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\begin{document}

\title{Responses of grassland vegetation and soil nutrients to desertification
in Qilian Mountains of eastern Qinghai-Tibetan Plateau}



\author[1]{qiang li}%
\author[1]{Guoxing He}%
\author[1]{degang zhang}%
\author[1]{xiaoni liu}%
\affil[1]{Gansu Agricultural University}%


\vspace{-1em}



  
  \date{May 26, 2024}


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\selectlanguage{english}
\begin{abstract}
To explore the change characteristics of vegetation and soil nutrients
in grassland desertification, the study intends to using the method of
replacing space with time, taking temperate steppe (TS), temperate
steppe desert (TSD) and temperate desert (TD) in Qilian mountains as the
research objects, and analyze the change characteristics of vegetation
and soil nutrients replacing succession of grassland desertification.
The study indicated that, from TS, TSD to TD, the total coverage, AGB,
Shannon-Weiner index and richness, total porosity, total N, soil organic
matter, C/N, C/P, N/P, alkaline phosphatase, catalase, sucrase were
decreased, and grass layer height, dominance, soil bulk density and pH
were increased. While the total P, total K and available P were first
increased and then decreased, and were opposite to evenness and urease
enzyme activity change. Meanwhile, the C/N, C/P and N/P of three
grassland types ranged from 5.06-17.27, 2.49-71.94 and 0.50-4.19.
Correlation analysis showed that, except for available P, available K,
and evenness, there was a significant correlation between other
vegetation and soil indicators. In conclusion, in the process of
grassland desertification, grassland vegetation and soil had undergone
significant changes, especially total coverage, AGB, Shannon-Weiner
index, soil bulk density, soil organic matter and total nitrogen. Based
on N/P, the nitrogen was the main factor limiting the productivity of
grassland, and the limitation was more prominent in the process of
grassland desertification.%
\end{abstract}%



\sloppy


\textbf{Responses of grassland vegetation and soil nutrients to
desertification in Qilian Mountains} \textbf{of eastern Qinghai-Tibetan
Plateau}

Qiang Li; Guoxing He; Degang Zhang; Xiaoni Liu11Corresponding author.
TEL:13809318054 E-mail address: liuxn@gsau.edu.cn (Liu Xiaoni).

(College of Grassland Science, Gansu Agricultural University/Key
Laboratory of Grassland Ecosystem of the Ministry of Education, Lanzhou
730070, PR China)

\textbf{Abstract:} To explore the change characteristics of vegetation
and soil nutrients in grassland desertification, the study intends to
using the method of replacing space with time, taking temperate steppe
(TS), temperate steppe desert (TSD) and temperate desert (TD) in Qilian
mountains as the research objects, and analyze the change
characteristics of vegetation and soil nutrients replacing succession of
grassland desertification. The study indicated that, from TS, TSD to TD,
the total coverage, AGB, Shannon-Weiner index and richness, total
porosity, total N, soil organic matter, C/N, C/P, N/P, alkaline
phosphatase, catalase, sucrase were decreased, and grass layer height,
dominance, soil bulk density and pH were increased. While the total P,
total K and available P were first increased and then decreased, and
were opposite to evenness and urease enzyme activity change. Meanwhile,
the C/N, C/P and N/P of three grassland types ranged from 5.06-17.27,
2.49-71.94 and 0.50-4.19. Correlation analysis showed that, except for
available P, available K, and evenness, there was a significant
correlation between other vegetation and soil indicators. In conclusion,
in the process of grassland desertification, grassland vegetation and
soil had undergone significant changes, especially total coverage, AGB,
Shannon-Weiner index, soil bulk density, soil organic matter and total
nitrogen. Based on N/P, the nitrogen was the main factor limiting the
productivity of grassland, and the limitation was more prominent in the
process of grassland desertification.

\textbf{Keywords:} Qilian mountains; desertification; grassland; soil
nutrient.

\textbf{Introduction}

''Desertification'' refers to land degradation caused by climate change
and human activities in arid, semi-arid and semi-humid arid regions
(Liu, 1998), which is a global environmental problem that plagues human
development, and one of the current ecological research hotspots (Gao et
al., 2019). Relevant studies have shown that the global arid area is
6.1\selectlanguage{ngerman}×10\textsuperscript{8} hm\textsuperscript{2}, among which China has
2.97×10\textsuperscript{8} hm\textsuperscript{2}, and the arid areas
have different degrees of desertification. From 1984 to 1991, the global
land desertification accelerated at an annual rate of 3.4\% (Office of
Desertification Prevention and Control, Ministry of Forestry of the
People's Republic of China, 1994). With global warming, the disorderly
development of resources and the urbanization process, desertification
may become a major ecological problem and challenge in the 21st century
(Li et al., 2019; Rong, 2019). Therefore, understanding the rules of
desertification, that is, the change characteristics and mutual
relationship of vegetation and soil, has important ecological
significance for slowing down the desertification process and vegetation
restoration and reconstruction in desertified areas.

Qilian Mountain is located on the eastern edge of the Qinghai-Tibet
Plateau in China, adjacent to the Mongolian Plateau and the Loess
Plateau (Gao et al., 2019). The Qilian mountains is the intersection of
the Qinghai-Tibet Plateau, the Loess Plateau and the Inner Mongolia
Plateau, with a fragile ecological environment, and is also one of the
most sensitive regions for global climate change (Wang et al., 2001).
Grassland is the largest vegetation type in Qilian Mountains (Li et al.,
2019). Therefore, the health of the grassland ecosystem in the Qilian
Mountains is not only related to the water source security in the Hexi
region, but also related to the ecological security and social stability
in northwest China (Yao et al., 2018). Related studies have found that
the climate change in the eastern part of Qilian is warm and humid,
while the central and western regions is warm and dry (Rong, 2019).
Therefore, climate change poses a serious challenge to the ecology of
the Qilian Mountains. The warming and humidification of the climate in
the eastern region had caused the degradation of alpine meadows and the
shift of the snow line, while the warming and drying of the climate in
the central and western regions led to the succession of temperate
steppe to temperate desert steppe and temperate desert (Rong, 2019).
Succession of grassland desertification will lead to a series of
grassland ecological changes, such as changes in vegetation community
structure, soil nutrients, microbial composition, etc. (Rong, 2019).
Tang et al. (2016) found that the effects of desertification on total
phosphorus and available phosphorus were not significant, and
desertification decreased productivity in the desert steppe as a result
of direct changes to soil physical properties, which can directly affect
soil chemical properties. Li et al. (2006) found that soil organic
matter is also reduced with desert development, which leads to
destruction of the stability of soil physical structure and nutrient
content, such as progressive N, P and K loss in surface and subsoil
layers, and in response to changes in soil properties, vegetation
altered as regards species composition, species diversity, coverage,
structure and life-form (Tang et al., 2016). Therefore, a systematic
understanding of the vegetation and soil change characteristics in the
process of grassland desertification is of great significance to the
restoration of desert grassland.

At present, most of the research on grassland desertification focused on
the degradation of desert grassland. According to the degradation
standard, the desert grassland is divided into several degradation
gradients to study the relationship between vegetation community
structure and soil nutrients (Rong, 2019; Li et al., 2006; Tang et al.,
2016). Because grassland desertification is a process of long-term
succession of grassland under warm and dry climate, this study used the
method of replacing time with space, and selects three types grassland
(temperate steppe, temperate steppe desert and temperate desert) as
research objects replacing desertification succession. The
characteristics changes and relationship of vegetation and soil factors
were analyzed during the succession of temperate steppe to temperate
desert, which could provide theoretical basis and data support for
curbing grassland desertification and inducing forward succession of
desert grassland.

\textbf{1 Materials and methods}

\textbf{1.1 Study area}

The study sites were located in the Qilian mountain nature reserve in
Gansu province, China (94°10\selectlanguage{english}'-103deg04\selectlanguage{english}'E, 35deg50\selectlanguage{english}'-39deg19\selectlanguage{english}'N). In the
horizontal distribution, the vegetation gradually deteriorates from
southeast to northwest, which results in the formation of four
vegetation zones in an order of forest, shrub, grassland and desert.
Vegetation at the vertical belt spectrum, from low altitude to high
altitude, was grassland belt, forest belt and alpine meadow grassland
belt. The main types of soil were mountain gray cinnamon soil, subalpine
meadow soil, alpine meadow soil, alpine cold desert soil. The annual
precipitation varied from 100 to 500mm, mostly from June to September,
the average annual temperature was -0.6-2.0degC, the average annual
relative humidity was 20\%-70\%, the average annual evaporation was
about 1200-1400 mm, the frost-free period was 90-120 days.

\textbf{1.2 Research methods}

\textbf{1} .2.1 Sample selection

This research area was mainly focused on the Qilian mountain in Gansu
province, China. The grassland types were temperate steppe, Temperate
Steppe Desert, temperate desert. The utilization of different grassland
was shown in Table 1.

\textbf{1.2.2 Sample collection}

The sampling time of this study was from July to August 2019, when the
plants were in full bloom. The central area of the typical distribution
area of the above 3 types of grassland (TS, TSD and TD) were selected
for the sampling site. We established 6 random sampling quadrates (1 m x
1 m) in each site. In each quadrate, plant species, coverage, height,
density of the respective species and aboveground biomass were measured
and recorded. Aboveground parts of the green plants of the respective
species were harvested by clipping to the soil surface, and collecting
all litter in each quadrate. All the aboveground plant samples were
placed into envelopes and then tagged, respectively. All the green plant
samples were immediately dried at 105\selectlanguage{english} for 30 min, then oven-dried at
60\selectlanguage{english}for 48 h and weighed. Meanwhile, a 60-meter sample line was set for
each sample site and the sample spots were set at a 20-meter interval.
Four soil samples were taken around each sample spot using soil drill at
the soil layers with a depth of 0-10 cm, 10-20 cm and 20-30 cm
respectively, and mixed four soil samples from each layer as one sample
respectively. The samples were put into a sample bag and taken back to
indoor for air-drying, which were measured for soil organic matter,
Total N, Total P, pH and soil enzyme activity.

\textbf{1.2.3 Sample determination}

Soil samples were air-dried at room temperature, and visible roots and
other debris in the soil samples were removed. Each composite soil
sample was sieved through a 2-mm sieve. SOC was determined by the
Walkley--Black method (Nelson \& Sommers, 1996). Total N was determined
by the Kjeldahl acid digestion method (Foss Kjeltec 8400, FOSS, DK)
(Nelson \& Sommers, 1996). Total P was measured by Mo-Sb colorimetry
(UV-2102C, UNICO, Shanghai, China) (Nelson \& Sommers, 1996). Soil pH
value: potential method (water/soil ratio: 2.5:1). Urease enzyme
activity was measured using the phenol sodium-hypochlorite sodium
colorimetric method (Soil Physics institute, 1978). Sucrase enzyme
activity was calculated according to the amount of glucose (mg) that was
generated in 1 g soil sample after cultivation at 37\selectlanguage{english} for 24h (Soil
Physics institute, 1978). Alkaline phosphatase enzyme activity was
determined using the method of disodium phenyl phosphate (Soil Physics
institute, 1978). Catalase was determined by
KMnO\textsubscript{4}titration (Soil Physics institute, 1978).

\textbf{1.3 Statistical analyses}

The mass ratio was used for the ratio C/N, C/P and N/P. Statistical
analyses were performed with SPSS version 19.0 (SPSS Inc., Chicago, IL,
USA). All results are presented as mean and standard deviations. One-way
ANOVA and least significant difference (LSD) tests (P\selectlanguage{english}<0.05). The IV of
each plant species was calculated by the following formula (Lindsey,
1956):

IV =\(\frac{relative\ coverage\ +\ relative\ height\ +\ relative\ density\ +relative\ dry\ weight}{4}\)

Plant diversity was estimated using the three standard multi-dimensional
biodiversity indices, i.e., Pielou Evenness index, Shannon-Weiner (H'),
and Simpson diversity index (D) was calculated based on the following
equations.

H\selectlanguage{english}'=\(-\sum_{i=1}^{s}P\)i\(\ln P\)i

When H' represents Shannon-Weiner index, P\textsubscript{i} the total
number of individual species proportion of ith species in the community
and S represents encountered species number. the number of species
encountered, P\textsubscript{i} the proportion of the total number of
individual species belonging to I th species in the community,
lnP\textsubscript{i} means the natural logarithm of P\textsubscript{i}.

D=1-\(\sum_{i=1}^{S}P\)i2

Where D reprensets Simpson diversity index, S represents encountered
species number. Pi the total number of individual species proportion of
i th species in the community.

J=H\selectlanguage{english}'/\(\ln S\)

Where J represents Pielou evenness index, H' represents Shannon-Weiner
index, S means the number of species

R=S

Where R represents richness index, S means the number of species.

\textbf{2 Result}

\textbf{2.1 Vegetation characteristics}

From TS, TSD to TD, the total coverage, AGB, Shannon-Weiner index and
richness were decreased (Table2). There were significant differences
among different treatments. Compared with TS, the coverage under TSD and
TD decreased by45.31\% and 60.41\%, respectively, and the AGB decreased
by 58.90\% and 70.24\%, respectively, and the Shannon-Weiner index
decreased by 38.94\% and 55.43\%, respectively, and the richness
decreased by 51.08\% and 58.77\%, respectively. The changes in dominance
was the opposite of total coverage. The grass layer height was in order
of TSD\textgreater{}TD \textgreater{}TS and evenness was in order of TD
\textgreater{}TS \textgreater{}TSD.

\textbf{2.2 Soil physical and chemical characteristics}

Soil bulk density, total porosity and pH for various treatments were
shown in Table3. The soil bulk density gradually increased in TS and
increased in TSD and TD with the increase of soil depth. Compared with
TS, the TSD and TD significantly increased the soil bulk density in the
0-30 cm soil depth. Soil bulk density was in order of TS \textless{} TSD
\textless{} TD in same soil layers. The soil total porosity showed the
opposite trend as changes in soil bulk density, and the pH showed the
same trend as changes in soil bulk density except the pH in 10-20cm soil
layer (TS \textless{} TD \textless{} TSD). Except TS, the total N
gradually increased with the increase of soil depth. From Table4,
compared with TS, the TSD and TD significantly decreased the soil total
N in the 0-30 cm soil depth, and soil total N in same soil layers was in
order of TD \textless{} TSD \textless{} TS. The SOC showed the same
trend as changes in total N (Table4). The total P in same soil layers
was in order of TSD \textless{} TD \textless{} TS (Table4), and the TSD
and TD significantly decreased the total P in the 0-30 cm soil depth
compared with TS. Available P showed the same trend as changes in total
P (Table3). The available K in same soil layers was in order of TSD
\textless{} TS \textless{} TD, except in 0-10cm soil layers (Table3).

\textbf{2.3 Soil Stoichiometric characteristics}

Soil C/N, C/P and N/P for various treatments were shown in Table4.
Compared with TS, the TSD and TD significantly decreased the C/N in the
0-30 cm soil depth. The C/N was in order of TD \textless{} TSD
\textless{} TS in same soil layers. The C/P and N/P showed the same
trend as changes in C/N.

\textbf{2.4 Soil enzyme activity}

Soil enzyme activity for various treatments were shown in Fig.1. With
the increase of soil depth, the Urease, alkaline phosphatase and sucrase
enzyme activity was decreased, and catalase enzyme activity was
increased. Compared with TS, the TSD increased and TD significantly
decreased the Urease enzyme activity in the 0-30 cm soil depth. The
Urease enzyme activity was in order of TD \textless{} TS \textless{} TSD
in 10-20cm and 20-30cm soil layers, but was in order of TD \textless{}
TSD \textless{} TS in 0-10cm soil layers. The alkaline phosphatase
enzyme activity in same soil layers was in order of TD \textless{} TSD
\textless{} TS, and the TSD and TD significantly decreased the alkaline
phosphatase enzyme activity in the 0-30 cm soil depth compared with TS.
The catalase (except in 20-30cm soil layers) and sucrase enzyme activity
showed the same trendas changes in alkaline phosphatase enzyme activity.

\textbf{2.5 Correlation}

Correlation analysis showed that, except for available P, available K
and evenness, there was a significant correlation between other
vegetation and soil indicators (Table 5).

3 \textbf{Discussion}

\textbf{3.1 Vegetation characteristics}

Species diversity is an important feature of ecosystem stability
maintenance and function operation, and vegetation productivity is an
important indicator to characterize the plant ecological function
stability and regional ecological carrying capacity (Pennekamp et al.,
2018; Campos-Herrera, et al., 2013). Tang et al. (2016) found that, with
the succession of grassland to desert, species richness, total
vegetation coverage, dominant abundance, richness index and aboveground
organisms all showed a downward trend. In this study, compared with TS,
the coverage, AGB, Shannon-Weiner index and richness decreased, which
was similar to the results of the Meng et al. (2018) study. Due to the
suitable water and heat conditions in the temperate steppe distribution
area, it was mainly composed of perennial herbs. The Temperate Steppe
Desert was used for water limitation, and part of the herbaceous plants
were degraded and formed mainly herbaceous and semi-shrub vegetation.
However, due to extremely rare rainfall, dwarf shrubs and subshrubs with
strong drought resistance were mainly found in temperate desert (Zhang
et al., 2015). Evolving from temperate steppe to temperate desert, the
vegetation structure gradually becomes uniform and the stability of the
ecosystem decreases (Sharma et al., 2009).

\textbf{3.2 Soil nutrient and stoichiometric characteristics}

Soil degradation was one of the important characteristics of
desertification. With the development of desertification, the
degradation of soil physical and chemical factors had gradually
increased (Wang et al., 2018). Related studies have found that, with the
succession and development of desertification, and the content of soil
organic matter and total N decrease significantly (Meng et al., 2018).
In this study, from TS, TSD to TD, the total porosity, total N and soil
organic matter were significant decreased, and soil bulk density and pH
were significant increased. But the total P, total K and available P
were in order of TSD \textless{} TD \textless{} TS. Due to the long-term
warming and drying of the climate, the succession of temperate steppe to
Temperate Steppe Desert has been induced. The vegetation type had
evolved from herbaceous to semi-shrub (Wu et al., 2017). The decrease in
vegetation litter amount and the decrease in decomposition rate
(decomposing bacteria activity decreased) were co-leading of the reduced
soil carbon, nitrogen, and phosphorus nutrients, which turned the
changes of soil pH, porosity and bulk density. At the same time, soil
changes had a negative effect on vegetation, which was caused the
heterogeneity absorption of trace elements by vegetation (Tao et al.,
2019). Soil ecological stoichiometric characteristics (C/N, C/P and N/P)
have a strong regulatory effect on the carbon fixation process in
terrestrial ecosystems (Zhang et al., 2016), which is an important
parameter to measure soil quality (Liu et al., 2021), reflecting the
ability of soil to release nitrogen and phosphate mineralized nutrients.
The optimal C/N required for microbial life activities was 25 (Tessier
and Raynal, 2019), which is the most favorable ratio for the conversion
of organic carbon. In this study, from TS, TSD to TD, C/N rate was
decreased, and C/N of TS, TSD to TD was 15.70, 10.13 and 5.80,
respectively. That was, TS was good for microbial life activities, but
TSD and TD was bad for microbial life activities. Soil C/P ratio was an
important indicator of soil phosphorus mineralization capacity, which
could measure the potential of soil organic matter mineralization to
release phosphorus or absorb phosphorus fixation (Zhang et al., 2018).
In this study, from TS, TSD to TD, C/N was decreased, and C/P of TS, TSD
to TD was 63.17, 10.04 and 2.98, respectively. The N/P ratio was used as
an index for determining the nutrient factors that limit productivity,
and N/P\selectlanguage{english}<10 and N/P\textgreater{}20 were used as indicators to evaluate
the productivity of vegetation limited by nitrogen or phosphorus(Li et
al., 2018). In this study, from TS, TSD to TD, C/N was decreased, and
N/P of TS, TSD to TD was 4.02, 1.00 and 0.53, respectively. That was,
productivity was increasingly limited by soil nitrogen from TS, TSD to
TD.

\textbf{3.3 Soil enzymes characteristics}

Soil enzymes decomposed complex organic compounds into smaller organic
compounds and inorganic nutrients (Grandy et al., 2007). As the most
active and sensitive components in soil, soil enzymes played an
important role in the nutrient cycle of soil and the supply of nutrients
needed for plant growth, and were an important index for evaluating soil
quality of different grassland types (Veronica et al., 2007). Its
activity and mainly affected by soil physical and chemical properties,
soil microorganisms and vegetation litters (Cui et al., 2019). In ours
research, with the increase of soil depth, urease, alkaline phosphatase
and sucrase enzyme activity decreased and catalase enzyme activity
increased. From TS, TSD to TD, the alkaline phosphatase, catalase and
sucrase enzyme activity were decreased. Because the soil surface layer
accumulates more litter and humus than the depth soil layer, the organic
matter content was high and the nutrient source was sufficient, which
was conducive to the growth and reproduction of microorganisms.
meanwhile water and heat in topsoil better and ventilation conditions,
plant hair root and litter turnover rapid, which further accelerated the
microbial growth and metabolism, making the soil Urease, Alkaline
phosphatase and Sucrase enzyme activity decreased with the increase of
soil depth, but the Catalase enzyme activity increased.

\textbf{3.4} \textbf{The evolution mechanism of grassland
desertification succession}

This research used space instead of time to analyze the evolution of
grassland vegetation and soil in the process of grassland
desertification, in the western Qilian Mountains- eastern Qinghai-Tibet
Plateau during the global warming process. Based on the steppe spatial
distribution pattern, the evolution from temperate steppe to temperate
desert, rainfall decreased and temperature raised, that was, climate
change was ''warming and drying'' (Rong, 2019). The complex of altitude
as a regional hydrothermal factor also showed a decreasing trend (from
temperate steppe to temperate desert was from 2800m to 1500m) (Xu et
al., 2021). In the grassland ecosystem, above-ground vegetation was a
sensitive indicator of climate change (Zhou et al., 2020), and
prioritized the perception of climate change and adjusted its growth
characteristics to adapt to environmental changes. For example, in the
process of grassland desertification, grassland vegetation coverage,
biomass and diversity index decreases, and the height of the grass layer
increased (Fig.2), and plant species evolved from herbaceous plants to
lignified shrubs to reduce water evaporation and adapt to rising
temperatures, which was a survival strategy for plants (Zhou et al.,
2020). When vegetation changed, soil, as a substrate and nutrient donor
for plants, would also change, such as physical structure (increased
bulk density, decreased porosity), decreased chemical nutrients (carbon,
nitrogen, and phosphorus), and decreased soil enzyme activities (Fig2)
(Wang et al., 2018). Meanwhile, changes in soil physical structure,
nutrient content and enzyme activity will adversely affect plants and
affect their growth. The ''coordination'' and ''antagonism'' of
grassland plants and soil promote the flow of material and energy in the
grassland ecosystem to adapt to changes in the environment (He et al.,
2021). The results could provide theoretical basis and data support for
curbing grassland desertification and inducing forward succession of
desert grassland.

\textbf{4 Conclusion}

The research used the method of replacing space with time, and analyzed
the change characteristics of vegetation and soil nutrients in
succession of grassland desertification found that, grassland vegetation
and soil had undergone significant changes, especially total coverage,
AGB, Shannon-Weiner index, soil bulk density, soil organic matter and
total nitrogen. Based on N/P, the nitrogen was the main factor limiting
the productivity of grassland, and the limitation was more prominent in
the process of grassland desertification. The results could provide
theoretical basis and data support for curbing grassland desertification
and inducing forward succession of desert grassland.

\textbf{Conflicts of interest}

The authors declare no conflicts of interest.

\textbf{Acknowledgements}

This study was supported by National Natural Science Foundation of China
(31160475; 61401439); A new round of grassland Reward and subsidy
Benefit evaluation and grassland ecological evaluation in Gansu Province
(XZ20191225).

\textbf{Data availability statement}

The dataset that associated with this study is available. When the paper
is accepted for publication, we store the data to the specified platform
according to the requirements of the journal.

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\end{enumerate}

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