1 Introduction
Climate change threatens natural ecosystems and socioeconomic systems, with carbon dioxide viewed as the main driving force
[1]. To promote ecological civilization construction and cope with global climate change, China first proposed the carbon peaking and carbon neutrality goals ("dual carbon" goals hereafter) in 2020
[2] and addressed to integrate it into the overall plan of ecological civilization construction in 2021
[3].
The achievement of "dual carbon" goals depends on both controlling carbon emissions and increasing carbon storage in ecosystems, and ecological restoration has been proven effective in enhancing ecosystem carbon sequestration
[4]~[6]. To increase carbon sinks via ecological restoration, it is essential to evaluate the current ones with scientific methods. At present, it is common to analyze the spatial pattern or conduct spatio-temporal evolution of carbon sinks based on remote sensing data
[7]. The measuring and calculating methods include the calculation of net ecosystem production (NEP)
[8], simulation with the InVEST Carbon model
[9], and evaluation with carbon sink coefficients
[10]. Based on their evaluation results, some scholars proposed strategies for zoning and managing ecological spaces
[11]. However, most existing research focuses on larger scales of national, regional, or provincial
[8][10][12], some focuses on cities or counties in developed regions
[11][13], while little studies counties in western China, particularly the arid areas of northwest China.
As for ecological restoration, existing research reviewed the impact factors of ecosystem carbon sequestration and the mechanism of carbon sink increase
[14]; analyzed the current situation of utilizing ecological restoration strategies to promote carbon sequestration and carbon sink
[15]; and explored approaches to restoring forest
[16], grassland
[17], cultivated land
[18], wetland
[19], desert
[20], and marine
[21] ecosystems based on field measurement, literature review, case study, and policy interpretation. Yet few studies proposed restoration strategies incorporating ecological spatial patterns.
Focusing on Wensu County in the Xinjiang Uygur Autonomous Region of China, this research evaluated current carbon sinks in the study area, identified spatial pattern of these carbon sinks, and developed ecological restoration strategies according to different land use spatial patterns. Unlike those in the developed Beijing–Tianjin–Hebei region and coastal southeast China, most counties in the arid areas of northwest China cover relatively larger areas with less development land and more ecologically vulnerable non-development land
[22][23], where ecological restoration becomes a pressing task. This research aims to put forward ecological restoration approaches to increasing carbon sinks in the counties in the arid areas of northwest China, promoting the integration of "dual carbon" goals in territorial spatial planning and providing a referable paradigm for areas with similar geographical conditions. This paper discusses the following questions.
1) What is the spatial pattern of carbon sinks in Wensu County?
2) What are the territorial ecological restoration approaches to increasing carbon sinks?
And 3) how do the findings of this research contribute to carbon sink management in the arid areas of northwest China and similar regions?
2 Overview of Study Area and Data
2.1 Study Area
Administered by Aksu Prefecture in the Xinjiang Uygur Autonomous Region, Wensu County (40°52'–42°21'N, 79°28'–81°28'E) is located at the south foot of the Tomur Peak in the middle of Tianshan Mountain and borders the north edge of the Tarim Basin. Covering a total area of 14,600 km2, Wensu County stretches 153 km from east to west and 167 km from north to south (Fig.1)①. With elevations ranging from 800 m to 8,000 m, this county is higher in the northern mountains and lower in the southern plains and hills. Wensu County has a temperate continental arid climate with little rainfall, high evaporation, abundant sunshine, an annual mean temperature of 10.10℃, annual mean precipitation of 65.4 mm, and annual mean frost-free period of 185 days.② The county has extensive natural areas indicating high carbon sink potential, with the major soil types being gypsisols, petrified gypsisols, and luvic calcisols; the primary vegetation types being desert and meadow; and the most common land use types being unused land, grassland, and farmland (Fig.1)③.
Fig.1 Elevation and current land use of the study area (data sources: elevation from Geospatial Data Cloud; satellite images from Bing Maps; and land use from the Resource and Environment Science and Data Center, Chinese Academy of Sciences). |
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① Part of Aral City is surrounded by Wensu County. Although not administered by Wensu County, this exclave was included in the study area for ecological integrity. Limited by availability of the data, the raster data of this research covers about 14,200 km2.
② Information about Wensu County was sourced from the official website of the Wensu Government.
③ This research adopted the land use classification system proposed in China's Multi-Period Land Use and Land Cover Remote Sensing Monitoring Dataset (CNLUCC) and related data sources from the Resource and Environment Science and Data Center, Chinese Academy of Sciences.
2.2 Data Sources and Preprocessing
The research evaluated the current carbon sinks in Wensu County and identified their spatial pattern based on multiple data sources. Considering the update frequency, availability, spatio-temporal synchronization, and data quality in recent years, the research team utilized the multi-source data of Wensu County from 2015 (Tab.1). All data were projected into the Xi'an Geodetic Coordinate System 1980 and transformed into raster layers (1-km resolution).
Tab.1 Research datasets and data sources |
| Dataset | Data type | Spatial resolution | Data source |
|---|
| Land use | Raster | 1 km | Resource and Environment Science and Data Center, Chinese Academy of Sciences |
| Net primary production (NPP) | Raster | 500 m | United States Geological Survey |
| Digital elevation model (DEM) | Raster | 30 m | Geospatial Data Cloud |
| Monthly mean temperature | Raster | 1 km | Resource and Environment Science and Data Center, Chinese Academy of Sciences |
| Monthly total precipitation | Raster | 1 km | Resource and Environment Science and Data Center, Chinese Academy of Sciences |
| Spatial distribution of population density | Raster | 1 km | WorldPop |
| Spatial distribution of vegetation types | Vector | 1∶1,000,000 | Resource and Environment Science and Data Center, Chinese Academy of Sciences |
| China's soil dataset from Harmonized World Soil Database (v 1.1) | Raster | 1 km | National Tibetan Plateau Data Center |
3 Research Methods
3.1 Research Framework
The research followed the framework of "current carbon sink evaluation–spatial pattern identification–restoration strategy development." The research team first evaluated the carbon sequestration, carbon storage, and importance level of the current carbon sinks in Wensu County, then identified the spatial patterns of these carbon sinks. Lastly, based on the above analyses and approaches to increasing carbon sequestration and carbon sinks in existing studies, the research team proposed ecological restoration strategies for local land use spatial patterns (Fig.2).
Fig.2 Research framework. |
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3.2 Current Carbon Sink Evaluation
Ecological restoration can disturb the land, affecting vegetation carbon sequestration and storage. Therefore, it is necessary to comprehensively assess the carbon sequestration capacity ("flow") and storage ("stock") patterns of current carbon sinks in Wensu County, thereby analyzing the spatial pattern of the carbon sink importance level. The evaluation tool adopted is ArcGIS 10.3.
3.2.1 Carbon Sequestration Capacity Evaluation
Carbon sequestration capacity was measured with NEP, which is the difference between the net primary production (NPP) and the soil heterotrophic respiration (
RH)
[24][25], i.e.,
where T is the monthly mean temperature (℃) and A is the monthly total precipitation (mm).
3.2.2 Carbon Storage Evaluation
Wensu County has a high soil carbonate content. The research team evaluated both soil and vegetation carbon storage. The soil carbon storage is composed of topsoil inorganic carbon (Pic), topsoil organic carbon (Poc), subsoil inorganic carbon (Sic), and subsoil organic carbon (Soc). Topsoil and subsoil are the soil within 30 cm depth and 30 ~ 100 cm depth, respectively. The total carbon storage of the soil within 100 cm depth (Csoil) can be calculated with Eq. (3):
As vegetation carbon density largely depends on vegetation types
[26], of which carbon densities were determined based on existing research and site conditions including location, elevation, and climatic characteristics
[27]~[29] (Tab.2, Fig.3).
Tab.2 Vegetation carbon density and carbon storage |
| Vegetation type | Vegetation subtype | Carbon density(t/hm2) | Area(×104 hm2) | Carbon storage(×104 t) |
|---|
| Coniferous forest | Cool temperate and temperate montane coniferous forest | 52.3 | 1.09 | 57.007 |
| Scrub | Subalpine deciduous broad-leaved scrub | 7.7 | 0.22 | 1.694 |
| Desert | Temperate shrub desert | 1.0 | 27.67 | 27.670 |
| Temperate steppe shrub desert | 1.0 | 0.80 | 0.800 |
| Temperate subshrub and dwarf subshrub desert | 1.0 | 13.49 | 13.490 |
| Temperate succulent halophyte dwarf subshrub desert | 1.0 | 4.16 | 4.160 |
| Steppe | Temperate bunch grass typical steppe | 2.1 | 9.05 | 19.005 |
| Temperate grass and forb meadow steppe | 2.1 | 2.11 | 4.431 |
| Temperate bunch short grass and dwarf subshrub desert steppe | 2.1 | 4.57 | 9.597 |
| Meadow | Temperate grass and forb halophyte meadow | 3.7 | 24.07 | 89.059 |
| Temperate grass, sedge, and forb marsh meadow | 3.9 | 0.07 | 0.273 |
| Alpine Kobresia and forb meadow | 1.8 | 3.50 | 6.300 |
| Alpine vegetation | Alpine cushion-like vegetation | 3.3 | 0.65 | 2.145 |
| Alpine talus vegetation | 3.3 | 18.20 | 60.060 |
| Cultivated vegetation | Two-year triple cropping or one-year double cropping dry farming and deciduous orchard | 5.7 | 11.46 | 65.322 |
| Non-vegetation area | | 0.0 | 21.04 | 0.000 |
| Total | | — | 142.15 | 361.013 |
Fig.3 Spatial pattern of vegetation carbon storage. |
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3.2.3 Carbon Sink Importance Level Evaluation
Addressing equal importance on carbon sequestration capacity and carbon storage, the researchers gave equal weight to those indicators. First, normalize NEP, vegetation carbon density, and soil carbon density according to Eq. (4). Second, overlay the obtained results to score the carbon sink importance level (I) with Eq. (5). Then, rate the results as "very high, " "high, " "moderate, " "low, " and "very low" with natural breaks classification method.
where X' is the normalization result; XNEP', Xveg', and Xsoil' are the normalization results of NEP, vegetation carbon density, and soil carbon density, respectively.
3.3 Spatial Pattern of Carbon Sinks
Referring to the relevant concepts of spatial pattern in landscape ecology, this research recognized the carbon sinks in Wensu County in three spatial categories based on their importance level, namely primary, secondary, and potential carbon sinks (Tab.3).
Tab.3 Spatial categories of carbon sinks in Wensu County |
| Category | Description |
|---|
| Primary carbon sink | · Continuous carbon sinks that are mostly rated as "very high" and "high" level of importance, critical to maintaining and increasing carbon sinks |
| · They are mostly covered by natural vegetation with little human intervention, featuring a healthy ecosystem and abundant natural endowments |
| · Ecological protection should be prioritized |
| Secondary carbon sink | · Continuous carbon sinks that are rated as "high" level of importance |
| · They are mainly covered by cultivated vegetation |
| · It requires site-specific ecological protection and restoration approaches |
| Potential carbon sink | · Areas between the primary and secondary carbon sinks, rated as "moderate, " "low, " and "very low" level of importance |
| · They are mainly covered by natural vegetation, surrounded by large areas of unused land such as Gobi desert and bare rock or gravel; the vegetation is in fair condition, easily to be disturbed and generate carbon emissions |
| · Key areas to increase carbon sinks with gradual ecological restoration while avoiding large amount of carbon emissions from disturbances |
Based on the importance level evaluation results of carbon sinks, areas rated as "very high" and "high" were identified as the primary and secondary carbon sinks that should be managed according to land use boundaries.
As natural land with higher connectivity can better perform ecological functions
[30][31], the research team proposed to identify the potential carbon sinks to connect the primary and secondary carbon sinks, which have high feasibility of restoring ecosystem and increasing carbon sinks at low cost and are prone to generate carbon emissions that need to be sequestered locally. Accordingly, the research team selected relevant resistance factors and calculated with the minimum cumulative resistance (
MCR) model. Areas with low resistance level were identified as potential carbon sinks. Areas featuring higher elevation, steeper slopes, lower annual accumulated temperature, or less annual precipitation are less suitable for plant growth
[32]~[34] and more vulnerable to human intervention
[35], making ecological restoration more challenging
[36]. In this sense, slope, elevation, land use, annual accumulated temperature, annual precipitation, and population density were chosen as the resistance factors and then graded based on site conditions. Weights of these resistance factors were drawn with the entropy weight method
[37][38] in Stata (Tab.4). The resistance level of each factor (Tab.5) was assigned a value before a weighted superposition was applied in ArcGIS (Fig.4, Fig.5). By setting the primary and secondary carbon sinks as source patches, the potential carbon sinks were identified with Eq. (6),
Tab.4 Entropy weight method results |
| Resistance factor | Information entropy (e) | Difference coefficient (d) | Weight (w) |
|---|
| Slope | 0.8898 | 0.1102 | 28.8111% |
| Elevation | 0.8758 | 0.1242 | 32.4634% |
| Land use | 0.9680 | 0.0320 | 8.3713% |
| Annual accumulated temperature | 0.9101 | 0.0899 | 23.4966% |
| Annual precipitation | 0.9881 | 0.0119 | 3.1203% |
| Population density | 0.9857 | 0.0143 | 3.7373% |
Tab.5 Levels of resistance factors |
| Resistance factor | Level | Value |
|---|
| Slope (°) | < 20 | 1 |
| [20, 40) | 3 |
| [40, 60) | 5 |
| [60, 80) | 7 |
| ≥ 80 | 9 |
| Elevation (m) | < 2,000 | 1 |
| [2,000, 3,000) | 2 |
| [3,000, 4,000) | 4 |
| [4,000, 5,000) | 6 |
| [5,000, 6,000) | 8 |
| ≥ 6,000 | 10 |
| Land use | Forest | 1 |
| Grassland | 1 |
| Farmland | 3 |
| Water area | 5 |
| Urban land, rural settlement, and other development land | 10 |
| Unused land | 5 |
| Annual accumulated temperature (℃) | ≥ 4,000 | 1 |
| [3,000, 4,000) | 3 |
| [2,000, 3,000) | 5 |
| [1,000, 2,000) | 7 |
| < 1,000 | 9 |
| Annual precipitation (mm) | ≥ 600 | 1 |
| [450, 600) | 3 |
| [300, 450) | 5 |
| [150, 300) | 7 |
| < 150 | 9 |
| Population density (people per square kilometer) | < 1, 000 | 1 |
| [1000, 2000) | 3 |
| [2000, 3000) | 5 |
| [3000, 4000) | 7 |
| ≥ 4,000 | 9 |
Fig.4 Resistance factors of the study area. |
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Fig.5 Resistance profile of the study area. |
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where
Dij is the physical distance between the ecological source patch
j and the landscape unit
i,
Ri is the resistance coefficient, and
f is the positive correlation between minimum cumulative resistance and ecological processes
[35][39].
3.4 Ecological Restoration Strategy Development
After analyzing the current land use in the primary, secondary, and potential carbon sinks, this research extracted typical spatial patterns of land use. Referring to existing research
[40]~[46], varied ecological restoration strategies, as well as management approaches were proposed specifically for different land use spatial patterns (Tab.6) to maintain carbon sinks in unused land, moderately increase those in forests, and significantly boost those in grasslands and farmlands.
Tab.6 Management approaches to increasing carbon sequestration and carbon sinks |
| Land use type | Land use subtype | Management approaches |
|---|
| Forest | Forest land, shrub land, open forest land, other forest land | Afforestation and reforestation, converting farmland to forest, natural forest protection, forest tending, naturalization of man-made forests, mixed forest plantation, agroforestry |
| Grassland | High coverage grassland, medium coverage grassland, low coverage grassland | Natural grassland enclosure, converting farmland to grassland, degraded grassland restoration, perennial mixed grassland interplanting, grazing intensity control, rotational grazing, deferred grazing |
| Farmland | Paddy land, dry land | Straw mulching, applying organic fertilizers, conservation tillage, farmland rotation, irrigation management, composite system intercropping, recycling wasted farmland resources |
| Water area | River and canal, lake, reservoir, glacier and perennial snowfield, floodplain | Wetland conservation, degraded floodplain restoration, wetland management, garbage disposal, artificial wetland construction |
| Unused land | Sandy land, Gobi desert, saline-alkali land, bare soil, bare rock or gravel | Soil remediation and maintenance, planting saline-tolerant and sand-fixing species, introducing new biotechnology to cultivate species (trees, grasses, or microorganisms) that can adapt to local conditions and efficiently sequestrate carbon |
4 Results and Analyses
4.1 Current Carbon Sinks in Wensu County
4.1.1 Current Carbon Sequestration Capacity
According to the results, the NEP of Wensu County ranges between –229.125 ~ 293.020 gC·m– 2·a– 1 and varies significantly across different areas, indicating a staggering pattern of carbon sequestration capacity. Gobi deserts and bare rock or gravel areas in southern Wensu County show low NEP with the weakest carbon sequestration capacity. The high northern areas of Tianshan Mountain exhibit poor NEP and carbon sequestration capacity due to year-round snow cover and scarce vegetation. The belt of the southern foothills of Tianshan Mountain with an elevation of 2,000 ~ 3,000 m, where the natural forest and grassland flourish, has a high NEP, suggesting the best carbon sequestration capacity in the county. In addition, the cultivated vegetation area (mainly farmland) in the southern Wensu County is characterized by a higher NEP with a great carbon sequestration capacity (Fig.6).
Fig.6 Spatial pattern of NEP. |
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4.1.2 Current Carbon Storage
The central area of Wensu County has the highest soil carbon storage, followed by the southern area, and the northern area has the least (Fig.7). Higher inorganic carbon content are found in both topsoil and subsoil in the central and some southern areas of the county, and in the southeastern alluvial plain (Fig.8, Fig.9). Topsoil in the southern foothills of Tianshan Mountain with an elevation of 2,000 ~ 3,000 m is richer in organic carbon (Fig.10). Overall, Wensu County subsoil contains little organic carbon, with higher content found only in the central (1,500 ~ 2,000 m in elevation) and a few southern plain patches in the county (Fig.11).
Fig.7 Spatial pattern of total carbon in soil. |
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Fig.8 Spatial pattern of inorganic carbon in topsoil. |
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Fig.9 Spatial pattern of inorganic carbon in subsoil. |
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Fig.10 Spatial pattern of organic carbon in topsoil. |
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Fig.11 Spatial pattern of organic carbon in subsoil. |
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Total vegetation carbon storage in Wensu County is 361.013 × 104 t (Tab.2). The temperate grass and forb halophytic meadows contribute the most (89.059 × 104 t), followed by cultivated vegetation, featuring two-year triple cropping or one-year double cropping dry farming and deciduous orchard (65.322 × 104 t). The cool temperate and temperate montane coniferous forest covers a small area but boasts the highest carbon density and high carbon storage (57.007 × 104 t). The carbon storage of the temperate steppe shrub desert is the lowest (0.8 × 104 t) due to the small area and low carbon density.
Combining vegetation and soil carbon storage, the plain in the southern county and the southern foothills of Tianshan Mountain have the largest total carbon storage. These areas are currently dominated by forest land, grassland, and farmland. Future ecological restoration should focus more on land development and transfer, and ensure that the carbon storage remains largely undisturbed.
4.1.3 Spatial Pattern of Carbon Sink Importance Level
According to the evaluation results, the importance level of carbon sinks in different geographical environments of Wensu County varies significantly. Carbon sinks in the southern foothills of Tianshan Mountain areas (2,000 ~ 3,000 m elevation) are of "very high" and "high" importance level; those continuous in the southern and southeastern plains are of "high" and "moderate" importance level; and those in the northern Tianshan Mountain areas elevated above 3,000 m and Gobi desert and bare rock and gravel areas in the central and southeastern county are of "low" and occasionally "very low" importance level (Fig.12).
Fig.12 Spatial pattern of carbon sink importance level. |
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4.2 Spatial Pattern Identification of Carbon Sinks
One primary, two secondary, and one potential carbon sinks are identified in Wensu County. The primary carbon sink (1,083.87 km2) lies in the southern foothills of Tianshan Mountain, with an elevation of 2,000 ~ 4,000 m. Two secondary carbon sinks (1,899.69 km2 in total) are located in the southern and southwestern plains of the county. The potential carbon sinks (2,259.81 km2) are found in the central part of the county. The land uses in the primary carbon sink are mainly high-, medium-, and low-coverage grasslands, occupying 48.60%, 11.48%, and 31.76% of the total area, respectively (Tab.7). The secondary carbon sinks are mainly dry land, which account for 86.63% of the total area (Tab.8). The potential carbon sinks feature the low-coverage grassland and bare rock and gravel, covering 51.28% and 24.81% of the total area, respectively (Tab.9, Fig.13).
Tab.7 Land use in primary carbon sink |
| Land use subtype | Area (km2) | Percentage | Total area (km2) |
|---|
| Dry land | 2.88 | 0.27% | 1,083.87 |
| Forest land | 41.29 | 3.81% | |
| Shrub land | 0.12 | 0.01% | |
| Open forest land | 36.54 | 3.37% | |
| High coverage grassland | 526.79 | 48.60% | |
| Medium coverage grassland | 124.45 | 11.48% | |
| Low coverage grassland | 344.26 | 31.76% | |
| River and canal | 5.70 | 0.53% | |
| Floodplain | 1.84 | 0.17% | |
Tab.8 Land use in secondary carbon sinks |
| Land use subtype | Area (km2) | Percentage | Total area (km2) |
|---|
| Paddy land | 4.65 | 0.24% | 1,899.69 |
| Dry land | 1,645.65 | 86.63% | |
| Forest land | 0.12 | 0.01% | |
| Other forest land | 51.65 | 2.72% | |
| High coverage grassland | 15.16 | 0.80% | |
| Medium coverage grassland | 12.21 | 0.64% | |
| Low coverage grassland | 78.79 | 4.15% | |
| River and canal | 8.18 | 0.43% | |
| Reservoir | 0.57 | 0.03% | |
| Floodplain | 13.55 | 0.71% | |
| Urban land | 2.21 | 0.12% | |
| Rural settlement | 65.66 | 3.46% | |
| Gobi desert | 0.31 | 0.01% | |
| Bare soil | 0.98 | 0.05% | |
Tab.9 Land use in potential carbon sinks |
| Land use subtype | Area (km2) | Percentage | Total area (km2) |
|---|
| Dry land | 16.13 | 0.71% | 2,259.81 |
| High coverage grassland | 5.11 | 0.23% | |
| Medium coverage grassland | 146.40 | 6.48% | |
| Low coverage grassland | 1,158.73 | 51.28% | |
| Gobi desert | 330.84 | 14.64% | |
| Bare soil | 41.88 | 1.85% | |
| Bare rock or gravel | 560.72 | 24.81% | |
Fig.13 Spatial pattern identification of carbon sinks. |
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4.3 Ecological Restoration Strategies
Based on the land use patterns in Wensu County, this research extracted eight typical spatial patterns of land use—one in the primary, two in the secondary, and five in the potential carbon sinks. To boost carbon sinks, the research proposed ecological restoration strategies for each typical land use spatial pattern, namely high-carbon reinforcement; carbon conservation, low-carbon development; carbon conservation expansion, replanting for carbon sequestration, restoring for carbon sequestration, maintaining high carbon storage, and carbon sequestration monitoring. Approaches to promoting ecological restoration and carbon sink benefits were proposed based on local conditions (Tab.10, Fig.14).
Tab.10 Land use spatial patterns, ecological restoration strategies and approaches |
| Category | Spatial pattern of land use | Characteristic | Ecological restoration strategy | Approach |
|---|
| Primary carbon sink | Forest–grassland | Continuous grassland scattered with forest patches | High-carbon reinforcement | Conserving existing carbon pools, restoring soil fertility, and increasing grassland carbon sink and storage by growing mixed forest on abandoned grasslands, forestation and reforestation, grazing intensity control, and natural grassland enclosure |
| Secondary carbon sink | Gobi–farmland–grassland | Continuous Gobi desert with floodplain, grassland, and farmland along the borders | Carbon conservation restoration | Increasing the above-ground biomass, restoring soil fertility, conserving existing soil carbon pools, and enhancing grassland carbon sink via boundary restoration, floodplain ecological conservation, degraded grassland reseeding and maintenance, and converting farmland to grassland |
| Farmland–forest land–rural settlement | Continuous farmland with scattered rural settlements and plantations | Low-carbon development | Recycling the agricultural, forest, and residential resources, reducing resource consumption, and enhancing farmland carbon sequestration and storage (soil and vegetation) via conservation tillage, agroforestry, and agricultural waste recycling |
| Potential carbon sink | Bare rock or gravel–grassland | Bare rock or gravel areas with belts of low-coverage grassland | Carbon conservation expansion | Maintaining natural succession, restoring soil fertility, and thus enhancing grassland carbon sink by soil mediation and maintenance of saline-alkali land, planting saline-tolerant and sand-fixing vegetation, and building shelterbelts along the boundaries |
| Grassland–bare soil | Continuous low-coverage grassland interspersed with few bare soil areas | Replanting for carbon sequestration | Increasing grassland carbon sink (soil and vegetation), the above-ground biomass, and then carbon sink and storage of the region via bare soil land management, grazing management, and ecological replanting |
| Floopplain–saline-alkali land | Floodplain near river and canals, bordered by saline-alkali land with sparse vegetation | Restoring for carbon sequestration | Improving soil fertility and maintaining soil carbon pool by soil mediation and maintenance of the saline-alkali land, planting saline-tolerant species, and introducing new planting technologies |
| Forest–farmland–water area | Shrub land–dry land–river and canal patches, with roads traversing through | Maintaining high carbon storage | Maintaining soil carbon pool and boosting vegetation carbon sink by conservation tillage, converting farmland to forest, naturalization of man-made forests, and mixed forest plantation |
| Grassland–highway | Narrow low-coverage grassland belts traversing bare rock or gravel areas, with highways in the center | Carbon sequestration monitoring | Maintaining grassland carbon pool (soil and vegetation) by natural grassland enclosure, rotational grazing, grazing intensity control, and shelterbelt construction; maintaining area carbon sink by planting along the highway |
Fig.14 Ecological restoration models. |
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5 Conclusions and Discussion
5.1 Conclusions
This research evaluated current carbon sinks in Wensu County based on data from multiple sources, identified the pattern of carbon sinks in the county, and proposed ecological restoration strategies to increase carbon sinks. According to the findings, carbon sequestration capacity varies across the county. The southern foothills of Tianshan Mountain, with an elevation of 2,000~3,000 m, have the best carbon sequestration capacity. The plain in the southern county and southern foothills of Tianshan Mountain have the largest carbon storage. Carbon sink importance level varies significantly across geographical environments of the county. That in the southern foothills of Tianshan Mountain and in the southern and southeastern plains of the county are grated higher than the others. Based on the carbon sink importance level evaluation, one primary, two secondary, and potential carbon sinks with a total area of 2,259.81 km2 are identified. Eight ecological restoration strategies are proposed based on local conditions to achieve the main goal of boosting carbon sinks.
5.2 Inspirations for Carbon Sink Management in the Arid Areas of Northwest China
The findings suggest that grassland and farmland are the key land use types when aiming at carbon sequestration and carbon sink enhancement in the arid areas of northwest China (Tab.7 ~ Tab.9). Although the carbon sink capacity of forests is higher than that of grasslands
[47], in Wensu County, where forests cover a small area due to the arid climate, their carbon sink benefits are therefore insignificant. Meanwhile, grassland and dry land are of higher carbon sink importance level and function as the main carbon sinks in the county. Existing research evaluating the vegetation carbon sink capacities in the arid areas of northwest China also concludes that temperate grasslands and high-quality farmlands function better as carbon sinks
[24], which further supports the carbon sink potential of grasslands and farmlands. In this sense, ecological restoration in the arid areas of northwest China should take characteristics of local vegetation into consideration to avoid changing vegetation types or maintaining non-local vegetation, which causes material and energy consumption that leads to carbon emissions. Also, it is important to avoid large disturbances in grasslands, optimize management measures, and increase organic matter in the soil to boost carbon storage
[48]. On farmlands, while photosynthesis helps carbon sequestration, respiration and production activities emit carbon
[49]. Agricultural activities such as cropping system adjustment and agricultural investment control that minimize soil disturbance, as well as management upgrade
[50][51], can further increase carbon sequestration and carbon sinks.
The identified spatial pattern of carbon sinks in Wensu County could be instrumental to carbon sink management in the arid areas of northwest China. Compared with more developed regions where most carbon sinks are found in the natural areas distant from downtown
[9][52], in Wensu County, apart from the southern foothills of Tianshan Mountain, the rural settlements and farmlands featuring intensive human activities in the southern county have shown significant carbon sink benefits. In other words, landscapes under human intervention play an essential role in increasing carbon sequestration and carbon sinks in arid areas. More research is expected to further evaluate carbon sink potentials of cultivated vegetation in arid areas, thereby guiding the efforts for increasing carbon sequestration and carbon sinks.
5.3 Values to the Territorial Ecological Restoration
China has launched a number of ecological restoration projects since the 1950s. Many forest, grassland, wetland, and farmland ecosystems have been restored, with their carbon sequestration capacity improved remarkably
[15]. Focusing on more than any singular factor, territorial ecological restoration aims to enhance the stability and security of the whole regional ecosystem
[53]. The proposal of "dual carbon" goals makes carbon sequestration and carbon sink increase an integral component of ecological restoration, boosting the number of related research
[17][18]. Yet, most research focuses on a single ecosystem and provides specific ecological restoration approaches, or makes theoretical discussions on carbon sequestration approaches only
[14]. With a spatial perspective, this research took Wensu County as the study area, and proposed ecological restoration strategies that increase carbon sequestration and carbon sinks by current carbon sink evaluation, spatial pattern analyses, and feasible restoration strategies development. The
MCR employed to identify the spatial pattern of carbon sinks can also be applied in territorial ecological restoration focused on biodiversity protection
[54] and mountain disaster prevention
[55]. The framework of this research integrates carbon sink increase with other ecological restoration goals, and the findings contribute to the holistic preservation, restoration, and governance of mountains, rivers, forests, farmlands, lakes, grasslands, and deserts.
5.4 Prospects
The application of MCR, basing the resistance of developing potential carbon sinks on restoration difficulties and demand for local absorption of carbon emission, is still in the early development stage. Further post-implementation validation is required. In addition, more measured data with better precision over a longer period are expected in future research, so as to make the carbon sink evaluation more comprehensive and accurate. Moreover, carbon sink increase should be integrated with other ecological problems to emphasize the carbon problems in territorial ecological restoration and holistic maintenance.
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Fig.1 Elevation and current land use of the study area (data sources: elevation from Geospatial Data Cloud; satellite images from Bing Maps; and land use from the Resource and Environment Science and Data Center, Chinese Academy of Sciences).
Tab.1 Research datasets and data sources
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