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Abstract ［Objectives］ This study was conducted to investigate the effects of Vitis heyneana cultivation on rocky hillsides on the variation of soil fertility, so as to provide theoretical support for economic development and the control of rocky desertification in the Dashi mountainous area.
［Methods］Taking V. heyneana planting base in Luocheng County, Hechi City, Guangxi Province as the research object, the methods of field investigation, regular sampling and experimental analysis were used to analyze seasonal variations of soil urease, sucrase and soil alkaline phosphatase activity of 15 different sample plots surveyed, and their correlation with soil physical and chemical properties was analyzed.
［Results］ ① In general, sucrase, urease and alkaline phosphatase were lower in summer and autumn, and higher in spring and winter, and the performance of the activity of the three enzymes was inconsistent. Among them, the activity of sucrase was in order of spring>autumn>summer>winter, and the activity of urease and alkaline phosphatase showed an order of winter>spring>summer>autumn. ② The seasonal variations of soil fertility in different sample plots were affected by various factors such as human disturbance, climate change, vegetation coverage, topography and landforms, cultivation and management measures, and although the change laws in different sample plots were different, the seasonal differences in soil fertility in the same place were extremely significant. ③ If the influence of artificial fertilization factors is excluded, the planting of V. heyneana on rocky hillsides will cause a significant decrease in soil enzyme activity, that is, a significant decrease in soil fertility.
［Conclusions］Related issues such as the effects of planting V. heyneana on the variation of soil fertility in rocky hillsides should arouse necessary attention of management departments and producers.
Key words Rocky hillside; Continuous planting of Vitis heyneana; Soil enzyme activity; Seasonal variation
Received: May 30, 2021 Accepted: July 29, 2021
Supported by Natural Science Foundation of Guangxi (GKZ 0832273); Laboratory of Comprehensive Prevention and Control of Rocky Desertification in Karst Areas in Northwest Guangxi (XZF［2016］91); High-level Talent Research Startup Fund of Hechi University (XJ2018GKQ016); Undergraduate Innovation and Enterpreneurship Training Program of Guangxi (201810605086). Peiqing LIU (1996-), female, P. R. China, devoted to research about land resource utilization and environmental protection in karst area.
*Corresponding author. E-mail: hcxyqyr@126.com.
Northwest Guangxi is a typical karst area in Guangxi, where the contradiction between man and land is prominent, and the problems of soil erosion and rocky desertification are serious［1］. In order to solve the survival problem, local people often adopt steep slope farming, which leads to further deterioration of the ecological environment［2-3］. The formation of fragile habitats in karst areas is the result of special geological background superimposed on strong human interference and destruction［4］. Vitis heyneana Roem. et Schult is a kind of crop with higher economic benefits. In recent years, Luocheng County has called on the masses to vigorously develop grape planting industry. In order to get rid of economic difficulties as soon as possible, many farmers opened up large areas of wasteland on rocky hillsides. However, due to their own lack of technology and poor management, the original land cover of rocky hillsides has been greatly reduced, which could easily lead to soil erosion and rocky desertification, resulting in serious damage to the ecological environment of karst areas. Karst habitats are very sensitive to human disturbance. The karst habitats in the degraded stage have extremely weak natural resilience［5］ and strong heterogeneity［3］. Once damaged by excessive disturbance, it is usually difficult to recover, and the consequences are often catastrophic［6-10］.
Soil is an extremely complex system, and soil microorganisms are the key driving force of material circulation and energy flow in the soil ecosystem［11］. Soil enzymes are mainly derived from the secretions of soil microorganisms and plant roots, and participate in a variety of biochemical reactions in the soil. Because they are more sensitive to changes in the soil environment, soil enzyme activity is used as a biological indicator for soil environmental quality monitoring［12-18］. Although scholars at home and abroad have conducted some researches on environmental problems in karst areas, there are few reports on the variation in soil quality of rocky hillsides continuously planted with V. heyneana. Therefore, this study selected 15 different locations in the V. heyneana planting base in Luocheng County, Guangxi, where soil samples were collected at fixed points in the four seasons of spring, summer, autumn and winter, and their soil physical and chemical indexes including invertase, urease and alkaline phosphatase activity were determined. Through comprehensive comparison and analysis, the effects of V. heyneana cultivation on rocky hillsides on the changes of soil fertility was analyzed［19］, so as to provide theoretical support for the development of characteristic industries and ecological environment protection in the karst areas of northwest Guangxi. Materials and Methods
General situation of study area
The study area is located in Luocheng Mulao Autonomous County, Hechi City, Guangxi Zhuang Autonomous Region. It has a subtropical monsoon climate zone. The average annual temperature is 24.7 ℃; the average temperature in July is 33 ℃; the frost-free period is 358 d; the annual sunshine duration is 1 270 h; the annual average wind speed is 1.4 m/s, and the most wind direction is ENE wind; the average annual precipitation is 1 242.5 mm; the annual evaporation is 1 342.1 mm; and the annual number of thunderstorm days is 59 d. The precipitation is mainly concentrated in April to August, with the most precipitation from mid-June to mid-July, and the dry season is from September to March of the following year. The outstanding features of the ecological environment in this area are steep slopes, bare rocks, shallow soil, and natural vegetation consisting of common stone mountain shrubs and herbaceous plants. In order to solve the livelihood problem, people usually destroy forests and open up wasteland, and plant grapes, soybeans, mulberries, uncaria and other crops in the slopes and rock crevices below the mountainside.
Sample collection and treatment
On the basis of multiple field surveys, 15 different plots of the V. heyneana planting base and surrounding areas in Luocheng County were selected (see Table 1 and Fig. 1 for basic information). The area of each plot was 50 m×50 m, and plots nearby where no V. heyneana was planted were used as controls. From mid-April 2018 to mid-January 2019, in the four different seasons of spring, summer, autumn and winter, according to the plum-shaped sampling method, the topsoil of each selected sample site was collected from 0 to 15 cm deep, sieved through a 2 mm standard sieve, and sampled by the quartering method. Then, the collected soil samples were stored in polyethylene ziplock bags, and relevant records were made. The samples were brought back to the laboratory as soon as possible, then air dried and crushed with a miniature soil grinder, and added into polyethylene ziplock bags, which were stored in a cool place for the determination of soil enzyme activity.
Experimental methods
Fresh soil moisture content was determined by gravimetric method; soil organic matter content was determined by potassium dichromate-concentrated sulfuric acid oxidation colorimetry; total nitrogen content was determined by semi-micro Kjeldahl method; available nitrogen content was determined by alkaline hydrolysis diffusion method; the determination of total phosphorus content used sodium hydroxide alkali fusion-molybdenum blue colorimetric method; available phosphorus was determined by sodium bicarbonate leaching-molybdenum-antimony anti-colorimetric method; total potassium content was determined by sodium hydroxide alkali fusion-flame photometer; and rapidly available potassium content was determined by ammonium acetate leaching-flame photometer method［20］. Soil sucrase activity was determined by the 3,5-dinitrosalicylic acid colorimetric method; soil urease activity was determined by the sodium phenate colorimetric method; and alkaline phosphatase activity was determined by disodium phenyl phosphate colorimetric method. Each sample was set with a blank control, a matrix-free control, and three replicates［21］. Data processing
The processing of the experimental data was performed with Office Excel 2010. Correlation analysis was performed with SPSS 24.0. Variance analysis of multiple comparisons was performed with the least significant difference method (LSD method). Graph drawing was performed with Origin 7.5 software.
Results and Analysis
Seasonal variations of soil sucrase activity
Soil sucrase participates in the carbon cycle of the soil. Sucrase plays an extremely important role in increasing soluble nutrients in the soil. Under normal circumstances, sucrase activity is positively correlated with soil fertility［22］. Therefore, sucrose enzyme activity is often used as a monitoring indicator for judging the degree of soil maturation and fertility［23］.
The seasonal variations of soil sucrase activity in different survey plots are shown in Fig. 2. It could be seen that soil sucrase activity varied greatly in different places. Soil sucrase activity in the same sample plot was mostly higher in spring and autumn, and relatively lower in summer and winter. From the annual average value of soil sucrase activity in different plots, the soil sucrase activity of the H plot was the lowest, and the soil sucrase activity of other plots was relatively high.
Seasonal variations of soil urease activity
The seasonal variations of soil urease activity in different survey plots are shown in Fig. 3. Because soil urease is highly specific and can promote the hydrolysis of urea, soil urease activity can characterize soil nitrogen status［23］. Among all the survey plots, plot M had the highest soil urease activity, while plot H had the lowest urease activity. From the annual average value of soil urease activity, control plots C, J, and M were relatively high, while the rest were generally low. The seasonal differences of soil urease activity between different places were extremely significant (P<0.01), but there were no obvious laws, which might be related to human interference (such as fertilization, weeding, and artificial care) and topography and climate changes in different places.
Seasonal variations of soil alkaline phosphatase activity
Soil phosphatase plays an important role in the conversion of phosphorus in the soil ecosystem［24］. According to the difference of soil pH, phosphatase can be divided into acidic, neutral and alkaline phosphatase. Soil alkaline phosphatase is a kind of non-specific phosphomonoesterase, which can catalyze the hydrolysis of most phosphomonoesters and exists in all organisms except higher plants. The seasonal variations of soil alkaline phosphatase activity in different survey plots are shown in Fig. 4. It could be seen that compared with most of V. heyneana planting plots, the soil alkaline phosphatase activity of control plots C, J, and M was relatively higher; and in all the survey plots, the alkaline phosphatase activity of control plot M was the highest, and the alkaline phosphatase activity of V. heyneana planting plot H was the lowest. The seasonal variations of soil moisture content of different survey plots are shown in Fig. 5. From comparison and analysis, it was known that the soil moisture content varied with seasons to a certain degree. The water contents of the autumn plots A, B, C, D, E, G, H, K, L, M, N were higher, and the water contents of control plots C, J, and M were significantly higher than those of other plots. Among them, plot H had the lowest soil moisture content. In various plots, the seasonal variation of soil moisture content had a certain regularity, usually spring>summer>autumn. Due to the abnormal weather and the influence of typhoons, sometimes soil moisture content was higher in winter.
Seasonal variations in soil pH
The seasonal variations of soil pH in different plots are shown in Fig. 6. From the annual average value, according to the classification standard of soil pH, the soil pH values of plots C and M was weakly alkaline, the soil pH values of plots H and I were weakly acidic, and the soil pH values of all other plots were neutral. The change of soil pH was not only related to its geological background, but also related to the long-term application of chemical fertilizers in the process of crop cultivation.
Correlation analysis
In order to illustrate the relationship between soil fertility changes and soil enzyme activity, the correlation between soil enzyme activity and soil physical and chemical indexes of the survey plots were analyzed. The specific results are shown in Table 2 to Table 5. It could be seen that in the 15 surveyed plots in this study, the main nutrients of the soil in different seasons had a certain correlation with the activity of soil alkaline phosphatase, urease and invertase. Specifically, in the three seasons of spring, autumn and winter, the contents of soil organic matter, total nitrogen and available nitrogen had a significant or an extremely significant positive correlation with the activity of the above three soil enzymes, and there was also significant or extremely significant correlation between the measured three soil enzymes. In summer, the situation was slightly different. Alkaline phosphatase had an extremely significant positive correlation with soil organic matter, total nitrogen, and available nitrogen. Urease had an extremely significant positive correlation with total nitrogen and available nitrogen. Sucrase had an extremely significant positive correlation with soil organic matter. In addition to urease and sucrase, there was also a significant or an extremely significant positive correlation between the three soil enzymes in their activity. ① In the table, OM, TN, AN, TK, AK, TP, AP, MC, APA, UA and IA respectively represent soil organic matter, total nitrogen, available nitrogen, total potassium, available potassium, total phosphorus, available phosphorus, moisture content, alkaline phosphatase activity, urease activity and sucrase activity; ②** means an extremely significant difference (P<0.01); * means a significant difference (P<0.05). The same as the table below.
Clustering analysis
In order to comprehensively compare the soil quality status of various survey sample plots and to thereby explain the impacts of different land use methods on soil fertility changes in the karst areas of northwest Guangxi, the 15 survey plots could be clustered based on the soil physical and chemical indexes determined during the investigation and research process (Fig. 7). Based on the results of the clustering analysis, and with reference to the measurement data of the main nutrients in the soils of different survey plots, the soil fertility of different survey plots could be simply sorted on the basis of comprehensive consideration of the influences of various external environmental factors. It can be seen from Fig. 7 that within the Euclidean distance of 2.5, the 15 survey plots were divided into 4 groups, of which plots A, F, D, E, B, and H were a group, and plots I, N , C, O were a group, sample plots G, J, K, L were a group, and sample plot M was a single group; within
Discussion
The impact of farming on rocky slopes on the soil quality in karst areas is a long-term process. Although the study time was only one year, the changes were still very obvious from the monitoring indexes of soil enzyme activity, which is consistent with related research results of other scholars. The area of this study is located in the karst areas of northwest Guangxi, where the local economy and culture are relatively backward. Due to historical reasons, the ecological environment has been severely damaged. The surveyed sample plots had been cultivated on steep slopes for many years, causing serious vegetation damage, greater human disturbance, and prominent problems in the predatory development of land resources. Rocky desertification or potential rocky desertification problems had appeared in many places. Therefore, in the process of economic development, the negative impact on the environment is also more serious. How to scientifically consider the relationship between economic development and ecological environment protection is an important issue facing people. The ecological environment of the karst areas is fragile, and once it is disturbed by the transition, irreversible damage will be caused, which will lead to soil erosion and serious decline in land productivity and rocky desertification. It could be seen from the data of this study that in the 15 sample plots in this study, the seasonal differences in soil enzyme activity in the same place were very significant. The activity of different soil enzymes were mostly low in summer and autumn, and high in spring and winter, but the performance of the three enzymes was not completely consistent. It can be seen that in addition to the influence of various natural factors such as atmospheric temperature, moisture (rainfall), slope aspect, topography and landforms, soil enzyme activity also has obvious influences on the changes of soil quality and fertility of rocky hillsides by human factors. Although the relationship between soil enzyme activity changes in different sample plots is complicated, various factors are superimposed on each other, and many problems need to be further clarified, the concept of ecological priority and green development is beyond doubt. The seasonal variations of soil enzyme activity are closely related to the changes in soil organic matter, nitrogen, phosphorus and potassium content and other nutrients, and can be used as monitoring indexes for changes in soil fertility. In this study, although only the activity of three soil enzymes including urease, sucrase and alkaline phosphatase was measured, it could be seen from the correlation analysis of experimental data that no matter what season, in the same plot, most of soil organic matter, total nitrogen, available nitrogen and other nutrients had a significant or an extremely significant positive correlation with the activity of these three soil enzymes, and the activity of the monitored three soil enzymes was also significant or extremely significantly positive correlated. The results of this study are consistent with the results of other related studies. It can be seen that in karst areas, soil enzyme activity can also be used as a monitoring index for changes in soil fertility. It is worth mentioning that in the course of this study, we found that the activity of acid phosphatase and neutral phosphatase in the soil of the survey plots was extremely low, and the correlation between soil nutrient contents and soil enzyme activity in summer was more complicated. Whether the reason is related to the geological background, temperature, rainfall and other climatic conditions and crop cultivation management measures in karst areas remains to be further studied.
Conclusions
Based on the above experimental data of soil enzyme activity and its correlation analysis results with other soil physical and chemical factors, comprehensively considering various related factors, the following conclusions could be preliminarily obtained: ① In general, sucrase, urease and alkaline phosphatase were lower in summer and autumn, and higher in spring and winter, and the seasonal differences in soil enzyme activity were more obvious. Among them, the activity of sucrase was in order of spring>autumn>summer>winter, the activity of urease showed an order of winter>spring>summer>autumn, and the activity of alkaline phosphatase was winter>spring>summer>autumn.
② The order of soil fertility of the 15 sample plots in this study was: M>G, J, K, L>C, I, N, O>A, D, E, F>B>H. The seasonal variations of soil fertility in different sample plots were affected by various factors such as human disturbance, climate change, vegetation coverage, topography and landforms, cultivation and management measures, and although the change laws in different sample plots were not obvious, the seasonal differences in soil fertility in the same place were extremely significant.
③ If the influence of artificial fertilization factors is excluded, the planting of V. heyneana on stone hillsides will cause a significant decrease in soil enzyme activity, that is, a significant decrease in soil fertility. Continuous planting of V. heyneana on rocky hillsides has a greater impact on the variations in soil fertility of sloping farmland. In order to maintain soil fertility, scientific management must be considered.
④ For planting V. heyneana on rocky hillsides, it is necessary to scientifically evaluate the ecological and economic benefits of industrial development, and try to adopt reasonable interplanting and diversified management strategies to reduce soil erosion, maintain soil productivity, and effectively prevent deforestation and land reclamation and the occurrence of rocky desertification.
References
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［Methods］Taking V. heyneana planting base in Luocheng County, Hechi City, Guangxi Province as the research object, the methods of field investigation, regular sampling and experimental analysis were used to analyze seasonal variations of soil urease, sucrase and soil alkaline phosphatase activity of 15 different sample plots surveyed, and their correlation with soil physical and chemical properties was analyzed.
［Results］ ① In general, sucrase, urease and alkaline phosphatase were lower in summer and autumn, and higher in spring and winter, and the performance of the activity of the three enzymes was inconsistent. Among them, the activity of sucrase was in order of spring>autumn>summer>winter, and the activity of urease and alkaline phosphatase showed an order of winter>spring>summer>autumn. ② The seasonal variations of soil fertility in different sample plots were affected by various factors such as human disturbance, climate change, vegetation coverage, topography and landforms, cultivation and management measures, and although the change laws in different sample plots were different, the seasonal differences in soil fertility in the same place were extremely significant. ③ If the influence of artificial fertilization factors is excluded, the planting of V. heyneana on rocky hillsides will cause a significant decrease in soil enzyme activity, that is, a significant decrease in soil fertility.
［Conclusions］Related issues such as the effects of planting V. heyneana on the variation of soil fertility in rocky hillsides should arouse necessary attention of management departments and producers.
Key words Rocky hillside; Continuous planting of Vitis heyneana; Soil enzyme activity; Seasonal variation
Received: May 30, 2021 Accepted: July 29, 2021
Supported by Natural Science Foundation of Guangxi (GKZ 0832273); Laboratory of Comprehensive Prevention and Control of Rocky Desertification in Karst Areas in Northwest Guangxi (XZF［2016］91); High-level Talent Research Startup Fund of Hechi University (XJ2018GKQ016); Undergraduate Innovation and Enterpreneurship Training Program of Guangxi (201810605086). Peiqing LIU (1996-), female, P. R. China, devoted to research about land resource utilization and environmental protection in karst area.
*Corresponding author. E-mail: hcxyqyr@126.com.
Northwest Guangxi is a typical karst area in Guangxi, where the contradiction between man and land is prominent, and the problems of soil erosion and rocky desertification are serious［1］. In order to solve the survival problem, local people often adopt steep slope farming, which leads to further deterioration of the ecological environment［2-3］. The formation of fragile habitats in karst areas is the result of special geological background superimposed on strong human interference and destruction［4］. Vitis heyneana Roem. et Schult is a kind of crop with higher economic benefits. In recent years, Luocheng County has called on the masses to vigorously develop grape planting industry. In order to get rid of economic difficulties as soon as possible, many farmers opened up large areas of wasteland on rocky hillsides. However, due to their own lack of technology and poor management, the original land cover of rocky hillsides has been greatly reduced, which could easily lead to soil erosion and rocky desertification, resulting in serious damage to the ecological environment of karst areas. Karst habitats are very sensitive to human disturbance. The karst habitats in the degraded stage have extremely weak natural resilience［5］ and strong heterogeneity［3］. Once damaged by excessive disturbance, it is usually difficult to recover, and the consequences are often catastrophic［6-10］.
Soil is an extremely complex system, and soil microorganisms are the key driving force of material circulation and energy flow in the soil ecosystem［11］. Soil enzymes are mainly derived from the secretions of soil microorganisms and plant roots, and participate in a variety of biochemical reactions in the soil. Because they are more sensitive to changes in the soil environment, soil enzyme activity is used as a biological indicator for soil environmental quality monitoring［12-18］. Although scholars at home and abroad have conducted some researches on environmental problems in karst areas, there are few reports on the variation in soil quality of rocky hillsides continuously planted with V. heyneana. Therefore, this study selected 15 different locations in the V. heyneana planting base in Luocheng County, Guangxi, where soil samples were collected at fixed points in the four seasons of spring, summer, autumn and winter, and their soil physical and chemical indexes including invertase, urease and alkaline phosphatase activity were determined. Through comprehensive comparison and analysis, the effects of V. heyneana cultivation on rocky hillsides on the changes of soil fertility was analyzed［19］, so as to provide theoretical support for the development of characteristic industries and ecological environment protection in the karst areas of northwest Guangxi. Materials and Methods
General situation of study area
The study area is located in Luocheng Mulao Autonomous County, Hechi City, Guangxi Zhuang Autonomous Region. It has a subtropical monsoon climate zone. The average annual temperature is 24.7 ℃; the average temperature in July is 33 ℃; the frost-free period is 358 d; the annual sunshine duration is 1 270 h; the annual average wind speed is 1.4 m/s, and the most wind direction is ENE wind; the average annual precipitation is 1 242.5 mm; the annual evaporation is 1 342.1 mm; and the annual number of thunderstorm days is 59 d. The precipitation is mainly concentrated in April to August, with the most precipitation from mid-June to mid-July, and the dry season is from September to March of the following year. The outstanding features of the ecological environment in this area are steep slopes, bare rocks, shallow soil, and natural vegetation consisting of common stone mountain shrubs and herbaceous plants. In order to solve the livelihood problem, people usually destroy forests and open up wasteland, and plant grapes, soybeans, mulberries, uncaria and other crops in the slopes and rock crevices below the mountainside.
Sample collection and treatment
On the basis of multiple field surveys, 15 different plots of the V. heyneana planting base and surrounding areas in Luocheng County were selected (see Table 1 and Fig. 1 for basic information). The area of each plot was 50 m×50 m, and plots nearby where no V. heyneana was planted were used as controls. From mid-April 2018 to mid-January 2019, in the four different seasons of spring, summer, autumn and winter, according to the plum-shaped sampling method, the topsoil of each selected sample site was collected from 0 to 15 cm deep, sieved through a 2 mm standard sieve, and sampled by the quartering method. Then, the collected soil samples were stored in polyethylene ziplock bags, and relevant records were made. The samples were brought back to the laboratory as soon as possible, then air dried and crushed with a miniature soil grinder, and added into polyethylene ziplock bags, which were stored in a cool place for the determination of soil enzyme activity.
Experimental methods
Fresh soil moisture content was determined by gravimetric method; soil organic matter content was determined by potassium dichromate-concentrated sulfuric acid oxidation colorimetry; total nitrogen content was determined by semi-micro Kjeldahl method; available nitrogen content was determined by alkaline hydrolysis diffusion method; the determination of total phosphorus content used sodium hydroxide alkali fusion-molybdenum blue colorimetric method; available phosphorus was determined by sodium bicarbonate leaching-molybdenum-antimony anti-colorimetric method; total potassium content was determined by sodium hydroxide alkali fusion-flame photometer; and rapidly available potassium content was determined by ammonium acetate leaching-flame photometer method［20］. Soil sucrase activity was determined by the 3,5-dinitrosalicylic acid colorimetric method; soil urease activity was determined by the sodium phenate colorimetric method; and alkaline phosphatase activity was determined by disodium phenyl phosphate colorimetric method. Each sample was set with a blank control, a matrix-free control, and three replicates［21］. Data processing
The processing of the experimental data was performed with Office Excel 2010. Correlation analysis was performed with SPSS 24.0. Variance analysis of multiple comparisons was performed with the least significant difference method (LSD method). Graph drawing was performed with Origin 7.5 software.
Results and Analysis
Seasonal variations of soil sucrase activity
Soil sucrase participates in the carbon cycle of the soil. Sucrase plays an extremely important role in increasing soluble nutrients in the soil. Under normal circumstances, sucrase activity is positively correlated with soil fertility［22］. Therefore, sucrose enzyme activity is often used as a monitoring indicator for judging the degree of soil maturation and fertility［23］.
The seasonal variations of soil sucrase activity in different survey plots are shown in Fig. 2. It could be seen that soil sucrase activity varied greatly in different places. Soil sucrase activity in the same sample plot was mostly higher in spring and autumn, and relatively lower in summer and winter. From the annual average value of soil sucrase activity in different plots, the soil sucrase activity of the H plot was the lowest, and the soil sucrase activity of other plots was relatively high.
Seasonal variations of soil urease activity
The seasonal variations of soil urease activity in different survey plots are shown in Fig. 3. Because soil urease is highly specific and can promote the hydrolysis of urea, soil urease activity can characterize soil nitrogen status［23］. Among all the survey plots, plot M had the highest soil urease activity, while plot H had the lowest urease activity. From the annual average value of soil urease activity, control plots C, J, and M were relatively high, while the rest were generally low. The seasonal differences of soil urease activity between different places were extremely significant (P<0.01), but there were no obvious laws, which might be related to human interference (such as fertilization, weeding, and artificial care) and topography and climate changes in different places.
Seasonal variations of soil alkaline phosphatase activity
Soil phosphatase plays an important role in the conversion of phosphorus in the soil ecosystem［24］. According to the difference of soil pH, phosphatase can be divided into acidic, neutral and alkaline phosphatase. Soil alkaline phosphatase is a kind of non-specific phosphomonoesterase, which can catalyze the hydrolysis of most phosphomonoesters and exists in all organisms except higher plants. The seasonal variations of soil alkaline phosphatase activity in different survey plots are shown in Fig. 4. It could be seen that compared with most of V. heyneana planting plots, the soil alkaline phosphatase activity of control plots C, J, and M was relatively higher; and in all the survey plots, the alkaline phosphatase activity of control plot M was the highest, and the alkaline phosphatase activity of V. heyneana planting plot H was the lowest. The seasonal variations of soil moisture content of different survey plots are shown in Fig. 5. From comparison and analysis, it was known that the soil moisture content varied with seasons to a certain degree. The water contents of the autumn plots A, B, C, D, E, G, H, K, L, M, N were higher, and the water contents of control plots C, J, and M were significantly higher than those of other plots. Among them, plot H had the lowest soil moisture content. In various plots, the seasonal variation of soil moisture content had a certain regularity, usually spring>summer>autumn. Due to the abnormal weather and the influence of typhoons, sometimes soil moisture content was higher in winter.
Seasonal variations in soil pH
The seasonal variations of soil pH in different plots are shown in Fig. 6. From the annual average value, according to the classification standard of soil pH, the soil pH values of plots C and M was weakly alkaline, the soil pH values of plots H and I were weakly acidic, and the soil pH values of all other plots were neutral. The change of soil pH was not only related to its geological background, but also related to the long-term application of chemical fertilizers in the process of crop cultivation.
Correlation analysis
In order to illustrate the relationship between soil fertility changes and soil enzyme activity, the correlation between soil enzyme activity and soil physical and chemical indexes of the survey plots were analyzed. The specific results are shown in Table 2 to Table 5. It could be seen that in the 15 surveyed plots in this study, the main nutrients of the soil in different seasons had a certain correlation with the activity of soil alkaline phosphatase, urease and invertase. Specifically, in the three seasons of spring, autumn and winter, the contents of soil organic matter, total nitrogen and available nitrogen had a significant or an extremely significant positive correlation with the activity of the above three soil enzymes, and there was also significant or extremely significant correlation between the measured three soil enzymes. In summer, the situation was slightly different. Alkaline phosphatase had an extremely significant positive correlation with soil organic matter, total nitrogen, and available nitrogen. Urease had an extremely significant positive correlation with total nitrogen and available nitrogen. Sucrase had an extremely significant positive correlation with soil organic matter. In addition to urease and sucrase, there was also a significant or an extremely significant positive correlation between the three soil enzymes in their activity. ① In the table, OM, TN, AN, TK, AK, TP, AP, MC, APA, UA and IA respectively represent soil organic matter, total nitrogen, available nitrogen, total potassium, available potassium, total phosphorus, available phosphorus, moisture content, alkaline phosphatase activity, urease activity and sucrase activity; ②** means an extremely significant difference (P<0.01); * means a significant difference (P<0.05). The same as the table below.
Clustering analysis
In order to comprehensively compare the soil quality status of various survey sample plots and to thereby explain the impacts of different land use methods on soil fertility changes in the karst areas of northwest Guangxi, the 15 survey plots could be clustered based on the soil physical and chemical indexes determined during the investigation and research process (Fig. 7). Based on the results of the clustering analysis, and with reference to the measurement data of the main nutrients in the soils of different survey plots, the soil fertility of different survey plots could be simply sorted on the basis of comprehensive consideration of the influences of various external environmental factors. It can be seen from Fig. 7 that within the Euclidean distance of 2.5, the 15 survey plots were divided into 4 groups, of which plots A, F, D, E, B, and H were a group, and plots I, N , C, O were a group, sample plots G, J, K, L were a group, and sample plot M was a single group; within
Discussion
The impact of farming on rocky slopes on the soil quality in karst areas is a long-term process. Although the study time was only one year, the changes were still very obvious from the monitoring indexes of soil enzyme activity, which is consistent with related research results of other scholars. The area of this study is located in the karst areas of northwest Guangxi, where the local economy and culture are relatively backward. Due to historical reasons, the ecological environment has been severely damaged. The surveyed sample plots had been cultivated on steep slopes for many years, causing serious vegetation damage, greater human disturbance, and prominent problems in the predatory development of land resources. Rocky desertification or potential rocky desertification problems had appeared in many places. Therefore, in the process of economic development, the negative impact on the environment is also more serious. How to scientifically consider the relationship between economic development and ecological environment protection is an important issue facing people. The ecological environment of the karst areas is fragile, and once it is disturbed by the transition, irreversible damage will be caused, which will lead to soil erosion and serious decline in land productivity and rocky desertification. It could be seen from the data of this study that in the 15 sample plots in this study, the seasonal differences in soil enzyme activity in the same place were very significant. The activity of different soil enzymes were mostly low in summer and autumn, and high in spring and winter, but the performance of the three enzymes was not completely consistent. It can be seen that in addition to the influence of various natural factors such as atmospheric temperature, moisture (rainfall), slope aspect, topography and landforms, soil enzyme activity also has obvious influences on the changes of soil quality and fertility of rocky hillsides by human factors. Although the relationship between soil enzyme activity changes in different sample plots is complicated, various factors are superimposed on each other, and many problems need to be further clarified, the concept of ecological priority and green development is beyond doubt. The seasonal variations of soil enzyme activity are closely related to the changes in soil organic matter, nitrogen, phosphorus and potassium content and other nutrients, and can be used as monitoring indexes for changes in soil fertility. In this study, although only the activity of three soil enzymes including urease, sucrase and alkaline phosphatase was measured, it could be seen from the correlation analysis of experimental data that no matter what season, in the same plot, most of soil organic matter, total nitrogen, available nitrogen and other nutrients had a significant or an extremely significant positive correlation with the activity of these three soil enzymes, and the activity of the monitored three soil enzymes was also significant or extremely significantly positive correlated. The results of this study are consistent with the results of other related studies. It can be seen that in karst areas, soil enzyme activity can also be used as a monitoring index for changes in soil fertility. It is worth mentioning that in the course of this study, we found that the activity of acid phosphatase and neutral phosphatase in the soil of the survey plots was extremely low, and the correlation between soil nutrient contents and soil enzyme activity in summer was more complicated. Whether the reason is related to the geological background, temperature, rainfall and other climatic conditions and crop cultivation management measures in karst areas remains to be further studied.
Conclusions
Based on the above experimental data of soil enzyme activity and its correlation analysis results with other soil physical and chemical factors, comprehensively considering various related factors, the following conclusions could be preliminarily obtained: ① In general, sucrase, urease and alkaline phosphatase were lower in summer and autumn, and higher in spring and winter, and the seasonal differences in soil enzyme activity were more obvious. Among them, the activity of sucrase was in order of spring>autumn>summer>winter, the activity of urease showed an order of winter>spring>summer>autumn, and the activity of alkaline phosphatase was winter>spring>summer>autumn.
② The order of soil fertility of the 15 sample plots in this study was: M>G, J, K, L>C, I, N, O>A, D, E, F>B>H. The seasonal variations of soil fertility in different sample plots were affected by various factors such as human disturbance, climate change, vegetation coverage, topography and landforms, cultivation and management measures, and although the change laws in different sample plots were not obvious, the seasonal differences in soil fertility in the same place were extremely significant.
③ If the influence of artificial fertilization factors is excluded, the planting of V. heyneana on stone hillsides will cause a significant decrease in soil enzyme activity, that is, a significant decrease in soil fertility. Continuous planting of V. heyneana on rocky hillsides has a greater impact on the variations in soil fertility of sloping farmland. In order to maintain soil fertility, scientific management must be considered.
④ For planting V. heyneana on rocky hillsides, it is necessary to scientifically evaluate the ecological and economic benefits of industrial development, and try to adopt reasonable interplanting and diversified management strategies to reduce soil erosion, maintain soil productivity, and effectively prevent deforestation and land reclamation and the occurrence of rocky desertification.
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