Introduction

Ecosystem carbon stocks play an important role in the carbon cycle, reducing carbon emissions and mitigating global warming. Land use change is closely related to terrestrial ecosystems1,2 and is one of the most important factors influencing carbon storage change3,4. The transformation of land use types directly affects the carbon sequestration capacity of soil and vegetation in the region, which in turn affects the carbon storage of the whole region5. With the continuous advancement of urbanization, people ignore the impact of land allocation on ecosystems in land development6,7, which has caused many ecological problems8. Therefore, exploring ecosystem carbon storage and its changes under land use is conducive to maintaining ecosystem stability9,10.

The causes of land use change are intricate and complex, influenced by nature, society, and many other aspects. For the study of changes in land use time series, scholars mainly use models such as the integrated dynamic attitude, the integrated degree index of land use, and the land use transfer matrix to analyze11,12; For the study of spatial change in land use, scholars mainly use models such as center of gravity shift, geologic information map, standard deviation ellipse, and kernel density13,14. For example, at the watershed scale, Du et al.15 used the PLUS model to analyze the land use change in the middle and lower reaches of the Yellow River, and concluded that "under different scenarios, the land use pattern changes in the sandy and coarse sand areas in the middle reaches of the Yellow River are significantly different". However, previous studies have lacked research on the scale of coastal economic zones16,17,18, which are sensitive to climate change and vulnerable to the ecological environment, contain huge carbon pools, and play a vital role in the carbon cycle and energy flow at regional and global scales. The study area selected in this paper is a typical coastal economic zone, and the study of this area enriches the relevant research on land use and carbon storage at the scale of a coastal economic zone.

Land use simulation is an important way to analyze land use laws. To protect the ecological environment, people have begun to explore the development trajectory of land use in the future, to realize the effective adjustment of land use planning. Land use simulation research has been widely carried out around the world, and the research scale involves national, provincial, municipal, and county levels19,20,21. Land use simulation models also tend to be diverse, with the more mainstream models including CA-Markov, CLUE-S, FLUS, and PLUS models22,23,24. The CA-Markov, CLUE-S, FLUS, and PLUS models can not only predict the amount of land use in a long series of long series, but also simulate the spatial change of land use with high accuracy. However, the CA–Markov model only considers the influence of cell number and structure on land use simulation, the CLUE-S model lacks nonlinear analysis of land use change-driven data, and the FLUS model requires the coordinate system, resolution, and row number of all raster data to be unified, so it also has its shortcomings25,26,27. The PLUS model can solve the problems of the above models, and at the same time, the simulation accuracy is higher, and the operation process is more simplified28. For example, Zhang et al.29 simulated the ecosystem on the northern slope of the Tianshan Mountains based on the PLUS model, and concluded that "the decrease of grassland and the increase of bare land are the main causes of carbon loss in this area". In this study, the PLUS model was used to analyze the law of land use change from 1980 to 2020, which improved the accuracy of land use simulation to a certain extent.

Terrestrial ecosystem carbon stocks are part of the global carbon stock and play a vital role in mitigating climate warming30. As the primary factor affecting the change of carbon stock, land use change affects the carbon cycle process of the whole system by changing the structure and function of the ecosystem31. There are abundant research results worldwide for the study of carbon storage. The more traditional research methods include the inventory method, the bookkeeping method, and the biomass method, which are not conducive to the calculation of carbon stocks on a large scale32,33. With the development of GIS technology, many scholars have estimated ecosystem carbon storage through the CASA model34 and InVEST model35 based on land use data, which are more accurate than traditional methods. With the popularization of the InVEST model, it has gradually become the mainstream method to analyze the correlation between land use change and ecosystem carbon storage due to its advantages of less data volume and easier operation36,37. In conclusion, a large number of previous research results have shown that the InVEST model can accurately estimate the carbon storage of ecosystems and promote the scientific ecological protection scheme. For example, Li et al.38 evaluated the impact of land use change on carbon storage in the Inner Mongolia section of the Yellow River Basin based on the InVEST model. They concluded that "the increase in grassland area is the main reason for the increase of carbon storage in the Inner Mongolia section of the Yellow River Basin". A large number of studies have shown that it is important to increase regional carbon storage capacity, such as Chen et al. who studied the carbon storage in areas with abundant ancient relics in China and sought a balance mechanism between heritage protection and carbon sinks39. Therefore, it is very important to study the balance between carbon sinks and regional development. However, large-scale coastal economic development zones are characterized by the high intensity of development of construction land, which continues to squeeze ecological land. The conflict of land use structure is serious, and the pressure of urbanization is huge40. The research in this paper focuses on the rational planning of construction land and the protection of other land use types, and this kind of targeted analysis of the rapid expansion of construction land is rare in previous studies, which is also one of the innovations of this paper.

Scholars at home and abroad have used the InVEST model to analyze the temporal and spatial changes of terrestrial ecosystem carbon storage and its influencing factors from the perspectives of land use in three periods: historical, current, and future41,42. In terms of the selected research objects, the research on land use change has expanded from the natural field to urban ecological construction, and the research scope and objects have been greatly expanded43. For example, the GeoSOS-FLUS, DTTD-MCR-PLUS, Markov-CA, and other models are coupled44,45 to analyze the temporal and spatial evolution of carbon stocks under multiple scenarios46,47. On the time scale, the InVEST model is still insufficient in the study of carbon stocks, and most scholars have studied the relationship between land use and carbon stock changes for a short time span48,49, most of which are less than 20 years. The short period ignores the potential long-term impact of land use on carbon stocks, and it is difficult to observe the impact of strong lag carbon stocks on land use and regional economy50. In this study, the carbon storage in the next 10 years is predicted through the 40-year land use change in the Guangxi Beibu Gulf Economic Zone, and the longer time series is conducive to improving the accuracy of carbon storage calculation.

Carbon peaking and carbon neutrality are inevitable choices to solve environmental problems and achieve sustainable development51. However, research on carbon storage methods or examples rarely involves large-scale national economic development zones. The Guangxi Beibu Gulf Economic Zone is an important strategic location for the development of the country's western region52, and the development of this region is conducive to the overall national strategic planning and can create a mutually beneficial and win–win open situation53. In this paper, the Guangxi Beibu Gulf Economic Zone is selected as the study area, which enriches the research on carbon storage in large-scale economic development zones. Therefore, to better respond to the national "dual carbon" strategy, this paper uses the PLUS-InVEST model to analyze the land use change and carbon storage change in this region. The specific objectives of this study: (1) By exploring the land use change law of Guangxi Beibu Gulf Economic Zone from 1980 to 2020, the spatial pattern of land use in the region under multiple scenarios in the next ten years was simulated. (2) Evaluate the spatiotemporal dynamic evolution of carbon storage in the Guangxi Beibu Gulf Economic Zone under different scenarios, and then analyze the relationship between land use change and ecosystem carbon storage change. (3) The changes in regional carbon storage, provide a valuable reference for the sustainable development of the ecosystem and regional carbon balance in the Guangxi Beibu Gulf Economic Zone, enrich the theoretical research on the carbon storage of the ecosystem in the region, and provide a certain reference for the construction of low-carbon cities in the region.

Data sources and methods

Research framework

In this study, we propose a framework for research on the impact of land use change on carbon storage in the Beibu Gulf Economic Zone of Guangxi (Fig. 1). Under this framework, the entire research process is divided into four main parts. The first part focuses on the specific issues studied, their global significance, and progress at home and abroad (Section "Introduction"). The second and third parts are the two main research themes that run through the text, and the second part discusses the content and methods of land use and carbon stock change research (Section "Data sources and methods"). Part III discusses the analysis of the results of land use and carbon stock change (Sections "Analysis of land use change from 1980 to 2030" and "Analysis of carbon stock changes from 1980 to 2030"). The fourth section focuses on the findings, conclusions, and recommendations of this paper (Sections "Discussion" and "Conclusions and recommendations").

Fig.1
figure 1

Research framework diagram.

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Overview of the study area

Guangxi Beibu Gulf Economic Zone is located on the southwest coast of China (20°26′N ~ 23°32′N, 106°33′E ~ 110°18′E), and is an important hub in the south of China's "Belt and Road" strategy53. Since 2008, the Chinese government has been developing and building the region as an important international economic cooperation zone for 16 years. The total area of the area is about 72,209.79 km2, and it is composed of Nanning, Qinzhou, Yulin, Chongzuo, Fangchenggang, and Beihai (Fig. 2). The region has a well-developed river system, a subtropical monsoon climate, abundant precipitation, and lush vegetation. The terrain is dominated by low hills, with high terrain in the northwest and low in the southeast. The economic zone has a large environmental capacity and excellent ecology, and the development density is low.

Fig. 2
figure 2

Location map of Guangxi Beibu Gulf Economic Zone.Note: Maps are based on standard map number GS (2019) 1823. It was downloaded from the website of the China Surveying, Mapping and Geographic Information Standard Map Service of the Map Technology Review Center of the Ministry of Natural Resources of China, and was made by ArcGIS 10.8 without modifying the bottom map boundary. (http:// bzdt. ch. mnr. gov. cn/) The following diagram is made in the same way.

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Data sources

The data involved in this study include land use data, carbon density data, and land use change driving data, in which land use data and carbon density data are combined to estimate ecosystem carbon stocks, and land use data and land use change driving data are used to simulate and predict land use under different development scenarios in 2030.

Land use data

The land use data of the fifth phase from 1980 to 2020 are from the Resource and Environmental Science and Data Center (http://www.resdc.cn) of the Chinese Academy of Sciences, with a resolution of 30m and a comprehensive accuracy of > 90%. Combined with the actual land use situation of Guangxi Beibu Gulf Economic Zone, it is divided into six categories: cultivated land, forest land, grassland, water area, construction land, and Unutilized land.

Land-use change driving data

The use of the PLUS model for simulation and prediction requires two types of data on land use and its change drivers. According to the actual situation and combined with relevant literature, a total of 13 driving data of land use change were selected from three aspects: natural conditions, socio-economic conditions, and location conditions54,55,56.

Among them, the elevation data comes from the geospatial data cloud platform (http://www.gscloud.cn), the slope data is calculated and obtained from the elevation data with the help of slope analysis tools, with a resolution of 30m, and the soil type, average annual temperature, average annual precipitation, population density and GDP data come from the Resource and Environmental Science and Data Center (http://www.resdc.cn) of the Chinese Academy of Sciences, with a resolution of 1000m. The location conditions include the distance from the expressway, railway, and other roads and the distance from the municipal government station, the expressway, railway, primary road, and secondary road are from OpenStreetMap (http://www.openstreetmap.org/), and the municipal government resident data is collected in the Gaode map (https://lbs.amap.com/), with the help of the Euclidean distance analysis tool to the point (government station), The spatial distribution data was obtained by analyzing the line (road network) elements(Fig. 3).

Fig. 3
figure 3

Drivers of land use change.

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Carbon density data

The carbon density data required in the InVEST model contain four basic carbon densities for different land use types, namely above-ground organisms, below-ground organisms, soil carbon density, and dead organic matter carbon density. To ensure the accuracy of the carbon density data, the carbon density data of this study mainly refer to the previous research results in Guangxi57,58,59,60,61, and obtain the land use carbon density table of the Guangxi Beibu Gulf Economic Zone (Table 1).

Table 1 Carbon density of different land types (t/hm2).
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Research methods

Single land use dynamics

The dynamics of a single land use reflect the intensity of various types of land use changes in a certain period43. The formula is:

$$K=\frac{{U}_{y}-{U}_{x}}{{U}_{x}\times T\times 100\%}$$
(1)

where: Uy is the area of land use at the end of the study; UX is the area of land use at the beginning of the study; T is the study period.

Land use transition matrix

The land use transition matrix reflects the direction and scale of various types of land use transition in a certain period of time43. The formula is:

$${P}_{ij=}\left(\begin{array}{ccc}{P}_{11}& \cdots & {P}_{1n}\\ \vdots & \ddots & \vdots \\ {P}_{n1}& \cdots & {P}_{nn}\end{array}\right)$$
(2)

where: Pij is the area of land use transition; n is the type of land use; In this study, n is 6 (1: cultivated land, 2: forest land, 3: grassland, 4: water area, 5: construction land, 6: Unutilized land).

InVEST carbon stock model

The carbon storage estimation module in the InVEST model involves four categories: aboveground organisms, belowground organisms, soil carbon storage, and dead organic matter carbon storage44. The formula is:

$$\text{C}={C}_{a}+{C}_{b}+{C}_{c}+{C}_{d}$$
(3)

where: C is total carbon stock; Ca is above-ground biomass carbon stock; Cb is below-ground biomass carbon stock; Cc is soil carbon stock; Cd is dead organic matter carbon stock.

PLUS model

The PLUS model is a cellular automata model based on raster data for land use simulation prediction, which can predict future land use demand with the help of the linear regression method and Markov model62. The LEAS (Land expansion analysis strategy) module uses a random forest classification algorithm to mine the probability of change in land use types, analyze the relationship between driving forces and land use, and calculate the part of land use expansion from the initial stage to the end stage63. The parameters of this module are set: number of regression trees is 20, sampling rate is 0.01, mTry is 5, and Thread is 3. The CARS (CA-based on multiple random patch seeds) module is a CA model based on multiple classes of random patch seeds, which is used to simulate the spatial variation of complex scenarios64; The parameters of the module are set as follows: Neighborhood size is 3, Thread is 3, Patch generation threshold is 0.5, Expansion coefficient is 0.1, Percentage of seeds is 0.0001, and the neighborhood weight of each land use type is determined by calculating the ratio of the expansion area of land use types to total land expansion based on land expansion65. Among them, cultivated land was 0.2288, forest land was 0.2779, grassland was 0.0786, water area was 0.0375, construction land was 0.3756, and Unutilized land was 0.0016.

Setup of multi-scenario simulation

The natural development scenario (NS) is based on the extrapolation of the existing development trend, the parameters are not adjusted, and the changing trend of various types in 2000 ~ 2020 is continued, only the land use changes caused by the natural state are considered, and the influence of possible factors such as policies is ignored, and the conversion between all land types can occur. In the cultivated land protection scenario (CS), considering the resource and environmental carrying capacity of the Guangxi Beibu Gulf Economic Zone, considering the cultivated land that guarantees the source of food, prohibiting the transfer of cultivated land to other land types, and increasing the probability of transferring forest land and grassland to cultivated land by 35%, and the probability of transferring Unutilized land to cultivated land by 15%. According to the ecological protection scenario (ES), according to the territorial spatial planning of Guangxi Zhuang Autonomous Region (2021 ~ 2035), "strictly control the increment, revitalize the stock, stabilize the forest coverage rate, and adopt the strategy of 'two guarantees', that is, to protect the forest wetland ecosystem and the ecological environment protection of the river source area", this scenario strictly restricts the conversion of forest land, grassland, and water area to other land types, and at the same time reduces the probability of arable land, forest land, and grassland turning to construction land by 20%, and increases the probability of arable land, grassland, and water area turning to forest land by 20%. According to the scenario requirements, a land use conversion cost matrix under the three scenarios in 2030 (Table 2) was constructed, and the conversion cost matrix was divided into 1 and 0 to represent whether the land use type to be changed was restricted from being converted to other land types66.

Table 2 Land use conversion cost matrix under multi-scenario simulation.
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Analysis of land use change from 1980 to 2030

Analysis of land use change from 1980 to 2020

From 1980 to 2020, the land use types in the Guangxi Beibu Gulf Economic Zone were mainly forest land and cultivated land (Fig. 4), and forest land and cultivated land accounted for about 89%. From 1980 to 2020, the construction land showed an upward trend, and the construction land area increased by 1240.67km2. Cultivated land, forest land, grassland, and water area all showed a downward trend, and the cultivated land area decreased the most, by 547.42 km2. In terms of the rate of change, the rate of change of construction land is the fastest, reaching a peak from 2010 to 2020 and the single dynamic degree is 3.10% (Table 3). To sum up, from 1980 to 2020, in order to enhance the process of industrialization and urbanization, the Guangxi Beibu Gulf Economic Zone converted a large number of other land types into construction land, and finally formed a trend of "one increase and many decreases".

Fig. 4
figure 4

Spatial distribution of land use from 1980 to 2020.

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Table 3 The area and single dynamic degree of various types of land use from 1980 to 2020.
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To further understand the land use transfer in the Guangxi Beibu Gulf Economic Zone from 1980 to 2020, the land use transfer matrix model found that the transformation relationship between various land use types in the Guangxi Beibu Gulf Economic Zone from 1980 to 2020 was significant, showing the characteristics of "one to many". Among them, the area of construction land transferred is the largest, with an area of 1443.51km2, which is mainly transferred from cultivated land and forest land. The area of cultivated land transferred out was the largest, with an area of 1988.83 km2, which was mainly transferred to construction land and forest land (Table 4). The area of cultivated land, forest land, grassland, and water area is greater than that transferred in, indicating that the Guangxi Beibu Gulf Economic Zone has developed rapidly in the past 40 years, and the construction land has been expanding and encroaching on other land types.

Table 4 Land use transition matrix/km2 from 1980 to 2020.
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Multi-scenario simulation analysis of land use in 2030

Land use simulation accuracy test

Based on the data related to land use in 2000 and 2010, the PLUS model was used to simulate and predict land use in 2020. Under the Confusion Matrixand From module in the PLUS model , the accuracy of the real 2020 land use and the simulated 2020 land use are verified. The results show that the overall simulation accuracy of the PLUS model is 0.93, the kappa coefficient is 0.91, and the coefficient is greater than 0.75, which proves that the simulation results of the PLUS model are reliable and can be used for future land use simulation prediction.

Analysis of land use multi-scenario simulation results

Based on the land use data in 2010 and 2020, the PLUS model was used to simulate and predict the land use under various scenarios in 2030 (Table 5).

Table 5 Area and rate of change of land use under multiple scenarios in 2030.
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Under the natural development scenario, the scale of cultivated land, forest land, grassland, and Unutilized land decreased, and the cultivated land decreased the most, with a decrease of 2.252%. The scale of the water area increased slightly, with an increase of 3.648%, and the construction land continued to expand, with an increase of 19.437%. Compared with the other two scenarios, the scale of construction land increased the most under the natural development scenario, while the scale of forestland, grassland, and cultivated land decreased the most.

Cultivated land protection scenarios to restrict the transfer of cultivated land types to other land use types. Under this scenario, the scale of cultivated land increased by 142.56 km2 compared with 2020, an increase of 0.691%. The size of forestland, Unutilized land, and grassland has decreased, and the scale of water has increased. The construction land still showed an expansion trend, but compared with the natural development scenario, the expansion scale of construction land decreased by 609.04 km2.

Ecological protection scenarios take into account the protection of ecological land, and reduce the probability of transfer of forestland, grassland and water to other land uses. In this state, the size of forest land increased the most, followed by water. There is still an expansion of construction land, but compared with the other two scenarios, the scale of construction land expansion is smaller, with an expansion of 29.95 km2.

Through the comparison of three different scenarios, it is found that cultivated land protection and ecological protection can effectively inhibit the expansion of construction land. Under the ecological protection scenario, although the cultivated land area decreased, the area of forestland, grassland, and water area with high carbon sequestration capacity increased greatly (Fig. 5). If cultivated land protection and ecological protection measures are adopted at the same time, the growth of construction land will be limited and the ecological environment of the Guangxi Beibu Gulf Economic Zone will be greatly improved. It is predicted that by 2030, the spatial distribution of land use will be similar to the traditional pattern, in which forest land and cultivated land are still the main land types, and the expansion rate of construction land is relatively fast, and the construction land is mainly distributed in the urban core area and the surrounding suburbs of the city.

Fig. 5
figure 5

Spatial distribution of land use by scenario in 2030.

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Analysis of carbon stock changes from 1980 to 2030

Analysis of carbon storage time series characteristics

Carbon storage in the Guangxi Beibu Gulf Economic Zone has been declining over the past 50 years (Fig. 6). From 2020 to 2030, the carbon storage in the natural development scenario decreased sharply, with carbon storage of 12.5354 × 108t, and the construction land in this scenario continued to expand, and the ecological land and cultivated land were greatly reduced. The carbon storage in the cultivated land protection scenario was 12.5857 × 108t, a decrease of 0.0123 × 108t or 0.0978% compared with 2020. In this scenario, cultivated land is protected and the expansion rate of construction land is curbed. The carbon storage in the ecological protection scenario was 12.6916 × 108t, an increase of 0.0936 × 108t or 0.7429% compared with 2020. In this scenario, ecological land such as forestland and grassland was protected, and the carbon storage of the region increased.

Fig. 6
figure 6

Statistical chart of carbon stocks from 1980 to 2030.Note: NS natural development scenarios, CS cultivated land protection scenario, ES ecological protection scenarios. The same applies hereinafter.

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The carbon storage in the Guangxi Beibu Gulf Economic Zone is dominated by forest land, followed by cultivated land (Table 6). In the past 50 years, the average carbon storage of forest land and cultivated land was 1096.9543 × 106 tons and 135.9925 × 106 tons, respectively. The carbon storage of forestland, cultivated land, and grassland showed a decreasing trend, with the largest decrease of 9.2605 × 106 tons of forest land, 3.5284 × 106 tons of cultivated land, and 2.4486 × 106 tons of grassland. The carbon storage of construction land showed an increasing trend, increasing by 1.6574 × 106t.

Table 6 Carbon stocks of different types of land use from 1980 to 2030 (unit: 106 t).
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From 2020 to 2030, the carbon storage of forestland, cultivated land, grassland, and Unutilized land decreased, and the carbon storage of construction land and water area increased under the natural development scenario. In the cultivated land protection scenario, the carbon storage of cultivated land increased by 2.9815 × 106t compared with 2020, the carbon storage of forest land, grassland, and Unutilized land still decreased, and the carbon storage of construction land and water area increased. Compared with 2020, the carbon storage of forest land, grassland, and water area increased in the ecological protection scenario, and the carbon storage of construction land increased less than that in the other two scenarios, only 0.0421 × 106t, and the carbon storage of cultivated land and Unutilized land decreased.

Analysis of spatial characteristics of carbon stocks

The spatial distribution of carbon storage in the region over the past 50 years was obviously heterogeneous, and on the whole, "the carbon storage in the middle area was low, and the carbon storage in the surrounding areas was high " (Fig. 7). The high-value carbon storage areas are distributed in the northern part of Nanning City, Fangchenggang City, Pubei County, and Lingshan County of Qinzhou City, and the southeast of Yulin City. These areas have high vegetation coverage and strong carbon sequestration capacity. The spatial distribution of the median carbon storage area was similar to that of cultivated land, mainly in the central and northern parts of Chongzuo City, the southwestern part of Beihai City, the northwest of Qinzhou City, Fangchenggang City and Yulin City, Hengzhou City and Binyang County of Nanning City, Wuming District of Nanning City and Longan County. The low-value areas of carbon storage are mainly distributed in the urban areas of various cities and the coastal areas of "Qinbeifang" in the form of points and clusters, and the distribution of low-value areas of carbon storage in the urban areas of Nanning City is the most obvious. It shows a trend of increasing year by year. In 2030, the low-value carbon storage areas under the natural development scenario will expand significantly, mainly concentrated in the urban area of Nanning, the northwest of Yulin City, and the coastal area of "Qinbeifang". Compared with the natural development scenario, the expansion rate of low-value carbon storage areas in the cultivated land protection and ecological protection scenarios is slower.

Fig.7
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Spatial distribution of carbon stocks from 1980 to 2030.

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Through the change pattern of carbon storage in the Guangxi Beibu Gulf Economic Zone from 1980 to 2030 (Fig. 8), the change of carbon storage in the economic zone is reflected spatially. From 1980 to 2020, the carbon storage in most regions remained unchanged, accounting for 95.65%, the area with a significant decrease in carbon storage accounted for 2.42%, and the area with a significant increase in carbon storage accounted for 1.92%. In addition, the significant increase in carbon storage was mainly distributed in the eastern part of Nanning City, the northeastern part of Qinzhou City, and the northern part of Yulin City. These areas effectively implemented ecological protection policies such as "returning farmland to forest and grassland" and "closing mountains for afforestation" from 1980 to 2020. The areas with significantly reduced carbon storage are mainly distributed in the urban areas of various cities and "Qinbeifang" coastal area. The construction land in these areas has expanded dramatically, occupying a large amount of ecological land, and significantly reducing carbon storage. Carbon stocks from 2020 to 2030 will remain largely unchanged (Fig. 8). In the natural development scenario, carbon storage is basically unchanged at 95.45%, significantly decreased by 2.39%, and increased significantly by 2.16%. In the cultivated land protection scenario, the carbon storage remained basically unchanged at 96.53%, decreased significantly by 1.85%, and increased significantly by 1.62%. In the ecological protection scenario, the carbon storage is basically unchanged at 96.12%, and the obvious increase is 2.26%. A significant decrease of 1.61%.

Fig. 8
figure 8

The pattern of carbon storage change from 1980 to 2030.

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The impact of land use change on regional carbon stock change

Since land use area and carbon density data are the main basis for analyzing regional carbon stocks, land use change significantly affects carbon stock changes. The most important conversion type of carbon storage increase in Guangxi Beibu Gulf Economic Zone is the conversion of cultivated land and grassland to forest land. And the most important conversion type of carbon storage decrease is the conversion of forest land to cultivated land and construction land, and the conversion of cultivated land to construction land (Table 7).

Table 7 Changes in carbon stocks due to land-use transition.
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From 1980 to 2020, from the perspective of land use type transfer, the conversion of land use type into forest land increased the most carbon storage, reaching 26.5404 × 106t, of which cultivated land was converted to forest land, thereby increasing carbon storage the most, reaching 16.1362 × 106t. The conversion of land use type to cultivated land reduced the carbon storage the most, reaching 19.7161 × 106t. In addition, the transfer of construction land reduced carbon storage by 13.9238 × 106t, of which forest land was converted into construction land, a decrease of 8.5691 × 106t, and cultivated land was converted into construction land, a decrease of 4.8151 × 106t. From the perspective of land use type, the carbon storage increased by 10.7099 × 106t was the largest, mainly due to the increase of carbon storage caused by the conversion of cultivated land to forest land.

By 2030, the main conversion type that leads to the increase of carbon storage in the natural development scenario is the conversion of cultivated land to forest land, and the carbon storage will increase by 22.4123 × 106t. The main conversion types that led to the reduction of carbon storage were the conversion of forest land to cultivated land and construction land, which decreased by 23.2662 × 106t and 4.2961 × 106t, respectively. In the cultivated land protection scenario, the main conversion types that lead to the increase and decrease of carbon storage are consistent with the natural development scenario, and the increased value of carbon storage caused by the conversion of cultivated land to forest land in this scenario is lower due to the protection of cultivated land, which increases by 15.7824 × 106t. Compared with the natural development scenario, the reduction value of carbon storage caused by the conversion of forest land to cultivated land under the cultivated land protection scenario was improved to a certain extent, which was only reduced by 18.3567 × 106t. In the ecological protection scenario, the main types of conversion that lead to the increase and decrease of carbon storage are consistent with other scenarios. Compared with other scenarios, the increase in carbon storage caused by the conversion of cultivated land to forest land in this scenario was the highest, with an increase of 22.4425 × 106t. Compared with the other two scenarios, the reduction of carbon storage caused by the conversion of forest land to cultivated land was the lowest, with a decrease of 15.4939 × 106t. In summary, when other land use types are converted to forestland and grassland, it can help to improve the carbon sequestration capacity of ecosystems.

Discussion

Based on the land use change in the Guangxi Beibu Gulf Economic Zone from 1980 to 2020, this study used the PLUS model to predict the land use structure of the region under three scenarios in 2030. From the perspective of land use assessment of three scenarios, including the natural development scenario, cultivated land protection scenario, and ecological protection scenario, the ecological environment damage in the region in the past 40 years has been relatively serious, and the expansion of construction land has accelerated, crowding out ecological land such as forest land. This rapid expansion of construction land is basically consistent with the research of other scholars58,59,60. The InVEST model was used to analyze the temporal and spatial changes in carbon storage in the region in the past 40 years, and the changes in carbon storage in 2030 were simulated. From 1980 to 2020, the urbanization process of the Guangxi Beibu Gulf Economic Zone accelerated, and carbon storage decreased significantly. The main reason for the decrease in carbon storage is the increase in construction land and the decrease in forest land and cultivated land, which is consistent with the characteristics of the carbon storage estimation in Guangxi studied by Chen et al.60. Therefore, in the future economic development of the region, attention should be paid to coordinating the relationship between the economy and the ecological environment.

Discussion on changes in carbon stocks under the three scenarios in 2030

In the carbon storage simulation under the three scenarios in 2030, the carbon storage in the Guangxi Beibu Gulf Economic Zone decreased compared with 2020 under the natural development scenario and the cultivated land protection scenario, but increased the carbon storage under the ecological protection scenario. From 2020 to 2030, the region will have the fastest rate of carbon storage reduction under the natural development scenario, which is basically consistent with the study of Zhang et al. 61. Under the natural development scenario, the land use change is simulated in 2030 based on the development trend from 1980 to 2020. In this scenario, the ecological environment is seriously damaged, and the contradiction between regional economic development and environmental protection is acute. Under the scenario of cultivated land protection, the Guangxi Beibu Gulf Economic Zone pays attention to the protection of cultivated land, restricts the conversion of cultivated land to construction land, and curbs the disorderly expansion of construction land to a certain extent. Under the ecological protection scenario, the types of land with strong carbon sink capacity, such as forest land, cultivated land, and water area, increased significantly, and the ecological environment was significantly improved, which greatly alleviated the contradiction between man and land. The simulation results of carbon storage in the three scenarios show that the Guangxi Beibu Gulf Economic Zone can effectively increase regional carbon storage by rationally planning construction land and paying attention to the protection of cultivated land and forest land. The cultivated land protection scenario is the best strategy to achieve a balance between economic development and environmental protection in the region.

The main reasons for the decline in carbon stocks

The carbon storage in the Guangxi Beibu Gulf Economic Zone is dominated by forest land. Despite the high quality of the local environment, the economy developed rapidly due to its location at an important hub of the Maritime Silk Road. Rapid economic development has led to the conversion of a large amount of forest land to construction land, which directly leads to the reduction of carbon storage in the region. According to the simulation results, the rapid decline in carbon stocks will remain constant under the natural development scenario. Under the cultivated land protection scenario, when a large amount of cultivated land is restricted from conversion, the disorderly expansion of construction land is curbed, and the carbon storage still shows a downward trend, but the decline rate slows down. It shows that there are great challenges to achieve carbon neutrality in the coastal economic belt, and the rational use of construction land, preventing the rapid expansion of construction land, and protecting land types with strong carbon sink capacity such as forest land and cultivated land are important measures to achieve carbon neutrality in the coastal economic belt. In summary, the disorderly expansion of construction land and the massive conversion of forest land and cultivated land in the Guangxi Beibu Gulf Economic Zone are the most important factors for the reduction of carbon storage. Therefore, this article preliminarily identifies five specific reasons:

(1) The process of urbanization is accelerating, encroaching on cultivated land and forest land. Guangxi Beibu Gulf Economic Zone is an important part of the South China Sea Economic Circle and the southern hub of the Belt and Road Initiative. As a result, the demand for construction land such as industrial, commercial, and residential land has increased rapidly in the region since 1980. In addition, the government's strong support for the modernization of the region has led to the rapid expansion of urban areas, resulting in a continuous decline in Cultivated land and forestland. (2) Local residents cut down trees at will. Guangxi's Beibu Gulf Economic Zone has a lot of forest resources, but the local villagers have a weak ecological concept, and to solve the problem of living energy, they cut down trees at will, resulting in serious damage to some forest land in the region and a decline in carbon storage. For example, in 2024, Pubei County, Qinzhou City, held several inspection hearings on cases of indiscriminate logging, and in 2024, Fangchenggang City announced the first batch of cases of indiscriminate deforestation, hoping to strengthen the protection of local forestry resources. (3) Unreasonable resource exploitation. On the one hand, the Guangxi Beibu Gulf Economic Zone is an important part of the South China Sea Economic Belt. The abundant forest resources in this region have made great contributions to the infrastructure construction of the South China Sea Economic Circle, but the overexploitation of forestry resources has led to accelerated degradation of forest land. For example, there are many small-scale forest farms in Qinzhou and Beihai, which can easily cause a waste of forestry resources. On the other hand, the Guangxi Beibu Gulf Economic Zone is rich in minerals such as ilmenite, kaolin, and clay. The rapid economic development of the region has led to a high demand for minerals and energy. Irrational mineral exploitation has caused great damage to arable and forestland. For example, although Qinzhou and Fangchenggang are rich in mineral resources, most of the mining enterprises are small and medium-sized enterprises with backward technology, and they have not been able to effectively deal with the forest land and cultivated land damaged in the mining process. (4) The phenomenon of illegal occupation of cultivated land by rural residents is serious. Due to the lack of urban construction planning, there is a serious problem of rural self-run housing in the local area. In the suburbs of developed cities in the Guangxi Beibu Gulf Economic Zone and the economic development zones with flat terrain and convenient transportation, a large amount of cultivated land and forest land have been illegally occupied, and the problem of illegal housing construction is very prominent. (5) Unreasonable regional planning has led to the decline of the carbon sink capacity of some lands. The local government's planning measures for land use in the region are greatly affected by the environment of the times, which makes the conversion between cultivated land, forest land, and construction land frequent in the short term. There are differences in the carbon sequestration capacity of secondary forests and primary forests, and the frequent conversion of cultivated land and construction land will also affect the carbon sequestration capacity of land.

The results show that the main factor for the change of carbon storage in this region is the conversion of cultivated land, forest land, and construction land. Therefore, from an ecological perspective, the efficiency of construction land use should be improved as much as possible for regional land use, and excessive encroachment of construction land on land types with high carbon storage, such as cultivated land and forest land, should be avoided. In the estimation of carbon stocks, the changes of carbon stocks are generally stable because different land use types have certain carbon storage capacities. Due to the limitations of the PLUS model and the InVEST model, the impact of policy planning and natural disasters on land use change cannot be taken into account when simulating land use scenarios, which may lead to differences in the simulation results. In the future, more accurate models will be used to simulate land use multi-scenario and estimate carbon stocks, to improve the research accuracy. Land use change has a great impact on the change of regional carbon storage, but the lifestyle of residents, economic development mode, and energy consumption type will also affect the change of carbon storage. Therefore, measures such as promoting the development of green industries, using clean energy, and energy conservation and emission reduction are also important links to achieve the "double carbon" goal.

This study combines theory and practice to analyze the impact of land use change on carbon storage in the Guangxi Beibu Gulf Economic Zone. This kind of analysis of carbon storage changes over a long period in important coastal economic zones of China has been less emphasized in previous land use studies. Guangxi Beibu Gulf Economic Zone is an important economic development zone in China, and realizing the ecological balance of Guangxi Beibu Gulf Economic Zone from the perspective of land use and increasing carbon storage is an urgent problem to be solved, and it is also an important topic to balance the relationship between territorial spatial planning and ecological concept.

Conclusions and recommendations

Conclusion

Under the vision of the "dual carbon" goal, Based on the data on land use change and carbon density, this paper uses the InVEST model and the PLUS model to simulate the land use in the Guangxi Beibu Gulf Economic Zone in 2030. The results show that from 1980 to 2020, the forest land in the Guangxi Beibu Gulf Economic Zone decreased significantly, and the construction land continued to increase. The continuous decrease of carbon storage under the natural development scenario and cultivated land protection scenario from 2020 to 2030 indicates that the region is facing great challenges in ecological environmental protection and carbon storage increase.

The spatiotemporal characteristics of ecosystem carbon storage in this region from 1980 to 2030 were analyzed, and the following conclusions were obtained:

(1) From 1980 to 2020, forest land was the main land use type in the Guangxi Beibu Gulf Economic Zone, but the construction land continued to expand, encroaching on a large number of other land types, forming a trend of "one increase and many decreases" in which the construction land increased and other land use types decreased. Guangxi Beibu Gulf Economic Zone has always been an important ecological reserve in the South China Sea Economic Circle. In 2013, China put forward the initiative to establish the "21st Century Maritime Silk Road", and the region has become an important foreign trade zone due to its prominent geographical location. The national and local governments have invested a lot of energy in the economic development of the region, and there has been a pattern of land use change of "one increase and many decreases". However, the large reduction of forest land in this area reflects the continuous decline of the quality of the local ecological environment, and the protection of forest land in this area should be strengthened to give full play to the ecological function of Guangxi Beibu Gulf Economic Zone in the Pearl River Basin.

(2) The forest land and cultivated land in the Guangxi Beibu Gulf Economic Zone are the most widely distributed, so the carbon storage in this region is dominated by forest land, followed by cultivated land. Grasslands, Waterland, and Unutilized land are sparsely distributed and sporadically interspersed between other land use types in the region. Based on the above findings, this paper concludes that the increase of carbon storage in this region should focus on the protection of forest land and cultivated land and the rational planning of construction land.

(3) In general, the conversion of cultivated land and grassland to forest land, and the conversion of forest land to cultivated land and construction land dominated the changes of carbon storage in the Guangxi Beibu Gulf Economic Zone. In 2030, there will be differences in land use scale and carbon storage under different development scenarios, and under the natural development scenario, the scale of construction land will increase the most, while the scale of forestland, grassland, and cultivated land will decrease significantly, and the carbon storage will decline sharply. Under the cultivated land protection scenario, the scale of cultivated land increases, the scale of forest land and grassland decreases, and the decrease in carbon storage is small. In the ecological protection scenario, the scale of forestland, water area, and grassland increased, and the carbon storage in the region was 12.6916 × 108t, an increase of 0.0936 × 108t or 0.7429% compared with 2020. As an important area for opening up to the outside world, the Guangxi Beibu Gulf Economic Zone should not only consider the ecological function of the region, but also consider the economic function of the region. Therefore, this paper concludes that in 2030, the region should refer to the development model of cultivated land protection scenario, appropriately develop the construction land in the region, reasonably protect cultivated land and forest land, and give full play to the economic and ecological functions of the region in the South China Sea Economic Circle.

(4) From the perspective of time, the carbon storage in the Guangxi Beibu Gulf Economic Zone showed a downward trend from 1980 to 2020, and the decline was the most significant from 2010 to 2020. Spatially, the high-value areas of carbon storage in this area are distributed in the northern part of Nanning City, the ecological reserves of Fangchenggang City and Chongzuo City, the hilly area of Qinzhou City and the southeast of Yulin City, with high vegetation coverage and strong carbon sequestration capacity. The low-value areas of carbon storage are mainly distributed in clusters in the urban areas of various cities and the coastal areas of "Qinbeifang". Therefore, the spatial distribution of carbon storage is obviously heterogeneous and characterized by "low in the middle and high in the periphery". Based on the above spatial distribution characteristics, this paper concludes that the spatial planning difficulty of increasing carbon storage in this region is low. Since the high-value areas of carbon storage are clustered, more attention can be invested in the northern part of Nanning, the ecological reserves of Fangchenggang and Chongzuo, the hilly areas of Qinzhou and the southeast of Yulin when the local government takes measures to increase carbon storage.

In the process of urbanization, important ecological regions, including the Beibu Gulf of Guangxi, are faced with the contradiction between economic development and ecological protection, which is consistent with the research of Chen et al.67. Under the strategic goal of achieving carbon peak and carbon neutrality, it is very important to achieve the healthy development of regional carbon sinks. This paper hopes to establish a balance mechanism between economic development and carbon sink enhancement through the study of carbon storage in the Guangxi Beibu Gulf Economic Zone.

In summary, the Guangxi Beibu Gulf Economic Zone should balance the relationship between economic development and ecological environmental protection, and the region should not only assume important economic functions in the South China Sea Economic Circle, but also always contribute to the ecological development of the South China Sea region. Therefore, the region should make efficient use of construction land to prevent its over-expansion, and rationally develop forest land and cultivated land, so that the region can not only provide food, energy, and forest resources for the surrounding developed areas, but also contribute to the healthy development of carbon sinks in the South China Sea Economic Circle.

Recommendations

Land use adjustment is an important factor affecting the change of carbon storage in regional ecosystems64,65. Based on the Chinese government's goal of peaking carbon dioxide emissions by 2030 and achieving carbon neutrality by 2060, and based on the results of multi-scenario simulations of land use and ecosystem carbon storage in the Guangxi Beibu Gulf Economic Zone in 2030, this paper proposes six suggestions to increase carbon sink capacity:

(1) Effectively protect cultivated land and strengthen the transformation of cultivated land protection.

The cultivated land in the Guangxi Beibu Gulf Economic Zone accounts for about 29% of the total area of the region, and strengthening the protection of cultivated land is conducive to enhancing the carbon sequestration capacity of the ecosystem in the Guangxi Beibu Gulf Economic Zone. It is necessary to persist in ensuring that the quantity and quality of cultivated land do not decline, and strictly control the occupation of cultivated land by other types of land use. At the same time, the agricultural sector should crack down on the illegal occupation of farmland.

(2) Adjust the layout of construction land and optimize the structure of construction land.

The construction land of the Guangxi Beibu Gulf Economic Zone is expanding year by year. The carbon sequestration capacity of construction land is low, so it is necessary to strictly follow the general requirements of land conservation, efficiently control the expansion of construction land, and coordinate the increment and stock of construction land. Prevent the disorderly expansion of construction land in the process of urbanization.

(3) The government should increase efforts to protect ecosystems and establish an ecosystem compensation mechanism.

We should firmly prioritize ecology and be green and low-carbon. Consolidate the ecological foundation barrier, protect the ecological core area and vulnerable areas, strictly abide by the ecological protection red line, and beware of the encroachment of cultivated land and construction land on forest land and grassland. Establish an ecosystem compensation mechanism to ensure the quality and stability of forest and grassland ecosystems, and actively implement ecological protection policies such as "closing mountains for afforestation".

(4) Rational development of mineral resources.

The government should enact strict mining regulations, with a particular focus on the rehabilitation of mining land. The government uses guidance to merge or shut down small mining areas with weak technology to prevent greater pollution of the land in the mining areas due to backward technology.

(5) Strengthen the management of local residents by grassroots organizations.

The government should pay attention to the role of grassroots organizations and carry out regular environmental protection publicity for local residents to raise their awareness of environmental protection. Government departments should also strengthen supervision and control of indiscriminate deforestation and illegal encroachment on farmland. At the same time, the local government should actively care about the residents' living energy problems, provide residents with clean energy, and prevent residents from indiscriminate deforestation.

(6) The local government should formulate an overall plan for carbon storage and carbon emissions, and improve the regional carbon sink capacity at the policy level.

The local government should implement a long-term plan to prevent the frequent conversion of arable land, forest land, and construction land in the short term. Pay attention to the protection of high-quality cultivated land and pristine forest land.