Introduction:
As an important part of the carbon pool of terrestrial ecosystems, soil
organic carbon affects the productivity of ecosystems and greatly
impacts the Earth’s carbon cycle and global climate change. It is not
only the ”source” but also an important ”sink” of greenhouse gases
(Cheng et al. 2023). The SOC pool is the largest carbon pool in
terrestrial ecosystems (Bai. 2018), approximately three times that of
vegetation and twice that of atmospheric carbon (Lal. 2004). Global
climate change has a robust effect on soil organic carbon (SOC) (Chen et
al. 2019; Li 2009). In the context of global change, small changes in
soil carbon pools can cause large changes in atmospheric carbon dioxide
concentration (Plaza et al. 2019). Changes in SOC dynamics can alter the
ecosystem’s carbon balance and determine whether soils are carbon
sources or sink at regional and global scales (Román et al. 2022).
Therefore, research was carried out on the changes in the
characteristics of SOC and soil organic carbon density (SOCD) and their
response to influencing factors. Understanding the carbon cycle in
grassland ecosystems and managing ecosystem carbon sinks is significant
(Breulmann et al. 2016).
China’s total grassland carbon pool was 79.24 ± 2.42 Pg C, of which
82.9% was stored in the soil, 16.5% in biomass, and 0.60% in litter
(Fang et al. 2010). The SOC content of China’s coastline ranges from
0.63 to 36.7 g/kg (Fang et al. 2010). The SOC content in the alpine
meadow is the largest, which is 51.54 g/kg, and the desert grassland
ecosystem is the smallest, with a value of 4.96 g/kg (Chen et al. 2019).
A study of different swamp wetland types in an arid area between the
Altai Jinshan and Qilian Mountains found seasonal saltwater swamp
> inland salt swamp > herb swape (Chen. 2020).
The SOCD of Leymus chinensis formations in Inner Mongolia is
higher than that of Stipa grandis and S. sareptanaformations (Feng et al. 2019). The SOCD of the main soil types in Henan
Province is 6.64–7.77 kg·m-2(Yu et al. 2008).
SOC content is controlled by output and input and above- and
below-ground biomass. Litter and root exudates are the main sources of
SOC input (Wang et al. 2015). Plant roots, microbial respiration (Shi et
al. 2012), and the oxidation of carbon-containing minerals (Trumbore
2006) are the main sources of SOC output. The factors that affect these
two processes also influence changes in SOC. SOC content is mainly
affected by the combined effects of climate, soil, and vegetation on a
regional scale (Chen et al. 2019). At the regional scale of the Tibetan
Plateau, SOC content mainly depends on total nitrogen and altitude (Dai
et al. 2022). Studies have shown that environmental factors have a
dimensional
effect on SOC(O’Rourke et al. 2015). SOC content is mainly affected by
climatic factors on a large
scale(Wang
et al. 2009). Climate affects SOC by affecting vegetation (Bot et al.
2005). The main factors affecting SOC on a small scale are land-use
methods and management (Zhang et al. 2020). There is a negative
correlation between soil pH and SOC (Yan et al. 2020). Grazing is the
main reason for the increase in SOC in northern Jiangsu (Lu et al.
2020). As a nutrient necessary for vegetation growth, an increase in
nitrogen content can promote the accumulation of SOC (Qi 2017). The
surface SOC storage of a typical grassland in Inner Mongolia is affected
by factors such as the type of community vegetation, aboveground
biomass, and average leaf area of the plant community (Feng et al.
2019).
As an important part of the Qinghai-Tibet Plateau, Qinghai Province is
one of the most sensitive and vulnerable regions to global climate
change. Its warming rate is approximately twice that of the global
average. It profoundly impacts the carbon cycle of terrestrial
ecosystems and environmental climate change in China (Wang et al. 2012).
Therefore, clarifying the distribution characteristics of organic carbon
in Qinghai Province is important for determining the role of Qinghai
Province and the Qinghai-Tibet Plateau in the carbon cycle of China’s
terrestrial ecosystem and global environmental changes. Previous studies
on SOC in Qinghai Province have been limited to local areas or specific
vegetation types (Wang et al. 2019;Liu et al. 2016). Our research was
based on 25 county-level regions in Qinghai Province, with 131 sampling
sites in the 0‒30 cm soil layer, differences in SOC and SOCD changes in
different vegetation and soil types, and the adjustment effect of SOC on
certain environmental factors. The purpose of this study was to explore
the distribution characteristics of SOC and its influencing factors in
the northeastern Tibetan Plateau, and to provide a scientific basis for
revealing the grassland carbon sink management and ecosystem service
optimization on the Tibetan Plateau.
Materials and method
Site description
The study area covers the Qinghai Province area and includes multiple
types of natural ecosystems, climatic conditions, soil conditions, and
vegetation community composition. There was apparent spatial
differentiation. The average annual temperature in the study area ranges
from -11.47 to 12.87 ℃. The average annual precipitation is 47.38‒551.38
mm. We selected 131 sampling sites in 25 county-level regions of Qinghai
Province, including Guinan, Tongde, Zeku, Henan, Maqin, Gande, Dari,
Banma, Jiuzhi, Maduo, Tianjun, Dulan, Wulan, Gonghe, Xinghai, Chengduo,
Nangqian, Zaduo, Zhiduo, Qumalai, Golmud City, Gangcha, Haiyan, Qilian,
and Menyuan. They include steppe, meadow, desert, and swamp vegetation
(Fig. 1).
Field sampling and sample testing
A total of 131 samples were collected during the growing season from
July to September in 2011 and 2012. Stratified soil samples were
collected at depth increments of 0‒5, 5‒10, 10‒20, and 20‒30 cm, and
sampling from each site was repeated five times. The SOC was determined
using the dry burning method (PE2400) and repeated five times.
The aboveground biomass was collected using the standard harvest method
(sample area: 50×50 cm) and repeated five times for each plot.
Belowground biomass was measured using the root drilling method and
collected from 0‒5, 5‒10, 10‒20, and 20‒30 cm layers. Sampling from each
plot was repeated five times, and the roots were washed using the
water-washing method. The samples were dried at 65 ℃ to constant weight,
weighed, and converted to a standard unit (g·cm-3).
The total nitrogen content was measured using an elemental analyser
(PE-2400). pH was measured using a pH meter.
The root drilling method was used to collect soil bulk density samples
at 0‒5, 5‒10, 10‒20, and 20‒30 cm in selected plots, with five
replicates for each plot. The samples were baked to a constant weight at
105 °C in a blast drying box to obtain soil bulk density.
The soil organic carbon density (SOCD)was calculated as
\begin{equation}
\text{SOCD}=\sum_{i=1}^{n}{(1-\theta_{i})}\times T_{i}\times v_{i}\times C_{i}/10\nonumber \\
\end{equation}In the above formula, \(T_{i}\) is the thickness (cm) of layer i,\(v_{i}\) is the bulk density (g·cm-3) of layeri , \(C_{i}\) is the SOC content (%) in layer i , and\(\theta_{i}\ \)is the volumetric percentage of fragments >
2 mm. The \(C_{i}\) content was converted from the organic matter
concentration using a conversion coefficient of 0.58 since the organic
matter was determined by wet combustion with
Cr2O72−(ZHANG et al.
2022).
Data analysis
Data were organised using Excel 2010. The R programme (4.2.1) was used
for significance testing and path analysis, and the figure was drawn
using Origin 2022. The semi-variance function of the experimental data
and ordinary kriging interpolation was calculated using ArcGIS 10.2.
Results
Spatial distribution characteristics of SOCD
The SOCD in the northwest was lower than that in the southeast on a
horizontal scale in the Qinghai Province (Fig. 2), and the changing
trend was the same on a vertical scale. The SOCD in most areas of the
0‒5, 5‒10, 10‒20, and 20‒30 cm soil layers are 0.28–2.31, 0.53–2.61,
1.00–4.20, 0.64–4.60 kg·m-2. The highest values of
SOCD in the 0‒5, 5‒10, and 20‒30 cm soil layers appeared in Menyuan
County, with values of 6.70, 14.96 and 6.05 kg·m-2,
respectively. The highest SOCD value of 11.603 kg·m-2appears in Zhiduo County in the 10‒20 cm layer. The lowest values of
0.758, 0.980, 1.237, and 1.632 kg·m-2 appear in Golmud
City regardless of the soil layer. Generally, the SOCD of the deep soil
layer (10‒30 cm) was higher than that of the 0‒10 cm surface soil layer.
Vertical distribution characteristics of SOC and SOCD of different
vegetation types
The SOC and SOCD show the same trend from the perspective of vegetation
types and were in the order of swamp > meadow
> steppe > desert (Fig. 3). The SOC content
decreased vertically in the lower soil layers and was mainly
concentrated in the surface soil. The SOCD of the steppe, swamp, and
meadow vegetation types showed an upward trend in the 0‒20 cm soil
layer, reaching a maximum in the 10‒20 cm soil layer, and that of the
desert reached a maximum in the 20‒30 cm soil layer. The SOCD ranges of
0‒5, 5‒10, 10‒20 and 20‒30 cm layers are 0.41–11.00, 0.39–18.08,
0.98–33.41, 1.57–41.49 kg·m-2, respectively.
The average values were 2.58,
2.77, 5.05 and 4.42
kg·m-2.
Vertical distribution characteristics of SOC and SOCD in different
soil types
The SOC and SOCD were the highest in the meadow soil. SOC gradually
decreased with the depth of the soil layer, while the SOCD gradually
increased. The SOC of the
different soil types are in the following order: meadow soil
> bog soil >
chestnut soil >
saline-alkali soil >
alpine steppe soil >
grey-brown desert soil (Fig. 4).
The SOC of bog and chestnut soils was the highest at 5‒10 cm. Alpine
steppe soil had the highest SOC in the 10‒20 cm soil layer. The 20‒30 cm
soil layer had the lowest SOC. The SOCD values of the different soil
types were as follows: meadow soil > bog soil
> chestnut soil > saline-alkali soil
> grey-brown desert soil > alpine steppe soil.
The SOCD value at 10‒20 cm was the highest. The order of SOCD was as
follows: 10‒20 cm > 20‒30 cm > 0‒5 cm
> 5‒10 cm in the vertical distribution.
Regulation mechanism of SOC
The structural equation model (SEM) indicated that all predictor
variables together accounted for 31% of the variations in SOC at the
0–30 cm depth (R2 = 0.31, Fig. 5). Total nitrogen
(TN) and belowground biomass (BGB) had direct and significant (P< 0.01) positive effects on SOC, with values of 0.454 and
0.147, respectively. Above ground biomass (AGB) and Litter directly and
negatively impact SOC; the values are -0.043 and -0.050, respectively.
BGB also had a positive indirect effect on litter and TN. AGB had a
strong indirect negative effect on litter and BGB. pH had a strong
indirect impact on TN and BGB.
Discussion
Our research suggests that the SOCD mainly has a lower spatial
distribution in the northwest than in the southeast of Qinghai Province.
China’s average SOCD is 7.97 kg/m-2, with the highest
in the northeast, followed by the central region, and the lowest in the
dry areas of the northwest (YANG et al. 2023). Compared with other
regions in Qinghai Province, Golmud is arid and has less rainfall, with
a large temperature difference between day and night (WANG et al. 2019).
The ecological environment is fragile, and vegetation development is
poor (WANG et al. 2017), which is not conducive to the accumulation of
SOC. The replenishment of dead plants, litter, and secretions is mainly
concentrated in the surface soil, leading to SOC accumulation (WANG et
al. 2021).
In this study, the swamps had the highest SOC and SOCD values. SOCD is
determined by SOC and bulk density (BD) (ZHANG et al. 2020), causing the
soil litter layer and organic matter input from the soil layer to have
corresponding differences in vegetation, quantity, and chemical
properties due to vegetation types (YANO et al. 2005). The soil organic
carbon content of the alpine meadow in northern Xinjiang was higher than
that of the alpine steppe and increased to 161.32 kg/ha/yr (WANG et al.
2022). The average SOC at 1 m depth in the semi-arid grassland in Gansu
Province was 7.09 kgm-2(TIAN et al. 2016). The swamps
had the highest SOC contents in our study. This is because the stagnant
water in the swamp prevents adequate aeration, there are fewer aerobic
bacteria, the mineralisation process is weakened, the input of organic
matter is far greater than the output, and long-term accumulation
increases the total organic carbon content of the soil (DONG et al.
2019). Research on the soil organic carbon storage of different
vegetation types on the Qinghai-Tibet Plateau found that the soil
organic carbon storage of alpine meadows is higher than that of alpine
grasslands and deserts (CHEN 2016). This finding is consistent with the
results of the present study.
Differences in soil type also affect SOC content. The SOCD of different
soil types shows a certain distinction and similarity on the vertical
change scale. The difference in water and heat conditions on the
Qinghai-Tibet Plateau caused by topography and atmospheric circulation,
and the influence of fixed factors in the ecosystem, caused the SOCD of
the Qinghai-Tibet Plateau to decrease from south to north and east to
west (WANG et al. 2019). In this study, the meadow soil had the highest
SOC and SOCD, possibly due to the low temperature, high humidity, the
slow decomposition rate of humus, and the high accumulation rate of
organic matter in the meadow soil (WANG and ZHOU 1999).
In this study, TN and BGB had extremely significant direct and positive
effects on SOC, whereas pH, AGB, and litter had negative effects. Soil
nitrogen is important for soil life, respiration, and plant
photosynthesis (GUO et al. 2016). TN positively affected SOC, whereas pH
negatively affected SOC in a Stipa crustella grassland of Inner
Mongolia (ZHANG et al. 2020). Nitrogen addition can change the soil
microbial community structure (WANG et al. 2014), improve microbial
carbon use efficiency, weaken microbial respiration, and increase
organic carbon accumulation(WANG et al. 2016). Studies have shown that
the soil pH in Alxa grasslands significantly reduces SOC (FU et al.
2004). Microbial activities, which play an important role in the
mineralisation of SOC, are suppressed when the soil environment is
acidic (TANG 2019). High salinity reduces the normal growth of
vegetation and the SOC content when the pH value is higher than 10(CHEN
et al. 2019). It mainly affects SOC content by affecting the litter
decomposition rate (ZHAO et al. 2018).
Conclusion
On the horizontal scale, the soil organic carbon density in the
northeastern part of the Qinghai-Tibet Plateau was low in the northwest
and high in the southeast. The vertical distribution of soil organic
carbon and soil organic carbon density varied significantly among soil
and vegetation types. Total nitrogen and belowground biomass were the
main factors affecting soil organic carbon content in the northeastern
Tibetan Plateau.