.431−0.601 **−0.477 *0.070.484 *
S-AP−0.266−0.561 *0.0330.873 **−0.658 **0.3190.34−0.337−0.504 *
S-CEL0.4580.622 **0.573 *−0.543 *0.453−0.556 *−0.2490.0590.209
S-PPO−0.2680.0880.1870.364−0.0830.013−0.162−0.486 *−0.059
S-CAT−0.477 *−0.548 *−0.632 **0.515 *−0.1290.906 **0.186−0.059−0.099
CPMS-UE0.725 **−0.815 **0.673 **0.668 **−0.095−0.3350.778 **0.399−0.786 **
S-SC−0.1870.316−0.345−0.240.215−0.15−0.4620.070.245
S-AP0.099−0.240.4060.153−0.168−0.0480.324−0.251−0.265
S-CEL−0.0940.144−0.083−0.0890.392−0.660 **−0.366−0.0440.005
S-PPO0.217−0.2010.475 *0.198−0.498 *0.210.313−0.117−0.273
S-CAT−0.1230.128−0.2910.0070.080.4290.0510.090.321
CCLS-UE−0.2370.205−0.3660.626 **0.4380.757 **−0.371−0.0080.336
S-SC0.0980.151−0.292−0.380.2360.044−0.240.1940.196
S-AP−0.41−0.13−0.1120.821 **0.3060.482 *−0.035−0.206−0.011
S-CEL0.862 **0.463−0.169−0.556 *−0.077−0.554 *−0.1570.656 **0.368
S-PPO−0.338−0.585 *0.835 **−0.077−0.781 **−0.4550.706 **−0.607 **−0.740 **
S-CAT−0.704 **−0.531 *0.512 *0.162−0.4360.3180.431−0.719 **−0.529 *
CBLS-UE−0.204−0.516 *0.420.315−0.614 **0.0190.589 *−0.451−0.502 *
S-SC0.657 **0.344−0.074−0.758 **0.717 **−0.560 *−0.2240.683 **0.27
S-AP−0.730 **−0.2830.0980.863 **−0.779 **0.617 **0.08−0.786 **−0.253
S-CEL0.664 **0.1490.27−0.553 *0.504 *−0.638 **−0.0990.443−0.019
S-PPO0.455−0.0890.789 **−0.34−0.088−0.755 **0.252−0.079−0.352
S-CAT−0.4320.162−0.0980.518 *−0.4430.3430.088−0.3810.197
Note: *: p < 0.05. **: p < 0.01.
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Zhao, H.; Yang, R.; Yuan, C.; Liu, S.; Hou, C.; Wang, H. Chemical Stoichiometry and Enzyme Activity Changes during Mixed Decomposition of Camellia sinensis Pruning Residues and Companion Tree Species Litter. Agronomy 2023, 13, 1717. https://doi.org/10.3390/agronomy13071717

AMA Style

Zhao H, Yang R, Yuan C, Liu S, Hou C, Wang H. Chemical Stoichiometry and Enzyme Activity Changes during Mixed Decomposition of Camellia sinensis Pruning Residues and Companion Tree Species Litter. Agronomy. 2023; 13(7):1717. https://doi.org/10.3390/agronomy13071717

Chicago/Turabian Style

Zhao, Hongjiu, Rui Yang, Congjun Yuan, Shaqian Liu, Chunlan Hou, and Haodong Wang. 2023. "Chemical Stoichiometry and Enzyme Activity Changes during Mixed Decomposition of Camellia sinensis Pruning Residues and Companion Tree Species Litter" Agronomy 13, no. 7: 1717. https://doi.org/10.3390/agronomy13071717

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