1. Introduction
Cultivated land, the result of comprehensive interactions between humans and nature [
1], is an important semi-natural resource for human survival and development [
2]. Being the most common type of land use, cultivated land ensures food security [
3], thus providing the material foundation for urbanization and industrialization while assuring multiple other non-productive functions recognized as positive externalities and public goods, such as regulation, support, and cultural benefits [
4,
5,
6]. In recent years, with the rapid development of urban agglomerations and urban circles, issues such as nonagriculturalization, nongrain production, fragmentation, marginalization, and abandonment of cropland caused by population transfer and construction occupation have grown [
7]; additionally, these issues have seriously constrained the process of CLIU and sustainable development [
8]. During construction of an ecological civilization in the new era in China, contradictions regarding the protection and intensive utilization of cultivated land have arisen in relation to the coordinated development of the population, the economy, nature, and the environment [
9]. Therefore, determining the spatial differentiation and driving mechanism of CLIU via scientific evaluation, revealing its driving mechanism, and providing a decision-making basis for zoning optimization and efficient utilization of cropland resources are important research topics for regional cropland protection and sustainable utilization.
Continuous population growth has driven improvements in agricultural efficiency [
10]. Since intensive agricultural land management was first proposed [
11], the law of diminishing land returns, theory of agricultural location [
12], theory of labor value [
13], and theory of land rental and price [
14] have laid a theoretical foundation for CLIU. Scholars have focused on the research and practice of assessing cropland use intensity. In recent years, China has increased its investment in chemical fertilizers, pesticides, and machinery in arable land [
15,
16], achieving a steady increase in grain production. However, 53.60% of the counties have had agricultural land use indices lower than 0.6 [
17], the inappropriate use of chemical fertilizers has led to an imbalance in soil fertility [
18], and the reduction in the use of organic fertilizers has also reduced the soil’s ability to sequester carbon [
19], indicating that the current efficiency and factors input structure of cultivated land needs to be further improved. In addition, the outflow of the rural population and increase in the urbanization rate has further limited the intensity of cropland utilization [
20]; the unreasonable use of this cropland can easily lead to overintensive utilization [
21]. Regarding cropland practices, the Dutch Skylark Group has developed a framework for farmer-initiated agricultural solutions; when farmers are incentivized by peers and supply chain partners, improvements in agricultural ecological efficiency and sustainable intensification are more likely to occur [
22]. China has implemented a strict farmland protection system, and scholars have evaluated the average land use intensity and relative standard deviation of land use intensity in counties [
17]. They have also calculated the dynamic balance between provincial cropland productivity and yield, as well as the relationship between cultivation distance and the sustainability of cropland use [
23]. Studies have shown that the total productivity of cropland in China has decreased, the north–south distribution has been uneven, the boundary center of grain production has moved northward [
24], and the intensity of arable land use and the proportion of sustainable compensated arable land in most areas are less than 0.7 [
23]. Additionally, more than 40% of cropland has begun to deteriorate, posing a threat to sustainability [
25]. The PMC-Index and PMC-Surface models have been used to evaluate the policy effects of cropland protection and intensive utilization by local governments in China, and the results show that there is significant room for improvement in the structure of policy tools, coordination of policy release agencies, and completeness of policy content in current local government public policies [
26]. Factors such as population size, land size, GDP, policy effects, farmer organizations, and agricultural capital productivity all affect CLIU; under the guidance of the DSG’s framework, the concept and paradigm of sustainable and intensive use of cultivated land have become new focuses.
On the connotation of CLIU, geographers have defined that effective input, sustained output, reduced environmental damage, and restored soil fertility are the core elements of cropland intensive use. Garnett et al. proposed that the sustainable intensive use of arable land, in response to the priority action goals of food security and food systems, has four premises: increased production, environmental sustainability, and multiobjective synergy [
27]. Firbank et al. argued that CLIU is associated with increases in the market dimension (agricultural production) without reductions in its nonmarket dimensions (environmental services); they used farm data in England and Wales to construct an indicator system from agricultural output and environmental output for empirical analysis [
28]. Xie et al. argued that adjusting the input–output relationship can improve the yield of farmland, reduce ecological damage, and maintain the resilience of cultivated land and that this approach is the key to CLIU; a case evaluation was conducted from an energy perspective [
29]. Influenced by the concept of sustainable intensification, scholars have conducted more comprehensive research on the evaluation and optimization of CLIU. Some ecologists analyzed influencing factors from aspects such as fertilizers, machinery, pesticides, labor, capital, energy, cultivated land productivity, cultivated land economic benefits, labor productivity, and land use mode transformation and used comprehensive evaluation and multiple regression models to evaluate China’s CLIU [
30]. The indicators reflected renewable environmental input, nonrenewable environmental input, nonrenewable industrial assistance, renewable organic assistance, agricultural output, and chemical pollution industrial input; the intensive use of farmland was evaluated via energy analysis and the livelihood framework to determine farmers’ livelihood types [
31]. Comprehensive factors reflected the levels of material productivity, the implicit flow of material output and stock, and the effects of environmental economics. Furthermore, the material flow method and Tobit model were used to evaluate CLIU at the household scale [
32]. In addition, the use of econometric models and spatial econometric models to explore the economic relationship between urbanization level and farmland utilization efficiency [
33], as well as economic agglomeration and sustainable intensification of farmland [
34], provides a new paradigm for exploring the transformation of CLIU.
In the evaluation of CLIU, much research has been conducted on the spatiotemporal evolution, driving mechanisms, obstacles, and sustainable utilization of cropland. Current hotspots of concern are manifested in two main topics. First are issues involving the relationship between CLIU and nongrain production; in the process of large-scale and intensive use of cultivated land, the phenomenon of nongrain fragmentation of cropland will inevitably occur. In some areas, the proportion of grain crop-sown area has decreased and, the density of land has increased [
35]; these changes will limit the effectiveness of CLIU. Second, from the perspective of input–output data, scholars have analyzed the causes and mechanisms of the nonagricultural transformation of farmland and believe that improving farmland irrigation facilities and increasing grain planting subsidies can increase enthusiasm for grain growth [
36]. To construct a framework for the interaction between cultivated land fertility and comprehensive utilization fertility, an evaluation index system was established, and the Tapio decoupling model was used to study the decoupling relationship between nongrain production and CLIU in China from in spatial and temporal dimensions [
37]. The spatial state of CLIU was evaluated via 3S technology. In some regions of China, the aggregation and cohesion index of farmland plots have been decreasing annually, while the density and fragmentation index of patches have gradually increased, which means that the methods and connotations of CLIU need to be transformed and upgraded [
38]. On the other hand, scholars have focused on both anti-intensive cultivated land use and sustainable cultivated land use; they have new approaches to CLIU that involve expanding scale [
39], strengthening investment [
40], protecting ecology [
41], maintaining sustainability [
42], and implementing sustainable intensive transformation. However, in economically developed regions of China, the phenomenon of anti-intensive land use is quite significant. Liang et al. believe that anti-intensification acts as a kind of counterforce manifestation of negative effects that hinder the positive effects of intensive utilization of arable land; they constructed an evaluation index system from three aspects—input degree, utilization degree and output efficiency—and conducted an empirical analysis of Jiangsu Province in China by using the geographical detector model [
43]. Another approach considers that CLIU results in energy flow between water resources, land, and food [
12], i.e., natural conditions can influence CLIU. Based on a framework of water, soil, energy, and food interactions [
44], Chen et al. constructed a sustainable utilization evaluation system for cultivated land that coordinates these four elements in CLIU and conducted a case study by using fuzzy comprehensive evaluation and coupling coordination models. The research findings showed that water resources are the key influencing factors for CLIU [
45].
The results of these existing studies are rich and provide important references for this research. However, there are still some points that can be further refined in terms of the research object, research level, and research content. First, the current research is insufficient, and the existing evaluation system for CLIU does not account for the roles of geographical location and natural factors. In analyses of the driving factors influencing CLIU, natural factors such as temperature and precipitation, which affect the productivity level of arable land, are missing. Spatial imbalance analyses of the level of CLIU and spatial non-stationary analyses of driving factors are needed. Second, research has not been sufficiently systematic; analyses of the level, influencing factors, and driving forces of CLIU have focused mainly on the internal aspects of the cropland use system, neglecting the impact of external land intensive use and natural conditions on cultivated land intensive use, which is a reflection of the driving force results of geographic exploration and geographic weighted regression analysis. Furthermore, interaction detection and driving mechanism analyses of the multiple factors that account for both internal and external factors are lacking; the integration of research methods also needs improvement. Third, research on urban agglomerations is lacking, though in the process of urbanization, a reduction in the amount of cultivated land caused by construction occupation is inevitable [
45]. Moreover, as the urbanization rate increases, declining interest in agricultural production has led to increasingly serious marginalization of arable land [
46]. Urban agglomerations are not only important carriers of urbanization development but also key areas for farmland protection and intensive utilization; evaluating the intensive utilization status of arable land in urban agglomerations promotes high-quality development of urbanization and sustainable, intensive utilization of arable land. The Beibu Gulf urban agglomeration is an important gateway to the 21st Century Maritime Silk Road, not only as a typical hub for the ASEAN-facing international corridor but also as a national-level model area for integrated land and sea development. In recent years, with the economic development of the Bay Area, the level of urbanization has increased annually, and the scale of construction land has expanded rapidly; thus, the protection and intensive use of cropland has been subjected to great challenges. This study contributes to the literature on CLIU with a systematic and cross-regional-scale analysis of cultivated land intensive utilization spatial heterogeneity, trying to address two scientific questions: (1) How do we understand the spatial heterogeneity of CLIU systematically and the manifestations at different scales? (2) What are the influencing factors and driving mechanisms behind the spatial heterogeneity of CLIU in urban agglomerations? The research findings may provide theory and policy references for the implementation of CLIU.
The Beibu Gulf urban agglomeration was considered as a case study in this study; methods such as principal component analysis (PCA), use of the Dagum Gini coefficient, geographic detection, and geographically weighted regression were used to investigate the spatial imbalance of CLIU, explore the spatial nonstationarity of key influencing factors, and uncover the driving mechanisms among multiple factors. The research findings may be helpful for the implementation of national sustainable intensification of cultivated land use in China and may also provide reference for future CLIU research in similar areas worldwide.
5. Conclusions
The case of the Beibu Gulf urban agglomeration in China shows that the spatial heterogeneity of CLIU is manifested by the spatial nonequilibrium of patterns and the spatial nonstationarity of factors, which helps to better promote inclusive spatial design and differentiated control systems.
On the spatial nonequilibrium scale, relatively high levels of CLIU were found in the Beibu Gulf urban agglomeration in the Jiangzhou District of Chongzuo city, the Fusui County of Chongzuo city, and the Port District of Fangchenggang city, while lower levels were found in the Hepu County of Beihai city, the Qinbei District of Qinzhou city, and the Lingshan County of Qinzhou city. The average level of CLIU in the region was not high; there was a significant spatial nonequilibrium, and the hypervariable density and provincial difference were the main factors.
On the spatial nonstationarity scale, among the independent effects of individual factors, the multiple cropping index, labor force index, and intensification of construction land had the largest impacts on CLIU and spatial differentiation; among the interaction effects of two factors, there were mainly nonlinear enhancements, with the interaction between the labor force index and the multiple cropping index being the most significant. The coefficients of temperature, multiple cropping index, and labor force index were relatively large and positive, while the coefficients of slope, precipitation, and urbanization were relatively small and negative; additionally, the driving forces of different factors on CLIU were spatially nonstationary.
On the driving mechanisms scale, the spatial heterogeneity of CLIU in the Beibu Gulf urban agglomeration was manifested by the spatial nonequilibrium of the pattern and the spatial non-smoothness of the factors; furthermore, the roles of multifactors were embodied as follows: the natural factors of cropland were the foundation, the factors of CLIU were dominant, and the factors of intensive land use were auxiliary.