**1. Introduction**

Cultivated land serves as an important prerequisite and guarantee for grain yield, and the material basis for human production, life and survival. Since the 21st century, the term "Anthropocene" has been widely mentioned and a series of risk events such as "gray rhinoceros" and "black swan" have evolved into global food crisis, environmental crisis, ecological crisis and major public events, making the whole society more and more concerned about cultivated land conservation. With the further increase of global population, increasingly higher requirements of quality of life, limited growth in cultivated land area, and a series of hazards of environmental pollution to the ecosystem, food security has become an important topic of global sustainable development [1–3]. According to the report "The State of the World's Land and Water Resources for Food and Agriculture in 2021: System at breaking point" released by the Food and Agriculture Organization of the United Nations (FAO), the state of the planet's soil, land and water resources has deteriorated dramatically over the past decade, increasing pressure to meet the food needs

**Citation:** Niu, S.; Lyu, X.; Gu, G. A New Framework of Green Transition of Cultivated Land-Use for the Coordination among the Water-Land-Food-Carbon Nexus in China. *Land* **2022**, *11*, 933. https:// doi.org/10.3390/land11060933

Academic Editors: Yongsheng Wang, Qi Wen, Dazhuan Ge and Bangbang Zhang

Received: 12 May 2022 Accepted: 15 June 2022 Published: 17 June 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of nearly 10 billion people worldwide by 2050. If agricultural water withdrawals are likely to increase by about 35% in order to achieve a 50% increase in grain yield, the world will face a major challenge in providing water, energy and food. Admittedly, the enormous pressure to increase grain yield also comes with the cost of global ecological degradation. Deforestation for agriculture is estimated to be a significant source of greenhouse gas emissions, accounting for 17% of global carbon emissions, as well as harms such as soil erosion and biodiversity reduction [4]. Moreover, the heavy use of chemical fertilizers, pesticides, and plastic films can lead to significant nitrous oxide emissions and affect global warming [5]; excessive use of nitrogen and phosphorus fertilizers can cause (leaching, erosion) pollution of water resources, and its production can result in CO2 emissions [6]; plastic residual films can damage the soil structure of the tillage layer, causing difficult infiltration of groundwater and secondary soil salinization [7]; pesticides present a risk if they are not used in a proper professional way, and even damage the ecological environment through the atmosphere, water bodies, soil, and food [8]. Therefore, there is an urgent need to find sustainable ways to increase food supply and protect the environment in agroecosystems.

As a traditional agricultural power, China feeds 22% of the world's population with 7% of the world's cultivated land [9]. Its grain yield has remained stable at over 1.3 trillion jin (a metric unit equal to exactly 500 g) for seven consecutive years [10], but there is also a "double high" phenomenon of high stocks and high imports. The rapid development of urbanization has led to the loss of cultivated land [11], and the number in hectares per capita of cultivated land areas is less than 1/2 of the world average; the pursuit of unit grain yield and excessive intensive production have brought serious negative environmental effects, and exposed cultivated land resources to the serious situation of declining soil organic matter, soil erosion, soil acidification, rapid decline of groundwater, and farmland pollution [12,13]. China's cultivated land system is on the verge of breaching its environmental boundary threshold. Relying on a large number of chemical inputs, China consumes nearly a third of the world's chemical fertilizers and uses 2.5 times more pesticides per unit area than the world average [14]. N and P losses in 2018 were 821.5 kilotons and 2137.8 kilotons, respectively, which fall in the high-risk zone [15]. China accounts for 13% of global irrigation water use, has a blue water footprint of 224.5 billion cubic meters and suffers from acute water scarcity [16]. China contributes 33% and 36% of global excess agricultural nitrogen and phosphorus and is responsible for 28% of all global N2O emissions from croplands [3]. In addition, 19.4% of China's cultivated land is subject to soil contamination, which adversely affects grain yield [17,18]. In the short term, China's food supply is relatively adequate and maintains a tight balance with the ecological environment. However, in the long term, the spatial distribution of China's water, land and food resources is uneven [12] and ensuring food security remains China's top priority. The "North-to-South Grain Transfer" has put the north, where water resources are insufficient, in a dilemma of water resource "overload" and ecological environment degradation, which is not conducive to sustainable socio-economic development. As China's 14th Five-Year Plan suggests, emphasis should be, once again, put on promoting green development, pushing comprehensive green transition of economic and social development, and modernizing harmonious coexistence between man and nature. As stated in Subsection 8, "The action for consolidating and enhancing carbon sink" of the Action Plan for Carbon Dioxide Peaking Before 2030, efforts shall be stepped up to promote carbon emission reduction and carbon sequestration in agriculture and rural areas, develop green, low-carbon and circular agriculture, and take action to improve the quality of cultivated land [19]. Therefore, the coordinated and sustainable development of water resources, land resources, energy (carbon) and food systems is a major issue that the country needs to address urgently.

Food, energy and water are the three basic human needs, and their coupling has become a major global concern [20–22]. The society also faces major challenges in providing water, energy and food [23]. Bonn 2011 Conference: The Water, Energy and Food Security Nexus first proposed using "nexus" to see the complex relationship between the three. In the 17 Sustainable Development Goals (SDGs) set by the United Nations,

SDG 2: Zero Hunger (sustainable food production), SDG 12: Ensure sustainable consumption and production patterns (transition to sustainable development), and SDG 15: Ensure the conservation, restoration and sustainable use of terrestrial ecosystems (protect terrestrial ecosystems and promote the sustainable use of ecosystems) relate to the sustainable use of each element of the WEF respectively. The WEF nexus has become the basis for achieving the SDGs [24]. In China, WEF-nexus-related studies are limited at the inter-regional level, mainly focusing on the safety evaluation and influence factors of WEF [25–27] or on one aspect or "water-land", "land-carbon", "water-carbon" and "food-water" relationships [28,29], which is not sufficient to coordinate the sustainable development of regional agriculture. Theoretically, Zhao [30] proposed a system coupling model of "water-land-energy-carbon" to reflect the matching relationship of various regional resources, the efficiency of exploitation and utilization, and the degree of human and social impacts on the environment, so as to achieve the goals of efficient utilization of regional water-land-energy resources, carbon emission reduction, and social production optimization. Subsequently, it was extended to water-land-food-energy nexus [31], landwater-food-environment nexus [32] or food-water-land-ecosystem nexus [33]. However, food security deserves further attention in practical studies of cultivated land-use.

The above studies show that the WEF nexus, based on broad and multiple perspectives, can make a significant contribution to the field of sustainability. However, there is still a scarcity about WLFC nexus; synergistic water-land-food-carbon development is rarely discussed and the complexity of policy formulation and management regarding GTCL is still in shortage. Therefore, the motivation and novelty of this study is to fill this gap in the WLFC-nexus field and to quantitatively assess the development of GTCL from the perspective of WLFC. The details of the objectives are as follows: 1. to construct a theoretical framework for WLFC-based GTCL; 2. to dissect the spatial and temporal differentiation of GTCL; 3. to explore the coupling coordination of WLFC in cultivated land systems; 4. to propose a ductile regulation strategy based on WLFC nexus.

#### **2. Theoretical Framework and Research Methods**

#### *2.1. Case Study and Data Sources*

Given the availability of data, choosing the panel data of 31 provinces, municipalities and autonomous regions of China from 2001 to 2021 as research units, this study treated 31 Chinese provinces, municipalities and autonomous regions. Figure 1 depicts the study area, but for the time being, due to data loss, Hong Kong, Macao and Taiwan are not included. The data in this paper are obtained from the 2001, 2006, 2011, 2016 and 2021 China Statistical Yearbook and China Rural Statistical Yearbook.

**Figure 1.** Study area.
