*3.3. Water-Land-Food-Carbon" Coupling Coordination Analysis*

From Figures 9 and 10, the coupling coordination degree of "water-land-food-carbon" of all provinces, municipalities and autonomous regions in China from 2000 to 2020 showed a trend of decreasing before increasing, and the lag of "water-land-food-carbon" system was improved. The coupling coordination in Northeast China first decreased and then increased, among which, the coupling coordination in Heilongjiang was higher than the other two provinces and changed from moderate coordination (0.55) to good coordination (0.74) with a rate of 35.15%; Jilin changed from low coordination (0.50) to good coordination (0.62); Liaoning changed between low coordination and reluctant coordination (0.44→0.38→0.46→0.38→0.49). In North China, Beijing's coupling coordination changed more steadily, all in a low coordination state; Inner Mongolia was between low and reluctant coordination (0.44→0.38→0.46→0.38→0.49) during 2000–2015, and the coupling coordination was higher than the other four provinces and municipalities in 2020, reaching a good coordination state (0.62). Anhui in East China changed from low coordination (0.50) to good coordination (0.57). Except for Anhui, the areas of Jiangsu, Zhejiang, Jiangxi and Shandong increased on the whole and were basically between low coordination and reluctant coordination, while Shanghai and Fujian decreased on the whole and the coupling coordination changed from low coordination (0.41, 0.44) to reluctant coordination (0.36, 0.39). Henan, Hubei and Hunan in Central China had less changes and were basically in the state of low coordination (0.48, 0.48, 0.49). Guangdong, Guangxi and Hainan in South China developed from low coordination (0.42, 0.45, 0.43) to reluctant coordination (0.36, 0.36, 0.31) from 2000 to 2005; from 2005 to 2020, Guangdong and Guangxi developed into low coordination (0.41, 0.43), and Hainan was still in reluctant coordination, but with a relatively large increase of 11.42%. Shaanxi, Gansu, Ningxia and Xinjiang in Northwest China all changed from low coordination (0.45, 0.43, 0.48, 0.45) to reluctant coordination, and then to low coordination before increasing to low coordination (0.45, 0.47, 0.48, 0.49); Qinghai changes were more volatile, developing from reluctant coordination (0.39) to verge of disorder (0.28, 0.30) before rising to low coordination (0.45). The coupling coordination of Tibet in Southwest China was relatively low, developing from reluctant coordination (0.39) to verge of disorder (0.28) from 2000 to 2005, changing to reluctant coordination (0.36) from 2005 to 2020, and showing an overall decreasing trend (0.39→0.36); Ningxia and Shaanxi showed a decreasing trend but were basically in a low coordination state; Gansu, Qinghai and Yunnan were in low coordination. The "water-land-food-carbon" system showed lagging development.

**Figure 9.** "Water–Land–Food–Carbon" coupling coordination degree. Note: BJ, TJ, HB, SX, IM, LN, JL, HLJ, SH, JS, ZJ, AH, FJ, JX, SD, HEN, HUB, HUN, GD, GX, HAN, CQ, SC, GZ, YN, ZX, SAX, GS, QH, NX, XJ is the abbreviation of Beijing, Tianjin, Hebei, Shanxi, Inner Mongolia, Liaoning, Jilin, Heilongjiang, Shanghai, Jiangsu, Zhejiang, Anhui, Fujian, Jiangxi, Shandong, Henan, Hubei, Hunan, Guangdong, Guangxi, Hainan, Chongqing, Sichuan, Guizhou, Yunnan, Tibet, Shannxi, Gansu, Qinghai, Ningxia, Xinjiang.

**Figure 10.** Development trend of coupling degree and coupling coordination degree of "Water–Land– Food–Carbon".

#### **4. Discussion**

#### *4.1. Theoretical Implications of GTCL*

At present, the Chinese government aims to promote green development and the overall green transition of economic and social development. The purpose of this paper is to realize a green, low-carbon, efficient and intensive spatial pattern of sustainable development of cultivated land-use based on the coupling coordination effect of "waterland-food-carbon" (Figure 2). From the perspective of a geographical pattern, the cultivated land-use system is not only a basic material space for economic and social development, but also an important part of rural habitat. In the process of promoting GTCL, priority shall be given to complex adaptive relationships between the whole and the local (element sharing and system coupling), conservation and development (sustainable use and food security), and equity and well-being (intergenerational balance and ecological welfare). For this reason, there is an urgent need to conduct theoretical and practical research on GTCL from the perspective of resource-factor linkage. GTCL is an inherent requirement for driving regional high-quality development, which refers to the sustainable use of cultivated land gradually shifting away from the dependence on high factor consumption and agroecological damage to a green development approach in which economic growth and resource conservation, carbon emission reduction and farmland ecosystem improvement are reinforcing each other. GTCL is not only the essential requirement of the natural properties of land, but also the inevitable requirement of sustainable utilization of resources and ecological civilization construction, as well as the value requirement of realizing the green transition of economic and social development in an all-round way. To promote GTCL, attention must be paid to the conceptual innovation from "independent" means of production to "systematic" resource community, as well as the transition from extensive use and excessive mode to a large-scale, intensive and sustainable way, thus moving towards the unification of value goals from farmers' personal interests to social public interests. Thus, there is a recognition that exploring the WLFC nexus under this framework (Figure 2) is a crucial first step toward effectively implementing its emerging policies for GDCL. For example, policymakers can better formulate legally binding development and protection objectives by analyzing the interaction between WLFC subsystems, and promote the combination of relevant policies and broader socio-economic activities in a timely manner, which will be beneficial to the common realization of the objectives of sustainable use of cultivated land, increase of food production, economic transformation, optimization of resources and environment, and mitigation of climate change.

#### *4.2. Impact of GTCL on the WLFC Nexus*

A compelling reason for analyzing GTCL from the "water-land-food-carbon" perspective is that the degree of water-soil matching, cultivated land reuse, grain yield, cultivated land-use and carbon emissions play a crucial role in achieving sustainable cultivated land-use. "Waterland-food-carbon" is the external condition that leads to changes in GTCL; GTCL is the internal basis that influences the synergy of "WLFC", and the external cause works through the internal cause. According to the analysis above, the development strategies in different periods have different emphasis on "WLFC". From 2000 to 2020, industrial pollution caused by the rapidly developing of industrialization had a negative impact on the resources and environment, the modern agricultural construction was started after the abolition of agricultural tax, and urban sprawl in the period of rapid urbanization led to the massive loss of cultivated land. After that, cultivated land protection policies were integrated into a larger natural resources management framework at national level, which improved the coupling coordination degree of WLFC. Furthermore, the in-depth adjustment of the global industrial chain, supply chain and value chain represented by grain trade has produced a transmission effect on many uncertain risks. However, the policy framework of cultivated land protection has been constantly improved, including regulations, policies, subsidies, standards, technical measures, etc. [49]. Overall, the "W"-shaped fluctuating uptrend of GTCL is observed. That is naturally why the average values of the comprehensive transition index exhibit decrease first and then increase with a

repeatedly periodic dynamic process. Other compelling reasons include the positive effects of the green transition in soil conservation, soil carbon sequestration, soil organic matter levels, water and nutrient retention, and biodiversity support, especially for farmland and pastures with low productivity or environmental sensitivity. In general, when the degree of soil-water matching is close to the critical range, green use plays a dominant role in controlling grain yield and carbon emissions, e.g., the Xinjiang Uygur Autonomous Region and the Inner Mongolia Autonomous Region in Northwest China (Figures 3e and 6); if GTCL is not properly regulated, then no matter how high the degree of land-water matching is, it cannot further increase grain yield or reduce carbon emissions, e.g., Zhejiang Province and Fujian Province in East China (Figure 8a,b and Figure 9). Since the coupling of "water-land-food-carbon" is complex and involves the overlapping combination of virtual resource flows and remote utilization patterns in the context of cross-food trade, the examination of the coupling coordination between the WLFC subsystems will help to deeply analyze the external effects of GTCL, mainly from two perspectives. First, the cultivated land-use system is a complex giant system with coupling interactions of single or multiple subsystems in the process of green transition, and its transition goal, transition direction, transition path and its internal mechanism are mainly governed by water, land, food and carbon. Second, whether it is the degree of water-land matching, cultivated land reuse and grain yield or carbon emission, they are all influencing factors in the process of GTCL. Based on the spatial and temporal patterns of GTCL and the results of "water-land-food-carbon" coupling coordination analysis, the influence of external factors can be fully reflected by monitoring data. Relying on the macroscopic analysis model established on the basis of the transition mechanism, the "details" of the process that are difficult to quantify can be greatly reduced, thus ensuring the scientific and reasonable research results.
