Study on Vegetation Characteristics & Soil Properties of Secondary

Chunxiu GUO Lide WANG Fanglan HE Dacheng SONG Hao WU Heran ZHAO Fanglin WANG Baoyi XU

Abstract ［Objectives］This study was conducted to investigate the vegetation community characteristics and soil properties of secondary grassland in abandoned farmland areas of different years in Minqin Oasis.

［Methods］ By the method of space-for-time substitution, the changes of vegetation community characteristics (species composition, structural characteristics and important values) and soil microbial and enzyme activity were determined, and the correlation between various soil factors was analyzed.

［Results］ ① There were 39 species belonging to 32 genera of 15 families in the research area, including 8 shrub plants, 15 perennial herbs and 16 annual herbs. ② Soil microbial biomass carbon, nitrogen and phosphorus contents were higher in the upper layer than in the lower layer, and the phenomenon of “surface aggregation” was obvious. The differences were different with the change of abandoned years. ③ Soil catalase content was the highest among different plots, and soil phosphatase, sucrase and urease all showed a trend of fluctuating decline with the increase in abandoned years. Soil enzyme changes were different in different soil layers.

［Conclusions］ This study can provide a theoretical basis for the restoration, reconstruction and rational protection and utilization of secondary grassland in abandoned farmland.

Key words Minqin Oasis; Different years; Secondary grassland; Abandoned area; Vegetation characteristics; Soil properties

Received: May 16, 2022  Accepted: July 18, 2022

Supported by National and Regional Fund Project (31760709, 31860175); Key R&D Program of Gansu Province (21YF5FA038, 20YF8FA105).

Chunxiu GUO (1979-), female, P. R. China, associate researcher, PhD, devoted to research about desert ecology and grassland microbial diversity.

*Corresponding author. E-mail: wld69tom.com.

Minqin Oasis is located in the lower reaches of Shiyang River. It used to be a natural "Chaiwan" with lush vegetation［1］. However, in the 1950s and 1960s, with the continuous expansion of agricultural production scale and the over-exploitation of groundwater by local residents, the groundwater level in the downstream continued to decline and the degree of mineralization increased, resulting in the death of large areas of natural vegetation in the oasis area.  In recent years, after Minqin County implemented ecological environment governance policies such as "closing wells and pressing fields" and "returning farmland to forests", a large area of land has been abandoned［2］, and the area is increasing year by year. If reasonable agricultural management measures and protection and utilization are not taken, these lands will easily form desert grassland with the increase of the years of farmland abandonment, which may lead to the development of secondary grassland to desertification and accelerate the rapid deterioration of soil ecosystems［3］, and affect the ecological security of oasis. It poses a great threat to the ecological security of the oasis, which should be highly valued by the state.

Cultivated land is an important land resource, and the vegetation restoration and soil management of cultivated land are fundamental measures for soil and water conservation and ecological environmental protection, as well as an important indicator to evaluate the ecological environment of a region. Soil provides nutrients for vegetation growth, and vegetation growth can improve soil structure and prevent soil fertility loss. The two influence each other and co-evolve. Soil microorganisms are very sensitive to environmental changes and are the regulators of energy flow and nutrient cycling in soil ecosystem［4］. They act as decomposers in the processes of organic matter conversion［5］, fertility improvement［6］, pollutant degradation and environmental purification［7］, and are an important indicator of soil environmental quality. Soil enzymes are biocatalysts of soil ecosystems, and their activity is an important indicator of soil fertility and an important indicator of soil health［8］. Zhao et al.［9］ and You et al.［10］ found that plant roots are the channel for material exchange and energy transfer between plants and soil［11］, and roots play an important role in community succession and soil structure changes. Therefore, studying the relationship between vegetation community characteristics and soil factors is of great significance for vegetation restoration and rational utilization of abandoned farmland. In this study, with secondary grassland in the abandoned farmland areas of Minqin as the research object, the community characteristics of vegetation, soil microorganisms, enzyme activity and their correlation were studied by the space-for-time substitution method, aiming to provide a theoretical basis for the restoration, reconstruction and rational protection and utilization of the secondary grassland in abandoned farmland.

Materials and Methods

Overview of the study area

The experimental area is located in the desert-returned farmland area of Xiqu Town, Minqin County. Its geographical coordinates are: latitude 39°01′44″-39°03′28″ north, longitude 103°35′58″-103°37′54″, with the average altitude of 1 306 m. It has a typical temperate continental desert climate, with scarce precipitation, an average annual rainfall of 116 mm and an average annual evaporation of 2 419.6 mm. The temperature difference between day and night is 25.2 ℃; the extreme maximum temperature is 41 ℃ and the extreme minimum temperature is -30 ℃, and the annual average temperature is 7.8 ℃. The area has sufficient sunshine, with average annual sunshine hours of 2 832.1 h. It is windy and sandy, and the annual average wind speed is 2.5 m/s. The non-zonal soil types are aeolian sandy soil and gray-brown desert soil. The vegetation is mainly salt-tolerant plants, and the shrub species mainly include Kalidium foliatum, Lycium ruthenicum, Reamuria soongoria, etc.; and herbs include Chenopodium album, Peganum harmala, Suaeda glauca, Phragmites communis, etc.［11］.

Sample plot layout and vegetation survey

In mid-July 2018, with the secondary grassland established by our study group in the lower reaches of the Shiyang River for different years (1, 2, 4, 8, 13, 20, 30 and 40 years) as the evolution sequence plots and uncultivated desert grassland as the control (CK), a total of 9 typical plots of (100×100) m2 were set. In each plot, five fixed quadrats of 10 m×10 m were set up using the 5-point method to investigate the characteristics of shrubs and sub-shrubs, and to record the species and individual numbers of vegetation communities, dominant species and associated species in the quadrats; and herbaceous vegetation was investigated with 1 m×1 m small quadrats. The fixed sample points in each quadrat were marked by GPS, and the altitudes, longitudes and latitudes were also recorded (Table 1).

Sample collection

In mid-July 2018, three sample points were selected according to the S shape to excavate soil profiles at each fixed sample point, and soil samples were collected in two layers of 0-20 and 20-40 cm. After mixing the three soil samples of the same soil layer evenly, the soil samples were divided into two parts by the method of quartering. One part was sealed in a sterile bag, marked, and taken back to the laboratory in an incubator with ice cubes, and plant roots and debris were removed for the determination of soil biological characteristics (stored at 4 ℃ for no more than 24 h). The other part was used for the determination of soil enzyme activity［12］.

Calculation of importance values (IVs) for measuring community population composition

The important values (IVs) of each plant species in the shrub layer and the herb layer were calculated respectively, and the calculation method was: IV=(Relative height+Relative coverage+Relative frequency)/3［13］.

Determination methods of soil-related factors

Determination of soil microbial biomass

Soil microbial biomass was determined by chloroform fumigation culture method［14-15］.

Determination of soil enzymes

Catalase activity was measured by the volumetric method［16］; the determination of urease activity adopted the indophenol blue colorimetric method; for sucrase activity, the C7H4N2O7 colorimetric method was performed; and phosphatase activity was measured by the C6H5Na2O4P colorimetric method［17］.

Data processing and analysis

Excel 2010 was used to organize the data, and construct charts; SPSS 21.0 software was used to conduct one-way (One-Way ANOVA) analysis on each index of the secondary grassland soil in the abandoned farmland areas of different years, with a significant level of P<0.05.

Results and Analysis

Community species composition and structural characteristics of secondary grassland in abandoned farmland areas of different years

Community plant family, genus and species structure of secondary grassland in abandoned farmland areas of different years

It can be seen from Table 2 that there were 39 species of plants in the study area, belonging to  32 genera of 15 families, among which Chenopodiaceae had the most species (12 species of 9 genera), followed by Poaceae (7 species of 7 genera), Zygophyllaceae (4 species of 2 genera), and Compositae (2 species of 2 genera), Tamariaceae and Solanaceae had 2 species of 1 genus, and Liliaceae, Convolvulaceae, Plumbaginaceae, Cruciferae, Umbelliferae, Borraginaceae, Leguminosae, Asclepiadaceae and Euphorbiaceae all included single species of single genus. It could be seen that the community structure in this area was simple, and the families and genera were relatively scattered. The proportions of Chenopodiaceae, Poaceae, Zygophyllaceae and Compositae in the secondary grassland in the abandoned farmland areas of different years were relatively high. The sum of these four major families was 85% of the total plant species in the community when the farmland was abandoned for 4 years, and the sums of these four major families were more than 50% of the total plant species in the communities in the farmland abandoned for 2, 8, 20, 30, and 40 years. In the CK plot, there were only Chenopodiaceae plants and no other plant species.

Chunxiu GUO et al. Study on Vegetation Characteristics & Soil Properties of Secondary Grassland in Farmland Areas Abandoned for Different Years in Minqin Oasis Life-form structure characteristics and important values of community species in secondary grassland in abandoned farmland areas of different years

It can be seen from Table 3 that in the secondary grassland plant communities in the abandoned farmland areas of different years, there were at most 16 species of annual herbs, belonging to 14 genera of 4 families. The second was perennial herbs including 15 species, belonging to 14 genera of 11 families. Shrubs were the least, including 8 species, belonging to 6 genera of 5 families. The three types of plants accounted for 41.03%, 38.46% and 20.51% of the total, respectively. The herb layer was rich in species and large in number, while the shrub layer was less in number and relatively single. The overall performance of important values for different life forms was: shrubs (211.84)>annual herbs (58.87)>perennial herbs (29.31). In the plots of the farmland areas abandoned for 20-40 years, the important values of species of the vegetation shrub layer were relatively large. Specifically, in the plots abandoned for 20 years, the important value of Lycium ruthenicum was larger, with a value of 98.37, and in the plots abandoned for 30 and 40 years, the important value of Kalidium foliatum was larger, reaching 97.46 and 136.29, respectively. In the farmland plots abandoned for 1 and 4 years, the important values of perennial herb species were relatively large. In the farmland plots abandoned for 1 year, the important values of Peganum nigellastrum, Convolvulus arvensis and Chenopodium were relatively large, reaching 37.53, 48.8, and 47.85, respectively. In the farmland plots abandoned for 4 years, the important values of P. nigellastrum, Leymus secalinus and Phragmites australis were relatively large, at 33.58, 20.18 and 22.04, respectively. The important values of annual herb species were larger in the plots abandoned for 1, 2, 4, 8, 13 and 20 years, but there were no annual herbs in the plots abandoned for 30 years. In the plant communities of the secondary grassland abandoned for different years, the shrubs were mainly Reaumuria songarica, L. ruthenicum, Nitraria tangutorum and K. foliatum, and the herbs were mainly wormwood and grasses.

Soil microbiological characteristics of secondary grassland in abandoned farmland areas of different years

It can be seen from Fig. 1 that the soil microbial biomass carbon, nitrogen and phosphorus contents in different soil layers of the secondary grassland in the abandoned farmland areas of different years were generally in order of upper layer (0-20 cm) > the deep layer (20-40 cm), showing an obvious "surface aggregation phenomenon", and their values ranged from 49.84 to 459.29 mg/kg, 21.16 to 69.51 mg/kg, and 3.20 to 49.47 mg/kg, respectively. With the increase of years of farmland abandonment, the overall trend of soil microbial biomass carbon content in different soil layers was "increasing-decreasing-increasing again". The nitrogen content of soil microbial biomass showed a fluctuating upward trend as a whole. In the 0-20 cm soil layer, the soil microbial biomass phosphorus content showed a fluctuating downward trend, and in the 20-40 cm soil layer, it showed a fluctuating upward trend. Compared with the control plot (CK), the soil microbial biomass carbon content was not significantly different from that of the farmland abandoned for 20-30 years (P>0.05), but was significantly different from farmland abandoned for other years (P<0.05). The soil microbial biomass nitrogen contents of different soil layers were the lowest in the control sample. Specifically, in the 0-20 cm soil layer, the farmlands abandoned for 2, 8, 20 and 40 years were significantly (P<0.05) higher than the control sample (CK), but there was no significant differences with farmland of other abandoned years (P>0.05); and in the 20-40 cm soil layer, the farmland plots abandoned for 13 and 40 years were significantly (P<0.05) higher than the control plot (CK). For the phosphorus content of soil microbial biomass, in the 0-20 cm soil layer, the farmland plot abandoned for 2 years was significantly (P<0.05) higher than that of the control plot (CK), and the plots of 4 and 8 years were significantly (P<0.05) lower than the control plot (CK); and in the 20-40 cm soil layer, the farmland plots abandoned for 13 and 40 years were significantly (P<0.05) higher than the control plot (CK), but not significantly different from farmland plots of other abandoned years (P&gt;0.05).

Soil enzyme activity of secondary grassland in abandoned farmland areas of different years

It can be seen from Fig. 2 that the changes of soil enzymes in different soil layers of the secondary grassland in the abandoned farmland areas of different years were different. The soil catalase content was the highest in all plots, and its values were between 0.795 and 0.847 mg/(g·d), with no significant difference. In the 0-20 cm soil layer, the soil catalase showed a fluctuating downward trend with the increase of years of farmland abandonment, and the opposite was true in the 20-40 cm soil layer. Soil phosphatase, sucrase and urease all showed a fluctuating downward trend with the increase of years of land abandonment, and their values ranged from 0.002 to 0.025 mg/(g·d), 0.10 to 0.59 mg/(g·d), and 0.001 to 0.036 mg/(g·d), respectively. The soil enzyme activity exhibited an order of the surface layer (0-20 cm) > the deep layer (20-40 cm), and the surface aggregation phenomenon was obvious. Soil phosphatase, sucrase and urease in different soil layers showed that soil enzyme activity were significantly higher (P<0.05) in the early stage of abandonment (CK-8 years) than in the later period (13-40 years).

Discussions

The soil conditions of plant communities are different, and the community family, genus, and species structure also differs［18］. A total of 39 species of plants appeared in the nine plots of secondary grassland in the abandoned farmland areas of different years in Minqin Oasis, belonging to 32 genera of 15 families, including 8 species of shrubs, 15 species of perennial herbs, and 16 species of annual herbs. Due to the influence of natural environment conditions, the species composition was simple, and many species belonged to a single genus of a single family.  Such structure is consistent with the research results of Dang et al.［19］ on the floras of the northwest desert regions and He et al.［20］ on the vegetation characteristics of salinized farmland in the lower reaches of the Shiyang River. The sample plots were dominated by dwarf shrubs such as R. songarica, L. ruthenicum, N. tangutorum and K. foliatum［21］, and the herbs were mainly wormwood and grasses. With the increase of years of farmland abandonment, the vegetation changed from herbaceous plants to shrub communities, which is consistent with the research results of Wang［22］ and Tian［23］. However, it is different from the results of Li et al.［24］ on the vegetation succession of abandoned land in meadow steppe, indicating that different climate types and soil factors have different vegetation succession results in abandoned land.

Soil microbial biomass can reflect the function and activity of microorganisms in the soil［25］, and is the main driver of the soil nutrient cycling process［26］, so it plays an active role in purifying the soil environment and bioremediation［27］. With the deepening of soil layers, the overall contents of soil microbial biomass carbon, nitrogen and phosphorus in the secondary grassland in the abandoned farmland areas of different years showed a gradually decreasing trend, and the "surface aggregation phenomenon" was obvious. The peak of soil microbial biomass carbon content appeared in the 13th year, and the peaks of soil microbial biomass nitrogen and phosphorus appeared in the 2nd year, which might be due to that with the invasion and apoptosis of annual herbs and shallow-rooted plants in the early stage of abandonment, a large amount of litter gathered on the soil surface, which led to rich nutrients, promoted biological activity of microorganisms and more active metabolism. Such conclusion is consistent with the research results of Song et al., Chai et al. and Wang et al.［15,22,28］. In the abandoned farmland of 13 years, perennial herbs were replaced by shrubs, which increased deep root exudates, and soil nutrients continued to accumulate, making the carbon content of soil microbial biomass reach a peak value.

Soil enzymes are one of active organic components in the soil［29］, and their strength can indicate the health of the soil, so it is an important indicator of soil fertility［30］. The results of this study showed that soil phosphatase, sucrase and urease in secondary grassland in the abandoned farmland areas of different years all showed a fluctuating downward trend with the increase of years of farmland abandonment, and the phenomenon of surface aggregation was obvious; and the soil enzyme activity were significantly (P<0.05) higher in the early stage of abandonment (CK-8 years) than in the later period (13-40 years). It was mainly because the soil water and fertilizer conditions of the various plots were better in the early stage of farmland abandonment, with sufficient nutrients, and annual herbs and shallow root plants grew rapidly, which led to increased root exudates, continuous accumulation of soil nutrients, active soil microbial metabolism and enhanced soil enzyme activity. Such conclusion is consistent with the research conclusions of Song and Wu［28,31］. Due to the continuous consumption of soil water and nutrients in the later stage of farmland abandonment, the types and numbers of vegetation decreased, and herbaceous plants were gradually been replaced by deep-rooted shrubs such as L. ruthenicum and N. tangutorum, which are salt- and drought-tolerant. Therefore, the litter produced by communities decreased and the organic matter content in the soil decreased, which led to decreased number of soil microorganisms, and weakened metabolism of microorganisms, so the activity of soil enzymes gradually decreased and tended to be stable［32］.  There was little difference in soil catalase among the plots, indicating that the years of farmland abandonment had little effect on soil catalase.

Conclusions

Due to the influence of natural environment conditions, the species composition of vegetation communities in the secondary grassland in the abandoned farmland areas of Minqin Oasis was simple, and many species appeared in a single genus of a single family. As the years of abandonment of farmland increased, the dominant species of communities gradually changed from herbaceous plants to shrub communities; and the characteristics of soil microbial biomass and enzyme activity had an obvious "surface aggregation effect", and soil microbial biomass and soil phosphatase, sucrase and urease activity all showed a fluctuating downward trend after 8 years of farmland abandonment.

In conclusion, 2, 8 and 20 years were the years when soil fertility and quality changed significantly after farmland abandonment, especially at 8 years of farmland abandonment when soil enzyme activity began to decline, which was a critical period for grassland restoration and management. Although the soil physicochemical, microbial and fertility levels were constantly improving during the natural restoration of the secondary grassland in the abandoned farmland, it is a long and complicated process. Therefore, natural restoration and supplemented by artificial measures can maintain the persistence and stability of the soil ecosystem, and when farmland is abandoned for about 8 years, great attention should be paid to it. Moreover, the construction and protection of cultivated land quality must be strengthened, and economic, ecological and social benefits should be taken into account, so that the improvement of cultivated land quality and the improvement of productivity level can achieve a virtuous circle.

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