Study on Characteristics of Soil Seed Bank of Natural Grassland in Nierong County

Xiuhua

Institute of Pratacultural Science, Tibet Academy of Agriculture and Animal Husbandry Sciences, Lhasa 850009, China

Abstract [Objectives] To explore the species, density, distribution and other characteristics of the soil seed bank of different degrees of degraded grassland in Nierong County, and introduce the species composition of the above-ground vegetation community. [Methods] The grassland in Nerong County was divided into non-degraded, mildly degraded, moderately degraded, and severely degraded grassland. Then the relationship between above-ground vegetation and soil seed bank and the role of seed bank in the construction of above-ground vegetation community was analyzed. [Results] The soil seed bank density of moderately degraded plots is the highest, and that of severely degraded grassland is the lowest. For the non-degraded and mildly degraded plots, the species in the soil seed bank are mainly Cyperaceae such as Kobresia pygmaea and Kobresia humilis. For moderately degraded plots, the species in the soil seed bank is dominated by Potentilla anserina in Rosaceae family. Besides, the soil seed bank of each plot is mainly concentrated in the 0-3 cm soil layer. The similarity between the soil seed bank and the vegetation community of the sample plots from high to low is: no degradation>mild degradation>moderate degradation>severe degradation. [Conclusions] In accordance with the present situation of grassland degradation, for moderately degraded grasslands, it is recommended to take appropriate enclosure and rest grazing; for severely degraded grasslands, no-tillage reseeding will be favorable for the restoration and protection of grassland ecological functions. In summary, it is intended to provide references for the management of alpine meadows and the restoration and reconstruction of degraded ecosystems in alpine regions.

Key words Seed soil bank, Degraded grassland, Plant community

1 Introduction

The soil seed bank refers to the sum of upper litter decay and all survival seeds in the soil. It is an important indicator of the response of plant communities to land use and climate change, and plays an important role in the protection and restoration of plant communities. The soil seed bank is a potential plant population or community, and also the material basis for the natural regeneration of vegetation. It is essential for the restoration of degraded ecosystems. Understanding the ecosystem functions of the soil seed bank is helpful to the management of the natural ecosystem, accordingly helpful for us making reasonable decisions on the sustainable management of vegetation.

Foreign scholars started research on the soil seed bank in the 1930s, and their studies were concentrated on the biological characteristics of the soil seed bank, such as species composition, density, the relationship between the soil seed bank and above-ground vegetation, and the impact of disturbance on the soil seed bank. From the 1970s, the studies started focusing on the relationship between seed bank, seed rain, seed germination and seedling survival. After the mid-1990s, international research reports on soil seed banks increased rapidly, especially in the field of restoration ecology. Studies of soil seed bank by foreign scholars involve many disciplines. Existing studies have revealed the composition, density, distribution pattern and dynamic changes of soil seed bank, as well as influencing factors, and formed a systematic theoretical framework. Chinese scholars started the research on soil seed bank in the 1980s. Until the 1990s, the research on soil seed bank in vegetation restoration and ecological diversity attracted the attention of experts, scholars and workers in restoration ecology in China. In addition, Chinese scholars carried out basic research on soil seed banks. Research on the spatial distribution, species composition, density, and the relationship between soil seed banks and above-ground vegetation has become the major points of China’s soil seed bank research. However, few studies have involved the influencing factors of soil seed bank, as well as its specific role and theoretical mechanism in vegetation restoration and regeneration. In view of these, by studying the characteristics of the soil seed bank of degraded grassland in Nierong County and its relationship with the above-ground vegetation, we elaborated the grassland plant community and the size of the soil seed bank with different degrees of degradation, and analyzed the interaction mechanism between the soil seed bank and the above-ground vegetation of grassland with different degrees of degradation, to provide a theoretical basis for proposing the theory and technology of restoring degraded grassland and restoring the ecological carrying capacity of degraded grassland.

2 Materials and methods

2.1 General situation of the study area

The study area is located on the southern slope of Tanggula Mountain, with a sub-frigid plateau semi-arid monsoon climate, long winter and no summer, and an average altitude of 4 800 m. In the study area, there are more than 70 large and small rivers, which belong to the Yangtze River system and the Nujiang River system. There are one of the four main source rivers of the Yangtze River. There is a highway leading to Nagqu Town. The economy is mainly animal husbandry. Natural disasters are frequent, and it is a disaster-prone area. The precipitation is mainly concentrated in the warm season (June to September), which accounts for more than 80% of the annual precipitation. The annual precipitation is 400 mm, the annual average temperature is -10 ℃, the annual sunshine time is about 2 850 h, the evaporation is 1 704 mm, and the relative humidity is 46%. The main grassland types are alpine grasslands and alpine meadows. The dominant species in alpine meadow grasslands are

Kobresia

humilis

, common companion species are mainly

Carex

moorcroftii

and

Poa

litwinowiana

,

Leontopodium

pusillum

,

Potentilla

bifurca

,

Potentilla

saundersiana

,

Androsace

tapete

,

etc.

The total grassland area in Nierong County is about 836 404 ha, and the usable grassland area is 803 761 ha; the degraded grassland area is 213 960 ha, accounting for 34.20% of the grassland area, of which: the mildly degraded grassland area is 170 191 ha, accounting for the grassland area, the moderately degraded grassland is 27.867 ha, accounting for 4.46% of the grassland area, and the severely degraded grassland is 15 901 ha, accounting for 1.94% of the grassland area.

2.2 Research methods

2.2.1

Selection of experimental plot. According to the coverage of the grassland, the proportion of fine forage, the bare ground, and the size of the bald patches, we determined four experimental plots of different degrees of degraded grasslands in Nierong County, namely non-degraded (enclosed grassland) and mildly degraded, moderately degraded, and severely degraded grassland, each plot area is 1 ha.

2.2.2

Sampling from the soil seed bank. Samples were taken from the seed bank after the winter seed dormancy and before the production of new seeds that year. In mid-April 2013, random sampling was used in each experimental plot. The soil seed bank sampler is a special 15 cm × 15 cm × 15 cm steel sampler. Selected a point at the center of each plot, and randomly selected 5 sampling points from this center. The soil sampling depth was divided into 0-3, 3-6, and 6-9 cm. Sampling did not include litter. A total of 15 soil samples from the soil seed bank were collected from each plot. The soil samples were put into cloth bags and brought back to the laboratory.

2.2.3

Identification of soil seed bank species. In this experiment, we adopted the germination method. The seeds of most plants in alpine meadow grassland are small, and the germination method can get a more reliable and effective estimate, and the identification of seedlings is easier than the identification of seeds directly. The seed bank obtained by the germination method is generally a soil germinated seed bank. The soil samples were evenly spread in the germination tray with a thickness of about 2 cm. The germination tray was placed in the room for seed germination and seedling species diagnosis. During the seed germination period, an appropriate amount of water was sprayed into the germination tray at regular intervals every day, and continued to observe for 3 months.

2.2.4

Survey of the plant community on the sample plot. In August 2013, we carried out a community survey on the vegetation of the study plots. We set up 1 m × 1 m plots randomly at the center of each plot with a spacing of 10 m. In the survey, we made records of the coverage, frequency, height, and density.

2.2.5

Data processing. The data were calculated with the aid of Excel software.

Important value=(Relative density+Relative coverage+Relative height+Relative frequency)/4

Relative density=(Number of individuals in a certain species/Number of individuals in all species)×100%

Relative height=(Average height of a certain species/Sum of average heights of all species)×100%

Relative frequency=(Frequency of a certain species/Sum of frequencies of all species)×100%

Relative coverage=(Coverage of a certain species/Sum of coverage of all species)×100%

Correlation coefficient: the similarity between the above-ground vegetation and the soil seed bank was calculated using Sorensen index by the formula:

SC

=2

C/

(

S

+

S

), where

SC

is the Sorensen index value, and

C

is the number of species shared in the vegetation and soil seed bank;

S

and

S

denote the number of species appearing in the above-ground vegetation and soil seed bank, respectively.

3 Results and analysis

3.1 Main species of soil seed bank and their density and distribution

From Table 1 that the species composition of the soil seed bank is also inconsistent due to different degrees of degradation of alpine meadows. The the soil seed bank of non-degraded grassland is dominated by

Kobresia

pygmaea

, including

Saussurea

japonica

,

Poa

pratensis

L.,

Festuca

rubra

L.,

Polygonum

viviparum

L.,

etc.

; the soil seed bank of mildly degraded grassland is dominated by

Kobresia

pygmaea

, accompanied with

Kobresia

humilis

and

Poa

pratensis

L.; the soil seed bank of moderately degraded grassland is dominated by

Potentilla

anserina

, including

Halerpestes

tricuspis

and

Poa

pratensis

L.; the soil seed bank of severely degraded is dominated by

Potentilla

saundersiana

,

Halerpestes

tricuspis

,

Poa

pratensis

L., and

Draba

nemorosa

,

etc.

Among degraded grasslands of different degrees, moderately degraded grasslands have the highest soil seed bank density, followed by non-degraded grasslands, and the severely degraded grasslands have the lowest soil seed bank. In terms of the distribution of soil seed bank, the horizontal distribution of different degrees of degraded grassland is the most in moderately degraded grasslands, followed by non-degraded grassland and moderately degraded grassland, and severely degraded grassland has the least distribution. In terms of vertical distribution, it can be seen from Table 1 that the soil seed banks of the 4 plots are distributed most in the 0-3 cm soil layer, and the soil seed banks are rarely distributed in the 3-6 and 6-9 cm soil layers, and no germinating seeds were seen in the 6-9 cm soil layer of the mildly degraded and severely degraded plots (Table 1).

Table 1 Main species and their distribution and density in the soil seed bank of different degrees of degraded grasslands cm

3.2 Plant community compositions and their important values

From Table 2 it can be known that alpine meadow grassland differs in its plant community composition due to different degrees of degradation. Non-degraded grassland and mildly degraded grasslands are dominated by

Kobresia

pygmaea

and

Kobresia

humilis

; the moderately degraded grasslands are dominated by

Kobresia

humilis

,

Carex

spp. and

Potentilla

anserina

; the severely degraded grasslands are dominated by

Potentilla

fruticosa

L. and

Leontopodium

nanum

. With the intensifying degree of degradation, the types and numbers of Cyperaceae decrease, and the Asteraceae, Poaceae, Cruciferae, and Rosaceae increase.

Table 2 Species composition and important values of plant communities in the soil seed bank plots of different degrees of degraded grasslands

3.3 Similarity analysis of soil seed bank and plant community

In the survey of vegetation communities in different degrees of degraded grassland, it is found that there were 17 species of plants in the experimental plot. The results of the germination test in the soil seed bank found that there are 11 kinds of plants in the soil seed bank. Especially,

Halerpestes

tricuspis

in the soil seed bank was not found in the above-ground vegetation community, and the other 10 plants in the soil seed bank were found in the above-ground vegetation community. The similarity of above-ground vegetation and soil seed bank of different degrees of degraded grassland decreases with the degree of degradation. The similarity from high to low is: non-degraded > mildly degraded > moderately degraded > severely degraded (Table 3).

Table 3 Relationship between soil seed bank and aboveground vegetation communities in different degrees of degraded grasslands

4 Discussion

4.1 Species composition and density of soil seed bank in different degrees of degraded grasslands

In this study, we found that the soil seed bank of non-degraded grasslands and mildly degraded grasslands contains high seed content of edible forage grasses of Cyperaceae such as

Kobresia

humilis

,

Kobresia

humilis

and

Carex

spp., while the soil seed bank of moderately degraded and severely degraded grasslands is dominated by Asteraceae and Rosaceae. This is consistent with the results of the Nagqu soil seed bank research conducted by Wang Honghui

et

al.

.Wang Xiangtao

et

al.

found that grazing caused a change in soil seed bank density to a certain extent, the density of heavily grazing soil seed bank was higher. It is believed that the above-ground vegetation community and soil seed bank of heavy grazing plots have the highest seed proportion of dicotyledonous weeds. These weeds are dominated by seed propagation, and the number of seeds is large, and most of them can stay in the seed bank for a long time. Therefore, under overgrazing conditions, a large amount of seeds can be obtained in the seed bank. Our findings are consistent with these. We found that the soil seed bank density of moderately degraded plots is the highest, and that of severely degraded grassland is the lowest. The moderately degraded grassland was caused by overgrazing, and the severely degraded grassland on the experimental plot had sparse vegetation, which was close to desertification and was not suitable for grazing, and the corresponding soil seed bank had low seed content. Moderately degraded grassland may be due to the reduction of edible plant species, which leads to a large number of seeds retained in the soil seed bank; the non-degraded grassland is protected by fences and there is no livestockt, so there are more seeds in the soil seed bank; the mildly degraded grassland is dominated by edible plants such as

Kobresia

pygmaea

and

Kobresia

humilis

, which are eaten by domestic animals and rats and rabbits, resulting in less seeds in the soil seed bank; the severely degraded grassland has low vegetation coverage on the ground due to severe degradation, accordingly the seed content in the soil seed bank is the lowest.

4.2 Relationship between soil seed bank and above-ground vegetation in different degrees of degraded grassland

There is a close relationship between the above-ground vegetation and the composition of plant species in the soil seed bank. On the one hand, the above-ground vegetation is the source of the seeds in the soil seed bank; on the other hand, the seeds in the soil seed bank can directly participate in the regeneration and succession of the above-ground vegetation. The similarity between the soil seed bank and the above-ground vegetation species composition is affected by such factors as succession stage, research methods, interference,

etc.

In addition, the relationship between the characteristics of the above-ground vegetation community and the characteristics of the soil seed bank has become more complicated due to differences in species reproduction strategies, seed yields, seed sizes,

etc.

The previous related studies have not reached a unified conclusion.

According to findings, the similarity between above-ground vegetation and soil seed bank is related to interference, especially under larger frequency interference, the similarity between the two is relatively high. The similarity between the soil seed bank of the fenced plot and the above-ground vegetation is low, and the similarity of the degraded plot is relatively high. By contrast, our results are opposite. The similarity between soil seed bank and above-ground vegetation is negatively correlated with the degree of grassland degradation. The similarity of the fenced plots is the highest, and the similarity of the severely degraded grassland is the lowest. This is possibly because we adopted the direct germination method of soil samples in this study. The soil samples contained rhizomes and other vegetative reproduction of the plots. The fenced plots and the mildly degraded plots were dominated by vegetative reproduction of Cyperaceae plants. Therefore, the similarity between soil seed bank and above-ground vegetation is relatively high.

5 Conclusions

Moderately degraded grasslands have the highest soil seed bank density and the most species, but the content of edible forages is reduced. It is recommended to take appropriate rest grazing to prevent its degradation. Severely degraded grasslands have sparse vegetation communities on the ground and low soil seed bank density. Thus, it is difficult to naturally restore grassland ecological functions. Instead, no-tillage reseeding can be considered to improve its utilization value.