Responses of plant biomass allocation and carbon storage characteristics to altitude gradient in alpine peat bogs

doi:10.21203/rs.3.rs-3244669/v1

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Responses of plant biomass allocation and carbon storage characteristics to altitude gradient in alpine peat bogs

https://doi.org/10.21203/rs.3.rs-3244669/v1

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The change in hydrothermal conditions caused by altitude gradient will affect plant growth. The study of plant biomass and carbon storage in peat bogs at different elevations is of great significance for further understanding plant tolerance to habitat stress and the uncertainty of plant carbon sinks. According to the distribution characteristics of peat bogs in Zoige Plateau, 3400–3800 m peat bogs in the Shouqu Nature Reserve of the Yellow River were selected as experimental samples. The characteristics of plant biomass allocation, carbon storage, and their main influencing factors were studied by single-factor analysis and path analtysis. The results showed that (1) The biomass distribution ratio of plants in peat bogs was root > leaf > stem, and the subsurface biomass of vegetation was higher than that of above-ground biomass. With the increase in altitude, the aboveground biomass decreased, the total biomass and underground biomass increased first and then decreased, and the root-shoot ratio increased. (2) The organic carbon content of plants in peat bogs was higher in stem > leave > root. The underground carbon storage of plants was higher than that of above-ground carbon storage, and the total carbon storage decreased with the increase in altitude. (3) Path analysis showed that AD, SWC, and TS had direct positive effects on plant biomass and carbon storage, while DEP and pH had direct negative effects on plant biomass and carbon storage. The biomass allocation patterns and carbon storage characteristics of plants in peat bogs reflect the adaptation rules of plants in heterogeneous habitats. It is of great theoretical and practical value to understand the environmental response mechanism of plants in peat bogs under the global climate background and to evaluate wetland plants carbon sink.

Alpine peat swamp

biomass allocation

plant carbon storage

path analysis

Yellow River Shouqu Nature Reserve

Peat bogs are mainly distributed in alpine areas, with the special anaerobic environment of water saturation, high community productivity, difficult decomposition of organic matter, and easy accumulation of peat layer, which are important carbon pools in terrestrial ecosystems (Luca, et al., 2006; Dyukarev et al., 2021). Plants are the main source of organic carbon in peat bogs (Julie, et al., 2010; Askari et al., 2022). On the one hand, plants fix CO₂ in the atmosphere through photosynthesis, which has a strong capacity for carbon fixation and carbon storage (Elmore, et al., 2016). On the other hand, plants affect the input of organic carbon in peat bogs by regulating aboveground biomass distribution, root deposition, and the quantity and quality of root litter. The distribution of biomass and carbon content among different organs is the core problem of plant ecology (Enquist, et al.,2002; Poorter et al.,2012). Biomass refers to the total amount of organic matter accumulated by an individual or community some time and is an important indicator of energy acquisition by an ecosystem (Poorter et al.,2012). Among the organs of plants, leaves are important organs for plants to absorb light for photosynthesis and fix carbon. Stems mainly provide mechanical support, but also an important channel for plants to transport water and nutrients. Roots are the organs through which plants absorb water and nutrients. The size (or proportion) of roots, stems, and leaves relative to biomass is called biomass allocation (Yin et al.,2019; Zhang et al., 2017). Plants absorb C0₂ in the atmosphere through photosynthesis and transport the carbon fixed by photosynthesis to various parts of the plant, and enter the soil in the form of organic matter such as root exudates, dead roots, and crop residues (Zheng et al., 2007). Plant organic carbon content reflects the ability of plants to fix and store carbon elements during photosynthesis, and is a measure of plant carbon storage (Ma et al., 2018). The distribution and accumulation of plant biomass and organic carbon not only reflect the growth and metabolism of plants and ecological strategies but also reflect the investment of photosynthetic products in different organs of plants (roots, stems, and leaves) to adapt to environmental conditions (Li et al., 2022;Hecht et al., 2019). Studying the biomass allocation and carbon storage characteristics of plants in peat bogs not only can master the life history strategies of plants but also has important significance for understanding the uncertainty of carbon sinks in peat bogs.

Terrain, as an important factor in the natural ecological environment, has a significant impact on surface material migration and energy conversion(John et al., 2023). As an important topographic factor, elevation affects the spatial redistribution of temperature and precipitation, creates a heterogeneous community environment, and has an impact on plant morphological characteristics and biomass allocation (Davidson et al., 2019). As an important topographic factor, elevation is the main factor controlling the distribution characteristics of water, heat and plants in mountains (Zhao et al., 2022). Abiotic factors such as soil temperature, water content and soil nutrient elements have obvious spatial heterogeneity on the altitude gradient, which affects the growth and development of plants and the distribution pattern of biomass (Prather et al., 2020; Du et al., 2020). As the elevation increases, surface water, soil temperature and resource availability of peat bogs decrease(Ma et al., 2010). To resist the environmental stress of high cold, plants improve their photosynthetic capacity, ensure the balance between water supply and demand, and maintain their growth and development by adjusting their morphological and structural characteristics and optimizing the above-ground and underground biomass allocation patterns (Liu et al., 2021). This strategy is an important embodiment of the ecological adaptation strategies of plants to environmental heterogeneity stress (Freschet et al., 2018). At present, domestic and foreign scholars have studied the biomass allocation model of dominant species under drought stress(Eziz et al., 2017; Wilschut et al., 2021), flooded habitat (Li et al., 2021), grazing (Wei et al., 2022), soil heterogeneity and other conditions (Poorter et al., 2015; Reich et al., 2014). However, studies on plant biomass allocation in peat bogs from the perspective of the plant community are relatively few, which cannot elucidate the adaptive strategies of plants in alpine peat bogs to the heterogeneous habitats on the altitude gradient. Most studies on plant carbon storage are based on remote sensing and carbon conversion coefficient to estimate ecosystem carbon storage. However, the carbon conversion coefficients of different plants are different, and the carbon conversion coefficients of above-ground parts and underground parts are different, which will lead to the deviation of the estimation in plant carbon storage (Gao et al., 2007). Therefore, based on the measured method, the biomass allocation model and carbon storage characteristics of plants in peat bogs at different elevations were quantitatively studied, and the influencing factors were discussed. It is of great significance for understanding plant adaptation and estimating carbon storage in the peat ecosystem.

The unique cold and wet climate and complex topographic conditions of the Gannan Plateau have formed a special pattern of water and heat. Due to poor surface drainage, excessive wetting, or long-term water accumulation, plant residues are not easy to decompose, which provides favorable conditions for the development of peat bogs (Pu et al., 2020; Loisel et al., 2017). Maqu is the largest, most primitive, and most representative alpine peat swamp wetland in Gannan Plateau. The landform of the peat bogs in the Huanghe Shouqu Nature Reserve is typical, with a variety of peat flora such as sedge, Gramineae and Asteraceae, and plant species diversity (Sun et al., 2017; Zhang et al., 2023). It is an ideal area to study the characteristics of plant biomass, carbon storage and environmental response mechanism of the peat bogs. At present, the researches on Maqu peat swamp at home and abroad mainly focus on wetland dynamic change (Zhang et al., 2022), plant photosynthetic physiology and leaf morphological characteristics (Li et al., 2015), community structure and plant diversity (Xiang et al.,2009; Ma et al., 2011). However, there are few studies on the response of plant biomass allocation and carbon storage characteristics in peat bogs. Thus, in view of this, 3400 m−3800 m peat bogs in the Huanghe Shouqu Nature Reserve of Maqu were selected as the test site. The biomass distribution and carbon storage characteristics of plants in peat bogs at different elevations were studied. Pearson correlation and path analysis were used to explore the main influencing factors of plant biomass and carbon storage.

We hypothesize that the allocation of plant biomass and carbon storage are affected by environmental conditions. We also hypothesized that biomass and organic carbon content of plant roots, stems and leaves at different altitudes were different, which further led to differences in plant carbon storage. Therefore, the aim of this study was to address the following questions from a community level perspective. In this study, we attempted to clarify: (1) What is the research strategy for plant biomass allocation in peat bogs under an elevation gradient? (2) Differences in plant organic carbon content and carbon storage in peat bogs with different elevation gradients? (3) What are the main factors affecting plant biomass and carbon storage in peat bogs?. The aim is to reveal the differentiation of plant biomass and carbon storage in peat bogs, To provide data support for the correct assessment of carbon pool in high peat bogs ecosystems.

2.1. Study sites

The Yellow River Shou Qu Wetland National Nature Reserve is located southwest of Maqu, Gannan Tibetan Autonomous Prefecture, Gansu Province. With geographical coordinates between E101°54′12″-E102°28′45″ and N33°20′01″-N33°56′31″, the protected area is 203,400 hectares. In the study area, winter is long and cold, the warm season is short, the annual temperature difference is small, and the daily temperature difference is wide. The average annual temperature is between 1.1–1.5℃, the extreme maximum temperature is 23.6℃, and the extreme minimum temperature is -29.6℃. The average annual precipitation is 635–655 mm, and the average annual evaporation is 1353.4 mm. The annual average sunshine duration is about 2580 hours, the annual average frost-free period is 19 days, and the soil continues to freeze for 190 days. The main soil types in the study area are meadow soil, swamp soil and peat soil. The marsh vegetation is mainly herbaceous plants of the sedge family, and the main vegetation species are Blysmus sinocompressus, Kobresia tibetica, Carex muliensis, Carex kansuensis Nelmes, Ranunculus japonicus Thunb, Potentilla chinensis Ser, Centella asiatica(L.) Urban, Deschampsia, Pedicularis chinensis and Polygonum viviparum.

2.2 Experimental Setup

Based on Google Maps and combined with Gaofen-1 remote sensing image data to identify peat bogs, and after several field investigations and accessibility of peat bogs in the Huanghe Shougu Nature Reserve, the distribution area of peat bogs with typical development and weak disturbance in Manzhima Township of Maqu County was selected as the research object. At the altitude of 3400–3800 m, gentle slope areas with more consistent slope and slope direction were selected to set a sample site every 100 m (Fig. 1). Each sample site was set with 20 1×1 m quadrates, totaling 100 quadrates. The altitude, latitude, and longitude of the sample site were measured by portable handheld GPS (MG838, UniStrong). The study area is mainly winter pasture, and the plant growth is good during the experiment, which can eliminate grazing interference factors, and the experimental data are comparable.

2.3 Community investigation and sample collection

The wetland community survey and soil sample collection were conducted from August 12 to 25, 2022, when the weather was clear and the soil condition was relatively stable. The sampling process is as follows: (1) community learning. according to the Flora of China "and" the plant list "(http://www.theplantlist.org/) for some of each sample to confirm the name and all species records, statistical samples of plant coverage, height, and density. (2) Collection of plant samples. After the community investigation was completed, the above-ground parts of plants in the sample plots were cut with the harvesting method, and impurities such as surface adhesion soil and gravel were removed, sealed into envelopes, and brought back to the laboratory. A soil block of 30 cm (length) ×30 cm (width) ×30 cm (height) was dug in each sample square (because plant roots of the wetland community were almost distributed in this soil layer), and the attached soil was washed by a mesh screen (aperture = 0.25 mm) near the river, and all the plant roots were put into an envelope and brought back to the laboratory. (3) Soil sample collection. 200g of 0-30cm soil sample was taken from the central area of each sample square and brought back to the laboratory for the determination of the soil's physical and chemical properties. A ring knife (100 cm³) was used to collect soil samples (0–30 cm), and the recorded data were weighed immediately on-site and brought back to the laboratory for the determination of soil bulk density. Moisture MeterHH2, Delta-T Devices Ltd., Britain was used to measure soil temperature in a stratified field. When the water level is stable, record the water table value (“+” for water above the surface and “-” for water below the surface). After soil sampling, the field soil profile was restored to its original state.

2.4 Analysis of laboratory experiment data

In the Ecohydrology Laboratory of Northwest Normal University, the above-ground part (divided into stems and leaves) and the underground part (roots) of the plant community collected in the field were first put into numbered envelopes. Then the green is removed at 105 ℃ for 30 min and then placed in an oven at 85 ℃ for 24 h to constant weight. Finally, the stem biomass, leaf biomass, and root biomass of the plant community were recorded by weighing with an electronic balance (0.0001 g). After weighing, the plants were crushed and sifted with a plant crusher (40 mesh), and the organic carbon content of the roots, stems, and leaves of the plants was analyzed with a carbon/nitrogen analyzer (Vario EL III, Germany).

Due to the excess moisture in the soil sample, the impurities in the soil were removed after drying for 1 week under cool and ventilated indoor conditions. Then put in the oven set at 70 ℃ constant temperature conditions for 48 hours until constant weight, physical grinding after 60 mesh sieve, divided into sealed bags for the determination of indicators. Soil pH value was determined by a PHS-3 pH meter (ELTA32, Mettler-Toledo, Germany). The content of total nitrogen in the soil was determined by the semi-micro Kjeldahl nitrogen determination method. Total phosphorus content in soil was determined by sulfate-perchloric acid deboiling and molybdenum-antimony resistance colorimetric method (Bao Shidan, 2018). Soil water content and soil bulk density were measured by the aluminum box method.

Table 1

Common parameters and their abbreviations
Parameter	Abbreviations	Units	Parameter	Abbreviations	Units
Leaf biomass	LB	g/m²	Leaf organic carbon	LCC	g·kg^-1
Stem biomass	SB	g/m²	Stem organic carbon	SCC	g·kg^-1
Root biomass	RB	g/m²	Root organic carbon	RCC	g·kg^-1
Aboveground biomass	AB	g/m²	Aboveground carbon storage	ACS	gC•m^-2
underground biomass	UB	g/m²	Underground carbon storage	UCS	gC•m^-2
Total biomass	TB	g/m²	Total carbon storage	TCS	gC•m^-2
Root-shoot ratio	RST	No	Vegetation coverage	FVC	%
average height	AH	cm	average density	AD	Strain/m²
Soil bulk density	SBD	g/cm³	Soil moisture	SWC	%
Soil total nitrogen	TN	%	Soil temperature	TS	℃
Soil total phosphorus	TP	g·kg^-1	Groundwater depth	Dep	Dep

2.5 Data analysis methods

In path analysis, the correlation coefficient riy is decomposed into direct path coefficient (the direct effect of an independent variable on the dependent variable) and indirect path coefficient (the indirect effect of the independent variable on the dependent variable through other independent variables) based on multiple regression(Du et al.,2010). The specific calculation formula of path analysis is as follows:

r_iyp_jy=r_ij×p_jy

r_iy=p_iy+r_iཙp_jy

Where :r_iyp_jy is the indirect path coefficient between the independent variable x_i and the dependent variable y. r_ij is the correlation coefficient between two independent variables. r_iy is the path coefficient between the independent variable x_i and the dependent variable y. r_iy is the simple correlation coefficient between the independent variable x_i and the dependent variable y. P_jy is the direct path coefficient between the independent variable x_i and the dependent variable y.

The experimental data were analyzed by Microsoft Excel 2021. One-way ANOVA was used to compare the biomass and carbon storage characteristics of plants at different altitudes, and SPSS17.0 software was used to analyze the significance of data differences. The data in the chart are mean ± standard deviation using Origin2018 software.

3.1 Characteristics of plant communities in peat bogs

The average height, average density and average coverage of plant communities in peat bogs at different elevations had significant differences (Fig. 2, P < 0.05). The height and density of the plant community were the highest at the altitude of 3400 m, with values of 34.5 cm and 1226 branch /m², respectively. With the elevation increasing, the height, coverage and density of plant communities in peat bogs decreased by 49.67%, 23.46% and 23.78%, respectively.

3.2 Physical and chemical properties of peat bogs at different elevations

Table 2 shows that there are significant differences in the physical and chemical properties of peat bogs at different elevations (P < 0.05). Peat bog soils are neutral or acidic, with soil pH between 6.25 and 6.82. Soil pH and soil temperature showed a decreasing trend with the increase in altitude, which decreased by 8.35% and 16.70%, respectively. The soil moisture, soil total phosphorus and soil total nitrogen increased first, and then decreased with the elevation, and the change ranges were 35.6%, 9.6%, and 22.13%, respectively. The soil bulk density decreased first and then increased, and the variation range was 10.25%. The average buried depth of the groundwater level is between − 10 cm and 5.35 cm.

Table 2

Soil physical properties and buried depth of water level in peat bogs at different elevations
Altitude	SWC(%)	SBD(g/cm³)	ph	TS (℃)	TP (g·kg^− 1)	TN (%)	Dep(cm)
3400 m	125.95 ± 17.60	0.39 ± 0.01	6.82 ± 0.40	20.47 ± 3.17	0.77 ± 0.03	1.03 ± 0.08	5.35 ± 1.19
3500 m	135.05 ± 17.22	0.38 ± 0.01	6.50 ± 0.22	19.60 ± 2.11	0.79 ± 0.04	1.19 ± 0.07	3.85 ± 1.28
3600 m	129.95 ± 12.25	0.35 ± 0.02	6.46 ± 0.13	18.45 ± 2.68	0.83 ± 0.04	1.22 ± 0.08	-2.2 ± 1.65
3700 m	107.90 ± 17.27	0.37 ± 0.01	6.33 ± 0.32	17.86 ± 2.86	0.78 ± 0.03	1.14 ± 0.17	-7.6 ± 2.29
3800 m	87.25 ± 20.00	0.39 ± 0.02	6.25 ± 0.20	17.05 ± 2.84	0.75 ± 0.02	0.95 ± 0.14	-10 ± 1.84

Different lowercase letters in the same column indicate significant differences among plots (p < 0.05). SWC, SBD, TS....see Table 1.

3.3 Plant biomass characteristics of peat bogs at different elevations

3.3.1 Biomass distribution characteristics of rhizomes and leaves in peat bogs

There were differences in the biomass allocation of rhizomes and leaves in peat bogs at different elevations (Table 3). Biomass allocation ratio of plant organs Root > leaf > stem. The proportion of plant root biomass was the largest, the lowest at 3400 m (1076.30 ± 110.93 g/m²), and the highest at 3800 m (1208.95 ± 70.62 g/m²). The root biomass of plants increased with the elevation, and the increase was 12.26%. The biomass of leaves and stems above ground was the highest at 3400 m, with values of 252.88 ± 24.76 g/m² and 72.18 ± 26.99 g/m², respectively. Leaf and above-ground stem biomass of plants at 3800m were the smallest, and their values were 190.30 ± 24.73 g/m² and 59.81 ± 14.28 g/m², respectively. With the increase in altitude, the biomass of leaves and stems decreased by 24.75% and 18.25%, respectively.

Table 3

Root, stem and leaf biomass and its proportion in alpine peat bogs
Altitude(m)	Biomass (g/m²)			Biomass proportion (%)
Altitude(m)	Leaf	stems	root	Leaf	stems	root
3400	252.88 ± 24.76	72.182 ± 26.99	1076.30 ± 110.93	18.05	5.15	76.80
3500	243.04 ± 34.30	68.89 ± 19.37	1153.22 ± 88.50	16.59	4.70	78.71
3600	222.38 ± 19.98	65.75 ± 7.74	1183.11 ± 64.74	15.12	4.47	80.42
3700	208.77 ± 21.84	60.84 ± 11.62	1198.33 ± 82.19	14.22	4.14	81.63
3800	190.30 ± 24.73	59.81 ± 14.28	1208.95 ± 70.62	13.04	4.10	82.86

Different lowercase letters in the same column indicate significant differences among plots (p < 0.05). AD, AH, FVC....see Table 1.

3.3.2 Aboveground and underground biomass characteristics of plants in peat bogs at different elevations

There are differences in aboveground and subsurface biomass of plants in peat bogs at different elevations (Fig. 3). With the increase in altitude, the total plant biomass increased first and then decreased. The total plant biomass was the highest at 3600 m (1471.23 g/m²) and the lowest at 3400 m (1410.36 g/m²) (Fig. 3c). The aboveground biomass of plants decreased with the increase in altitude. The aboveground biomass of plants was the highest at 3400 m (325.06 g/m²), and the lowest at 3800 m (250.11 g/m²) (Fig. 3a). The underground biomass of plants increased with the increase in altitude (Fig. 3b). The root-to-shoot ratio was the smallest (3.31) at the altitude of 3400m and the largest (4.83) at the altitude of 3800 m. The root-to-shoot ratio showed an increasing trend with an increase of 45.92% (Fig. 3d).

3.4 Carbon storage characteristics of plants in peat bogs at different elevations

3.4.1 Characteristics of plant organic carbon content in peat bogs

There were differences in the organic carbon content of plant roots, stems and leaves at different elevations in peat bogs (Fig. 4). The organic carbon content of plant organs was in the order of stem > leaf > root in descending order. At the altitude of 3800 m, the organic carbon content of plant stems and leaves was the highest, and the values were 447.68 g/kg and 443.90 g/kg, respectively. At the altitude of 3400 m, the organic carbon content of plant stems and leaves in peat bogs was the lowest, and the values were 420.94 g/kg and 410.94 g/kg, respectively.

With the increase in altitude, the organic carbon content in stems and leaves increased by 6.47 and 8.03%, respectively. The organic carbon content of plant roots was relatively low, with the lowest (337.20 g/kg) at the altitude of 3800 m and the highest (368.99 g/kg) at the altitude of 3400 m. With the increase in altitude, the organic carbon content of plant roots showed a decreasing trend of 8.61%.

3.4.2 Above-ground and subsurface carbon storage characteristics of plants in peat bogs

There are differences in above-ground carbon storage of plants in peat bogs at different elevations (Fig. 5a,b,c). The total carbon storage of plants was the maximum (547.09 ± 37.33 gC•m^-2) at 3500 m and the minimum (518.36 ± 26.24 gC•m^-2) at 3800 m. With the increase of altitude, plant carbon storage showed a trend of first increasing and then decreasing, with a change range of 5.25% (Fig. 5a). Underground carbon storage of plants in peat bogs was higher than above-ground carbon storage at each elevation gradient. Plant underground carbon storage was highest at 3600 m (420.81 gC•m^-2) and lowest at 3400 m (397.00 gC•m^-2). With the increase of altitude, the underground carbon storage of plants increased first and then decreased, and the change amplitude was 5.99% (Fig. 5b). The mean above-ground carbon storage of plants was highest at 3400 m (136.86 gC•m^-2) and lowest at 3800 m (111.03 gC•m^-2). With the increase in altitude, the above-ground carbon storage of plants decreased by 23.29% (Fig. 5c).

3.5 Analysis of influencing factors of plant biomass and carbon storage in peat bogs

The normal distribution of TM and TCS, plant community characteristics (AH, FVC, AD), and soil environmental factors (TN, TP, SBD, SWC, pH, TS, Dep) were tested after dequantization. The results show that all of them conform to the normal distribution and can be analyzed by correlation and path analysis. Figure 6 shows the correlation between plant biomass, carbon storage, plant community characteristics and soil environment obtained by using the Pearson correlation analysis method. The characteristics of the plant community (AH, FVC, and AD) were positively correlated with total biomass (TM), and total carbon storage (TCS) (P ≤ 0.05). The correlation coefficient between AD and biomass and carbon storage was the highest (R² = 0.37 and R² = 0.51, P ≤ 0.01). There was a significant positive correlation between TS, TN, plant biomass (TM) and carbon storage (TCS) (P ≤ 0.01). SWC and TP were positively correlated with plant biomass and carbon storage (P ≤ 0.05). Dep was significantly negative with plant biomass (R²=-0.25, P ≤ 0.05). SBD and pH were negatively correlated with plant biomass and plant carbon storage, but the correlation coefficient was low.

Since there is a strong correlation between soil factors and the variation range of each soil factor is different, collinearity may occur in regression analysis. Therefore, in this paper, variance expansion factor (VIF) and tolerance (Tol) are used for multicollinearity diagnosis. As can be seen from Table 3, independent variables such as Dep, TS, SWC, AD and pH had significant effects on the dependent variables (plant biomass and carbon storage) (P < 0.05), while other environmental factors had no significant effects on the dependent variables (P > 0.05) and were excluded in the step-based regression. When VIF > 10 and Tol < 0.10, it indicates that there is serious multicollinearity among independent variables. According to the regression coefficients of plant biomass (TM) and carbon storage (TCS), VIF < 10 and Tol > 0.10 are found, so there is no multicollinearity problem among the selected environmental factors (Table 4).

Multiple regression equations of environmental factors and plant biomass were established through stepwise regression analysis:

TM = 1605.67-45.02pH + 13.62TS-3.24Dep + 0.2AD

TCS = 682.14-54.34pH + 3.46TS + 0.33SWC + 0.09AD

Table 4

Output results of regression coefficients of plant biomass (TM) and carbon stock (TCS)
Item	index	Unnormalized coefficient		Standardization coefficient	t	P	Collinearity statistics
Item	index	B	Std	Beta	t	P	allowance	VIF
TM	constant	1605.67	154.64		10.38	0.00
	TS	13.62	2.26	0.49	6.03	0.00	0.77	1.29
	Dep	-3.24	0.69	-0.36	-4.70	0.00	0.85	1.18
	AD	0.20	0.05	0.38	4.40	0.01	0.69	1.45
	pH	-97.08	25.24	-0.31	-3.85	0.00	0.78	1.28
TCS	constant	682.14	64.92		10.51	0.00
	AD	0.09	0.02	0.41	4.13	0.00	0.39	0.31
	pH	-54.34	10.92	-0.41	-4.98	0.00	-0.46	-0.37
	TS	3.46	1.03	0.29	3.35	0.00	0.33	0.25
	SWC	0.33	0.15	0.20	2.11	0.04	0.21	0.16
Note: B stands for regression coefficient; Std is a significance test for regression coefficients (p < 0.05). TM. TCS. see Table 1.

Stepwise regression analysis can not intuitively reflect the contribution of each environmental factor to plant biomass and carbon storage. The path coefficient was calculated by the method of the standardized regression coefficient, and the correlation coefficient was decomposed into direct path coefficient and indirect path coefficient, which could directly reflect the influence of various environmental factors on plant biomass and carbon storage(Wang et al., 2014; Song et al., 2016)。

Table 5 showed that the correlation coefficients between plant biomass, plant community characteristics, and soil environmental factors are TS > AD > DEP > PH in order from large to small. The main contribution of AD and TS to plant biomass is the direct positive effect (the direct path coefficient is greater than connecting path coefficient between them). The main contribution of pH and DEP to plant biomass was directly negative. The correlation coefficients of plant carbon storage with plant community characteristics and soil environmental factors were AD > SWC > TS > PH in order from large to small. The main contribution of AD, SWC and TS to plant carbon storage is the direct positive effect (direct path coefficient is greater than connecting path coefficient), while the main contribution of pH to plant carbon storage is the direct negative effect.

Table 5

Decomposition of simple correlation coefficients of plant biomass, carbon storage and soil factors in peat bogs
Item	index	Correlation coefficient	Direct path coefficient	Indirect path coefficient
Item	index	Correlation coefficient	Direct path coefficient	AD	SWC	pH	TS	Dep	合计
TM	AD	0.37	0.38			-0.12	0.22	-0.11	-0.01
	pH	-0.15	-0.31	0.15		0.00	0.13	-0.13	0.16
	TS	0.50	0.49	0.17		-0.08	0.00	-0.08	0.01
	Dep	-0.25	-0.36	0.11		-0.11	0.11	0.00	0.12
TCS	AD	0.51	0.41		0.12	-0.16	0.13		0.10
	SWC	0.46	0.20	0.25		-0.13	0.13		0.26
	pH	-0.10	-0.41	0.17	0.06		0.08		0.31
	TS	0.45	0.29	0.19	0.09	-0.11			0.17

Different lowercase letters in the same column indicate significant differences among plots (p < 0.05). TM.TCS .AD....see Table 1.

4.1 Differences in plant biomass distribution in peat bogs at different elevations

The cold and high-humidity environment of alpine peat bogs directly or indirectly determines the growth and distribution pattern of vegetation communities in peat bogs. Optimal allocation theory and growth balance hypothesis suggest that plants tend to preferentially allocate resources to organs with access to limited resources to obtain more limited resources and thus maintain a maximum growth rate (Mccarthy et al., 2007). In this study, it was found that the biomass of plant organs in peat bogs was root > leaf > stem, and the underground biomass was greater than the above-ground biomass (Fig. 3, Table 3). This is mainly because the plants in the peat swamp are mainly perennial sedge plants, and rhizoid plants such as Blysmus sinocompressus, Kobresia tibetica, Carex muliensis are the main ones, forming a single-plant community with absolute superiority. The vegetative propagation and competitiveness of plants are strong, and the roots of plants are developed, spread laterally, carry out vegetative propagation, and have strong self-propagation ability, forming a dense cluster root structure (Ji et al., 2006). In the process of plant growth, the need for low-ground stem and leaf input can meet the basic light and physiological needs. To improve the water and nutrient absorption and storage system of roots, plants invest more photosynthetic products in the construction of underground roots. This is consistent with Luo's research conclusion, the underground biomass of plants (roots) is higher than the above-ground biomass (leaves and stems)(Luo et al., 2012).

Biomass is the main embodiment of plant tissues and organs to obtain energy from the ecosystem, reflecting the accumulation of plant materials and the utilization of environmental resources (Hovenden et al., 2014). The biomass distribution patterns at different altitudes reflected the adaptability of plant organs and tissues to the environment. It was found that with the increase in altitude, the total biomass of plants increased first and then decreased, the underground biomass and root/shoot ratio increased, and the above-ground biomass decreased (Fig. 3). This is mainly influenced by the heterogeneous habitat created by the redistribution of water and heat conditions on the altitude gradient. At an altitude of 3400 m, the peat bog has higher soil temperature, large surface water, and higher soil moisture (Table 2). The decomposition rate of organic matter in the soil surface is accelerated, which provides sufficient hydrothermal conditions for plant growth and promotes the growth and development of plants in the peat bog. Plant cluster distribution in this region is highly dominant, with strong neighbor interference and interspecific competition among plants. Plants perform photosynthesis by increasing plant height and leaf area, so more resources are invested in the aboveground part (stem and leaf growth), and the aboveground biomass of plants is higher(Fig. 2, Fig. 3). With the increase in altitude, soil temperature decreases, and the thickening of permafrost blocks the hydraulic connection between atmospheric precipitation, surface water and groundwater (Zhao et al., 2019). As a result, surface water area decreases, soil moisture content decreases, humus decomposition in the soil is slow, and the effective fertility of soil total nitrogen and total phosphorus decreases (Table 2). The height, density and above-ground biomass of the plant community gradually decrease (Fig. 2, Fig. 3). The aboveground biomass of plants was lower in the altitude area of 3800 m, and the ratio of underground biomass to root shoot was the largest. This is mainly due to the strong daily freeze-thaw cycle in the high altitude area, the formation of clumps and ridge network grass mounds on the frozen ground, the reduction of surface water, and the lower soil moisture content and soil temperature. The low-temperature xerification environment restricts the growth of above-ground parts (leaves and stems) of plants in peat bogs, and plants invest more photosynthetic products in the construction of underground roots and rhizomes to improve their viability. By absorbing more water and nutrients, plant roots can improve the efficiency of nutrient utilization, so as to adapt to extreme environments such as high mountain wind, low temperature and poor soil, so as to better complete reproductive growth (Luo et al., 2012). This is consistent with Jackson's findings (Jackson et al., 1996). Plants at high altitudes improve root-shoot ratio by reducing leaves, above-ground stems and increasing roots (especially < 2 mm thin roots), which is a result of the adaptive evolution of plants in alpine peat bogs to low soil nutrient supply in low-temperature habitats.

4.2 Response of plant organic carbon content and carbon storage characteristics to altitude in peat bogs

In the process of plant growth, leaf stomata absorb carbon dioxide and fix it into organic compounds through photosynthesis, thus realizing the process of carbon sequestration. The difference in organic carbon content in plant organs comes from their own functions and physiological characteristics. It was found that the average organic carbon content of each organ of plants in alpine peat bogs was in descending order from the upper stem (444 g/kg) > leaves (431 g/kg) > roots (365 g/kg)(Fig. 4), which was consistent with the results of Zheng (Zheng et al., 2007). Plant stems contain more lignin, and the carbon content of lignin is higher, so the carbon content of stems is higher than that of the leaves. The leaves contain abundant chloroplasts and a lot of organic matter, so the carbon content is higher than that of the roots, which mainly exchange water and inorganic salts. Therefore, the carbon content of the stem (ear, flower) was the highest, and the carbon content of the root was the lowest. In addition, the plants in the peat swamp in the study area were mainly sedge plants such as Blysmus sinocompressus, Kobresia tibetica, Carex muliensis. The dry mass and leaf area index of the plants were large, and the plants had strong photosynthetic effects and showed strong photosynthetic carbon fixation ability, which was conducive to the accumulation of vegetation carbon. Therefore, the organic carbon content of the above-ground part (stems and leaves) of wetland plants is greater than that of the underground part (roots), which is similar to the results of Xie (Xie et al., 2015).

Plant organic carbon content reflects the ability of plants to fix and store carbon elements during photosynthesis, and is a measure of plant carbon storage. Plant carbon storage is the result of the interaction between plant biomass and plant carbon content. The results showed that above-ground carbon storage was lower than underground carbon storage, and the total carbon storage increased first and then decreased with the increase in altitude. The highest carbon storage of plants was found at the altitude of 3600 m(Fig. 5), mainly because suitable temperature and soil water conditions were conducive to the photosynthesis of plants, thus effectively improving net primary productivity. Therefore, the total biomass of plants reached the maximum and the carbon storage of plants was the largest. Soil temperature can affect various physiological processes such as organic synthesis, transpiration, respiration, and photosynthesis in plants. Soil temperature has a significant positive correlation with plant carbon storage (P ≤ 0.01) (Fig. 6). As the altitude increases, soil temperature decreases, soil nutrients, water, and other resources decrease (Table 2), the available energy of plants decreases, and the low-temperature environment restricts the growth of above-ground parts of plants (leaves and stems). As plant height and biomass decrease (Fig. 2 and Fig. 3), self-shading is enhanced as a result of plant height decrease, plant photosynthesis decreases, and plant biomass and carbon sequestration capacity decline, resulting in the lowest carbon storage of plants in peat bogs at an altitude of 3800 m. Therefore, the "low-high-low" pattern of plant carbon storage in peat bogs presents with the elevation increase. This is consistent with Xiao's research (Xiao et al., 2021). At each elevation gradient, the underground biomass of plants was higher than that of underground biomass, and the loss rate of underground biomass was relatively low (Table 3, Fig. 3). This is conducive to the accumulation of root carbon storage, so the underground carbon storage of plants is higher than the above-ground carbon storage. This is consistent with the conclusion of Wu (Wu et al., 2013). Ceiling and Steve et al. found that C₄ plants need to consume additional energy both to achieve a relative increase in carbon sequestration efficiency at high altitudes and low CO₂ partial pressure conditions and to achieve growth at lower temperatures (Cerling et al., 1997;Terrer, et al., 2021). Especially at low temperatures, C₄ plants need to consume a lot of light energy for photosynthesis. The Gannan Plateau has strong sunlight, long sunshine time, and large total radiation amount, which provides a large amount of available energy for plants to grow at high altitudes and low CO₂ partial pressure and low-temperature conditions, making up for the shortage of energy required for photosynthesis in a low-temperature environment. This led to an increase in the relative carbon sequestration efficiency and low-temperature tolerance of plants in high-altitude peat bogs (Youkhana et al., 2017).

Under the climate background of global warming, the unique topographic and hydrological conditions of the Maqu Peat swamp play an important role in regional climate regulation and ecological diversity protection. The biomass distribution pattern and carbon storage characteristics of plants at altitude gradient reflect the adaptability of plant organs and organs to the alpine environment. It was found that the biomass distribution ratio of plants in peat bogs was from large to small as roots > leaves > upper stems. The organic carbon content of plants was from large to small in the upper stems > leaves > roots. The underground biomass and carbon storage of plants in peat bogs were higher than those above ground. With the increase in altitude, the biomass and carbon storage of plants increased first and then decreased. Soil temperature and moisture are the most important environmental factors affecting plants in peat bogs, and have direct positive effects on plant biomass and carbon storage. In this paper, we analyzed the response of plant biomass and carbon storage allocation patterns to elevation only from the perspective of the community level. From the perspective of plant species, it is important to study the adaptation mechanism of plant species' morphological characteristics, biomass allocation, and carbon sequestration ability in heterogeneous habitats.

The manuscript has been approved by all authors. I would like to declare on behalf of my co-authors that the work described is original research that has not been published previously, and is not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the enclosed manuscript.

Funding

This work was supported, in part, National natural science foundation of China (Grant NO. 41461013 and NO. 41861009).

Declaration of interests

Declarations of interest: none. All authors have approved the manuscript. I declare on behalf of all authors that the work described is original research that has not been published previously, and is not under consideration for publication elsewhere, in whole or in part.

Availability of data and material

All study-related data has been provided in the supporting information

Authors contributions

Conceptualization: Man-Ping Kang.

Formal analysis: Man-Ping Kang.

Funding acquisition: Cheng Z Zhao.

Investigation: Man-Ping Kang

Methodology: Man-Ping Kang, Min Ma.

Resources: Cheng Z Zhao, Min Ma.

Supervision: Cheng Z Zhao,

Validation: Man-Ping Kang.

Writing – original draft: Man-Ping Kang.

Writing – review & editing: Cheng Z Zhao.

Acknowledgements

We thank Robert McKenzie, PhD, from Liwen Bianji, Edanz Group China (www.liwenbianji. cn/ac), for editing the English text of a draft of this manuscript. Besides, we are very grateful to Xinyu Ma, Suhong Wang, Peixian Zhang, DawWei Wang, Xiawei Zhao, Xuqian Bai and Zhiwei Zhang for their assistance in the feld and laboratory work.

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Responses of plant biomass allocation and carbon storage characteristics to altitude gradient in alpine peat bogs

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Abstract

Figures

1 Introduction

2 Materials and methods

2.1. Study sites

2.2 Experimental Setup

2.3 Community investigation and sample collection

2.4 Analysis of laboratory experiment data

2.5 Data analysis methods

3 Result Analysis

3.1 Characteristics of plant communities in peat bogs

3.2 Physical and chemical properties of peat bogs at different elevations

3.3 Plant biomass characteristics of peat bogs at different elevations

3.3.1 Biomass distribution characteristics of rhizomes and leaves in peat bogs

3.3.2 Aboveground and underground biomass characteristics of plants in peat bogs at different elevations

3.4 Carbon storage characteristics of plants in peat bogs at different elevations

3.4.1 Characteristics of plant organic carbon content in peat bogs

3.4.2 Above-ground and subsurface carbon storage characteristics of plants in peat bogs

3.5 Analysis of influencing factors of plant biomass and carbon storage in peat bogs

4 Discussion

4.1 Differences in plant biomass distribution in peat bogs at different elevations

4.2 Response of plant organic carbon content and carbon storage characteristics to altitude in peat bogs

5 Conclusion

Declarations

References

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