The Use of Cultivated Land for Multiple Functions in Major Grain-Producing Areas in Northeast China: Spatial-Temporal Pattern and Driving Forces

Abstract

The increasing scarcity of cultivated land resources necessitates the continuous change in cultivated land functions. Cultivated land has gradually changed from being used for a single function to multiple functions. The use of cultivated land for multiple functions has become an important way to achieve the sustainable use, management, and protection of cultivated land. In this, the development of different functions of cultivated land must be coordinated. Thus, clarifying the evolution trend of the use of cultivated land for various functions, calculating the coupling and coordination degrees of these multiple functions, and identifying the driving factors in these uses play important roles in realizing the orderly development of cultivated land multifunctionality. This paper defined multifunctioning cultivated land as containing a production function, a social function, and an ecological function. Based on the socioeconomic panel data and geospatial data of Heilongjiang, Jilin, and Liaoning, which are the major grain-producing areas of northeast China, in the years 2005, 2010, 2015, and 2020 we calculated the multiple function coupling coordination degree of cultivated land using the Coupling Coordination Degree Model and identified the driving forces in the evolution of the spatial-temporal pattern of cultivated land multifunctionality using Geodetector. The results show that from 2005 to 2020, there were significant regional differences in terms of the production, social, and ecological functions of cultivated land in the research areas. The multifunctional coupling coordination degree of cultivated land in the study areas was gradually improved. The spatial-temporal evolution of the multifunctional coupling coordination degree of cultivated land was found to mainly be influenced by the level of agricultural development, such as the level of per capita disposable income and the rate of effective irrigation of cultivated land. The government should attempt to guarantee the comparative benefits of agricultural production to increase the income level of farmers; increase investment in agricultural infrastructure construction to improve the level of agriculture development; and implement a strict farmland protection policy to achieve the continuous improvement of the productivity of cultivated land, realize the ordered development of coupling, and improve the coordination of the use of cultivated land for multiple functions. The results of this study are applicable not only to northeast China but also to other major grain-producing areas that are under pressure to protect their cultivated land and achieve the suitable use of cultivated land.

1. Introduction

Healthy cultivated land use systems have functional continuity [1]. The function of cultivated land refers to the ability of cultivated land to provide products and services, and it evolves in a complex process from a single function into multiple coordinated functions [2,3]. The multifunctionality of cultivated land has become an essential attribute of its use [3].

Cultivated land has traditional production and service functions necessary for human society [4]. With the development of the economy and technology, cultivated land is now not limited to traditional functions and has transitioned from having a single function to multiple functions [3,5]. As a basic resource and material guarantee for human survival, the multiple functions of cultivated land include its use to provide products and services necessary for human survival and development [6]. These functions of cultivated land include production functions, such as providing food, vegetables, and oil; ecological functions, such as regulating the atmosphere and maintaining water and soil; social functions, such as ensuring farmers’ livelihood and maintaining national food security; cultural functions, such as providing farming landscapes and spatial landscapes; and non-commercial functions, such as for building space reserves and other space-bearing reserve functions [7]. Thus, the utilization of cultivated land has led to its gradual expansion from production functions to social security functions and ecological functions [3,5,7]. Cultivated land is an irreplaceable basic resource needed for human survival and development and a core element contributing to food security and regional sustainability [8,9,10]. Therefore, the multifunctional use of cultivated land is an important concept that must be considered when assessing reginal cultivated land use changes and their effects on the sustainable use of cultivated land. Thus, the coupling coordination level of cultivated land used for multiple functions should be determined, as it can provide a feasible reference for optimizing the efficiency of the regional utilization of cultivated land [11].

The multiple functions of cultivated land start from its use for agricultural functions [12,13,14,15]. The Global Land Project (GLP) takes the multifunctionality of land as the basic framework with which to analyze the coupling of natural, ecological, social, and economic systems [16]. The multiple functions of cultivated land have been widely considered by scholars. The existing research on this concept mainly focus on the assessment of multiple functions of cultivated land [1,3,17,18,19]. The assessment of the multiple functions of cultivated land has been widely discussed since the implementation of the Land Use and Land Cover Change (LULL) program [3,20]. The current studies focusing on the assessment of the multiple functions of cultivated land concentrate on two major aspects: the assessment of each land use function individually and the comprehensive assessment of the multiple functions of cultivated land. Studies focused on the assessment of individual land use functions mainly aim to quantify each function of cultivated land, such as its production, ecological, social, or economic functions [21,22,23,24,25]. In the research on comprehensive cultivated land multifunction assessment, a growing number of studies have obtained the total intensity level of different function of cultivated land by summing the value of each cultivated land function [19,26] and some studies have analyzed the driving forces behind the use of cultivated land for multiple functions [3,27,28].

However, the emphasis of most studies was often on a single function or the impact issues of separate functions. While the comprehensive function of cultivated land is the result of coupling the coordinated development of the multiple functions, the multifunctional coupling coordination degree of cultivated land has rarely been reported on, much less the spatial-temporal pattern and driving forces of the coupling coordination degree of cultivated land used for multiple functions. Thus, our understanding of the multiple functions of cultivated land is poor. Furthermore, long-term studies can be used to examine the evolution mechanism of cultivated land used for multiple functions and offer a policy reference to increase the use of cultivated land for multiple functions, an area which lacks attention. As it stands, in the research of cultivated land multifunctionality, less focus has been provided to major grain-producing areas. In order to improve the shortcomings of the existing research, a better understanding of the spatial-temporal pattern of cultivated land used for multiple functions in major grain-producing areas is needed, and the spatial-temporal pattern, driving forces, and influencing mechanisms of the multifunctional coupling coordination degree of cultivated land need to be identified as well. Through this, effective policies can be promoted to improve the use of cultivated land to achieve the goal of sustainable cultivated land use, especially in the major grain-producing areas.

As an important grain base, northeast China is a major grain-producing area, shouldering the responsibility of ensuring national food security. However, in face of the challenges of cultivated land degradation [29] and reduced agricultural production efficiency [30], the sustainable use of cultivated land in northeast China is now seriously threatened. Thus, based on the main functional positioning of northeast China as a major grain-producing area, identifying the spatial-temporal evolution of the multifunctional coupling coordination degree of cultivated land is of great importance in order to take full advantage of cultivated land resources, maximize cultivated land utilization efficiency and yield, ensure ecological security, and maintain social stability in northeast China. Additionally, it is of strategic significance for ensuring the national food security of China.

Considering the research gaps highlighted above and the strategic positioning of the major grain-producing areas in northeast China as a national breadbasket, this study used Heilongjiang province, Jilin province, and Liaoning province as its research area to analyze the spatial-temporal pattern of cultivated land used for multiple functions, including its production, social, and ecological functions; assess the multifunctional coupling coordination degree of cultivated land; identify the spatial-temporal pattern trend and driving forces of the multifunctional coupling coordination degree of cultivated land; and finally put forward measures for the optimization of the usage of cultivated land for multiple functions. The remainder of this paper is structured as follows. Section 2 describes the materials and methods. The Comprehensive Index Model is used to calculate the evaluation value of cultivated land functions. The Coupling Coordination Degree Model is used to assess the multifunction coupling coordination degree of cultivated land. Geodetector is used to explore the driving forces behind the evolution of the multifunctional coupling coordination degree of cultivated land. Section 3 discusses the empirical results of the models. Section 4 and Section 5 present the discussion and conclusions of this study, respectively.

2. Materials and Methods

2.1. Study Area

Heilongjiang province, Jilin province, and Liaoning province are located in the northeast of China, in the hinterland of northeast Asia, with the geographical coordinates of 38°43′–53°33′ north latitude and 118°53′–135°05′ east longitude (Figure 1). The three provinces are surrounded by mountains and water with plains in the middle. Topographically, this region has a high altitude in the southeast, north, and northwest and a low altitude in the northeast, southwest, and central areas. The terrain of this area is mainly composed of mountains, hills, plains, and water bodies. The northeast plain, which consists of Sanjiang Plain, Songnen Plain, and Liaohe Plain, is the largest plain in China, and has extensive agricultural development. There are many rivers in the region, such as Yalu River, Songhua River, Mudanjiang River, Heilongjiang River, and Wusuli River. Most of the region is characterized by a temperate continental monsoon climate, featuring concurrent rain and heat. The region is widely covered by Black soil and it is a highly fertile area, making it suitable for crop growth. In 2021, the total economic output of the three provinces was CNY 5569.9 billion, and their total grain output was 144,456,400 tons, accounting for 21.2% of the total national grain output. The area of sown grain was 23,815,900 hectares, accounting for 20.3% of the total national area sown with grain. The total land area of the three provinces is 8.1 × 10⁷ hm², and the total cultivated land area is 3.0 × 10⁷ hm², accounting for 37.4% of the total land area. Thus, it is an important grain-producing area in China and a national grain base. By taking the three provinces as our study areas in order to analyze the spatial-temporal evolution and the driving forces of the multifunctional coupling coordination degree of cultivated land, we aimed to not only optimize the multifunctional coupling coordination of cultivated land in the three provinces, but also provide a useful reference for the multifunctional utilization of cultivated land in other major grain-producing areas.

2.2. Data Sources

The statistical data used in this study mainly came from the Liaoning Statistical Yearbook, Jilin Statistical Yearbook, Heilongjiang Statistical Yearbook, China Statistical Yearbook (County-Level) in 2006, 2011, 2016, and 2021, as well as the officially released statistics for national economic and social development for Liaoning, Jilin, and Heilongjiang. The geographic information data mainly came from http://www.gscloud.cn/ (accessed on 12 July 2022), while the vector data, land use data, and annual average precipitation data for Liaoning, Jilin, and Heilongjiang came from https://www.resdc.cn/ (accessed on 17 July 2022). In this study, 36 cities (districts) in Liaoning, Jilin, and Heilongjiang were selected as the study units.

2.3. Methods

In this subsection, the methods used in this study are presented. Section 2.3.1 presents the multifunctional evaluation index system used for cultivated land and introduces the reasons for the selection of the index used for each function. Section 2.3.2 presents the indicator standardization method used and the process used for the calculation of indicator weight and function weight. Section 2.3.3 describes the Comprehensive Index Model used to calculate the value of each cultivated land function and the Coupling Coordination Degree Model used to assess the multiple function coupling coordination degree of cultivated land. Section 2.3.4 displays the calculation process followed by Geodetector, which was used to explore the driving forces of the evolution of the multifunctional coupling coordination degree of cultivated land.

2.3.1. Establishment of Multifunctional Cultivated Land Evaluation Index System

As the cultivated land system is an open and complex system, this study focused on the production, social, and ecological functions of cultivated land and comprehensively considered other factors such as nature, society, economy, and ecology in order to select multifunctional indicators of cultivated land in major grain-producing areas in northeast China. On the basis of the comprehensive consideration of the systematicness, scientificity, objectivity, and data availability of multifunctional indicators of cultivated land, twelve indicators were selected based on three aspects, production, social, and ecological functions, to establish a multifunctional evaluation index system at the city scale in the study areas (shown in

Table 1. Multifunctional cultivated land evaluation index.

Function	Indicator	Calculation Method	Trend	Indicator Weight (%)	Function Weight (%)
Production function	Cultivated land reclamation rate	Cultivated land area/total land area (%)	+	20.1	46.2
	Per capita cultivated land area	Cultivated land area/total population (hm2/person)	+	26.5
	Grain crop yield	Grain output/total sown area of grain crops (kg/hm2)	+	20.5
	Per hectare agricultural output	Gross agricultural output/cultivated land area (CNY 10,000/hm2)	+	17.6
	Per hectare mechanization level	Total power of agricultural machinery/cultivated land area (kW/hm2)	+	15.3
Social function	Food self-sufficiency rate	Grain output × (permanent resident population × 400 kg/person)−1 (%)	+	35.9	26.2
	Agriculture contribution to GDP	Gross agricultural output/regional GDP (%)	+	25.1
	The income ratio between urban and rural residents	Rural per capita disposable income/urban per capita disposable income (%)	+	12.7
	The land-bearing capacity for the rural labor force	Number of rural agricultural employees/cultivated land area (person/hm2)	+	26.3
Ecological function	The ecological advantage of cultivated land	Paddy field area/cultivated land area (%)	+	33.3	27.6
	Farmland eco-diversity index	$-\sum a_i\ln a_i$$, \mathrm{where} a_i$ is the ratio (%) between the sown area of various crops and the total area sown with farm crops	+	38.3
	Fertilizer load of cultivated land	Fertilizer application amount/cultivated land area (t/hm2)	-	28.4

). The reasons for indicator selection were as follows:

• Production function

The production function represents the use of cultivated land to produce food crops and cash crops, which is the basic function of cultivated land [7,31]. Cultivated land’s production function provides agricultural products for society and economic income for farmer households [32]. The cultivated land reclamation rate is the ratio of reclaimed land area to the total land area and can effectively reflect the development and utilization of cultivated land resources in a certain area. The higher the cultivated land reclamation rate is, the higher the intensity of land development and utilization is [25,26]. The per capita cultivated land area, grain crop yield, per hectare agricultural output, and per hectare mechanization level reflect the availability of cultivated land resources per capita. Moreover, the grain output per unit area, gross agricultural output per unit area, and mechanization level of cultivated land utilization reflect the production function of cultivated land. Keeping other factors constant, the more inputs of productive factors there are per unit area of cultivated land, the more output is generated [32,33]. In the same token, the higher the degree of agricultural mechanization is, the higher the production capacity of cultivated land resources is [34,35]. Therefore, the five indicators of cultivated land reclamation rate, per capita cultivated land area, grain crop yield, per hectare agricultural output, and per hectare mechanization level were selected to comprehensively express the production function of cultivated land under certain natural conditions and production factor inputs.

• Social function

Social function refers to the role played by cultivated land in farmers’ livelihood and employment security [7]. With regard to the function of guaranteeing farmers’ basic livelihood, it is necessary to comprehensively consider the grain supply capacity and economic income supply capacity of cultivated land, as well as its capacity to guarantee stable employment. The food self-sufficiency rate reflects the ability of the output of cultivated land to meet the regional population’s food demand. The agricultural contribution to Gross Domestic Product (GDP) and the income ratio between urban and rural residents reflect the influence of agriculture on the national economy, its contribution to the national economy, and its ability to guarantee the livelihood of rural residents [25,34,36]. Meanwhile, the land-bearing capacity of the rural labor force reflects the ability of agriculture to guarantee employment for the rural population [37]. Therefore, in this study we selected four indicators, namely, food self-sufficiency rate, agriculture contribution to GDP, the ratio of income between urban and rural residents, and the land-bearing capacity of the rural labor force, to comprehensively express the social function of cultivated land under certain social and economic conditions.

• Ecological function

Ecological function relates to the role played by cultivated land in climate regulation, soil and water conservation, biodiversity maintenance, and relieving environmental pressure [7,38]. If a paddy field area is larger, its biodiversity level is higher and its ecological security maintenance function is stronger [39]. The ratio between the area of paddy field and cultivated land indicates the ecological advantage of the cultivated land category. The farmland ecosystem diversity index reflects the richness of crop varieties and the strength of ecological functions [40]. The farmland ecosystem diversity index is the ratio of the area sown with major food crops such as wheat, paddy rice, corn, soybean, and potato to the area sown with major cash crops such as oil and cotton out of the total area sown with staple farm crops in the study areas. The larger the index value is, the higher the crop variety is and the stronger the ecological functions are [41]. In this study, the fertilizer load of cultivated land was selected as a negative indicator of the environmental pressure on cultivated land. This is calculated as the ratio of the amount of fertilizer applied to the total cultivated land area. The larger the indicator value is, the greater the environmental pressure is. Therefore, in this study we selected three indicators, ecological advantage of cultivated land, farmland ecosystem diversity index, and fertilizer load of cultivated land, to comprehensively express ecological functions in a specific area.

2.3.2. Indicator Standardization and Weight Calculation

• Standardization of Indicators

In order to eliminate the influence of indicator unit dimensions and ensure the comparability of each indicator, the range standardization method was used to standardize each indicator. The calculation formulas of positive and negative indicators are as follows:

$$\mathrm{Positive} \mathrm{indicator}: X_{ij}=\frac{x_{ij}-x_{jmin}}{x_{jmax}-x_{jmin}}$$

(1)

$$\mathrm{Negative} \mathrm{indicator}: X_{ij}=\frac{x_{jmax}-x_{ij}}{x_{jmax}-x_{jmin}}$$

(2)

where $X_{ij}$ is the value after standardization, $x_{ij}$ is the actual value of the $j$ indicator in $i$ city, $x_{jmax}$ is the maximum value of $j$ indicator, and $x_{jmin}$ is the minimum value of $j$ indicator. The trend of each indicator is listed in Table 1.

• Calculation of indicator and function weight

Subjective and objective determination methods were used to comprehensively determine the indicator weight. The Yaahp software was used to determine the subjective weight, the entropy method was used to determine the objective weight, and the comprehensive weight was obtained by the weighted average of the subjective weight and objective weight. The calculation formulas for objective weight and comprehensive weight are:

$$Y_{ij}=\frac{X_{ij}}{{{\displaystyle \sum }}_{i=1}^m{X}_{ij}}$$

(3)

$$e_j=-k{{\displaystyle \sum }}_{i=1}^m\left(Y_{ij}·\ln Y_{ij}\right)$$

(4)

$$d_j=1-e_j$$

(5)

$$W_{kj}=\frac{d_j}{{{\displaystyle \sum }}_{j=1}^n{d}_j}$$

(6)

$$W_j=\frac{\left(W_{kj}+W_{zj}\right)}{2}$$

(7)

where $Y_{ij}$ is the ratio of $j$ index in $i$ city, $X_{ij}$ is the standardized value, $e_j$ is the index information entropy, $d_j$ is the redundancy of information entropy, $W_{kj}$ is the objective weight of $j$ index, $W_{zj}$ is the subjective weight, and $W_j$ is the comprehensive weight. In $k=1/\ln m$, $m$ is the number of cities evaluated and n is the number of indicators. The indicator weight and function weight are shown in Table 1.

2.3.3. Assessment of Multifunctional Cultivated Land

• Calculation of evaluation value of multifunctional cultivated land

The Comprehensive Index Model was applied to calculate the values of the production, social, and ecological functions of each city in the study areas. The formula is:

$$U={{\displaystyle \sum }}_{j=1}^{\mu }\left(X_{ij}·W_j\right)$$

(8)

where $U$ is the evaluation indicator of each function of cultivated land, $j$ is the evaluation indicator, and $\mu$ is the number of evaluation indicators for this function.

• Calculation of multifunctional coupling coordination degree of cultivated land

The production, social, and ecological functions of cultivated land are not independent of each other but rather are mutually restricted and influenced. Thus, the comprehensive assessment of the utilization of cultivated land can be achieved by calculating the multifunctional coupling coordination degree of cultivated land. The Coupling Coordination Degree Model can be used to calculate the multifunction coupling coordination degree of cultivated land. The formulas needed are:

$$C=\sqrt[3]{\frac{U_1{U}_2{U}_3}{{\left(\frac{U_1+U_2+U_3}{3}\right)}^3}}$$

(9)

$$T=W_1{U}_1+W_2{U}_2+W_3{U}_3, W_1+W_2+W_3=1$$

(10)

$$D=\sqrt{C·T}$$

(11)

where $C$ is the compatibility degree and $U_1, U_2,$ and $U_3$ are the production, social, and ecological function evaluation values of cultivated land, respectively. $T$ is the multifunctional comprehensive evaluation indicator of cultivated land. Additionally, $W_1$, $W_2,$ and $W_3$ are the undetermined coefficients of each function, namely, the comprehensive weight values of each function, where $W_1$ = 46.2%, $W_2$ = 26.2% and $W_3$ = 27.6%$.$ Finally, $D$ is the multifunctional coupling coordination degree of cultivated land.

2.3.4. Identification of Driving Forces of the Multifunctional Coupling Coordination Degree of Cultivated Land

As an open system, cultivated land is influenced by human production and economic activities. The spatial-temporal evolution process of the coupling and coordinated development among the production, social, and ecological functions of cultivated land is jointly influenced by the endowment of cultivated land resources, the level of agricultural development, and social and economic factors [3,25]. In order to reveal the factors influencing the change in the multifunctional coupling coordination degree of cultivated land in the study areas, in this paper we selected ten influencing factors, as shown in

Table 2. Selection of factors influencing the multifunctional coupling coordination degree of cultivated land in the study areas.

Influencing Factors	Indicators	Unit	Symbol
Cultivated land resource endowment	Slope	°	X1
	Altitude	m	X2
	Annual precipitation	mm	X3
Agricultural development level	Contribution of primary industry to GDP	%	X4
	Average salary level of agriculture, stockbreeding, forestry, and fishery	CNY	X5
	Rural per capita disposable income level	CNY	X6
	Effective irrigation rate of cultivated land	%	X7
Socioeconomic factors	Fiscal expenditure related to agriculture	%	X8
	Contribution of industry to GDP	%	X9
	Urbanization level	%	X10

. Geodetector was used to explore the driving forces from the perspective of single factors and dual factors [42] in order to analyze the driving forces behind the multifunctional coupled coordinated development of cultivated land in the study areas.

In this study, the natural discontinuity grading method was used to discretize the influencing factors, and then the differentiation, factor detection, and interaction detection module in Geodetector were used to analyze the driving forces. The formula for the expression of differentiation and factor detection is:

$$q=1-\frac{1}{N{\sigma }^2}{{\displaystyle \sum }}_{h=1}^L{N}_h{\sigma }_h^2$$

(12)

where $q$ represents the degree to which the influencing factor explains the dependent variable, with the range of [0,1]. The larger the value of $q \mathrm{is}$, the stronger the ability of the influencing factor to explain the dependent variable is, and vice versa. $N$ is the total number of cities in the study areas; ${\sigma }^2$ is the total variance of the multifunctional coupling coordination degree of cultivated land in each city in the study areas; $L$ is the number of layers of influencing factors; and $N_h$ and ${\sigma }_h^2$, represents the number of cities (districts) and the discrete variance of layers $h$ divided by the influencing factors, respectively. The detection of influencing factor interaction refers to the detection of the difference between $q\left(x_1\right)$ and $q\left(x_2\right)$ when two influencing factors $x_1$ and $x_2$ act on the dependent variables separately and $q\left(x_1\cap x_2\right)$ when the interaction of two influencing factors $x_1$ and $x_2$ act together on the dependent variables. This can be used to determine whether two influencing factors $x_1$ and $x_2$ acting together enhances or weakens the dependent variables or whether there is no combined impact on the dependent variables at all.

2.3.5. Summary

In the above subsections, the methodological process is illustrated. We further summarize the correspondence between methodological process and research results. Based on the multifunctional cultivated land evaluation index system, we chose twelve indicators to characterize the different functions of cultivated land. In order to eliminate the influence of indicator unit dimensions and ensure the comparability of each indicator, we standardized and weighted each indicator. As functions of cultivated land are not independent of each other but are mutually influenced, we applied the Comprehensive Index Model and the Coupling Coordination Degree Model to calculate the evaluation value of cultivated land functions ($U$) and the multifunction coupling coordination degree of cultivated land ($D$) of each city in the study areas in 2005, 2010, 2015, and 2020.

Based on the calculation results of the multifunctional cultivated land evaluation values, the functional level of cultivated land used for multiple functions was divided into five grades (

Table 3. Grade and level of evaluation values of the functions of cultivated land.

$\mathbf{Evaluation} \mathbf{Values} \mathbf{of} \mathbf{Cultivated} \mathbf{Land} \mathbf{Functions} \left(\mathit{U}\right)$	Functional Grade	Functional Level
(0.0~0.2)	1	Low level
(0.2~0.4)	2	Relatively low level
(0.4~0.6)	3	Middle level
(0.6~0.8)	4	Relatively high level
(0.8~1.0)	5	High level

); the degree of multifunctional coupling coordination of cultivated land was also divided into five grades (

Table 4. Grading and level of multifunctional coupling coordination degree of cultivated land and related quantitative distribution by year, province, and city (district).

Coupling Coordination Degree D	Coordinated Grade	Coupling Coordination Level	2005			2010			2015			2020
			HLJ	JL	LN	HLJ	JL	LN	HLJ	JL	LN	HLJ	JL	LN
(0.0~0.2)	1	Serious disorder	4	2	4	2	1	1	1	2	3	2	1	3
(0.2~0.4)	2	Moderate disorder	9	2	4	6	3	3	5	2	1	0	2	1
(0.4~0.6)	3	Barely coordinated	0	5	6	5	5	10	7	5	7	4	6	7
(0.6~0.8)	4	Basically coordinated	0	0	0	0	0	0	0	0	3	7	0	3
(0.8~1.0)	5	Well-coordinated	0	0	0	0	0	0	0	0	0	0	0	0

Note: HLJ: Heilongjiang; JL: Jilin; and LN: Liaoning.

). Additionally, the number of cities (districts) in each province in the different grades by year is shown in Table 4.

The calculation results of both $U$ and $D$ are visualized by using the hierarchical graphs in Section 3.1 and Section 3.2, respectively, and the spatial-temporal evolution characteristics of every single function and the multifunctional coupling coordination of cultivated land are also explained in these two subsections, respectively. In order to explore the driving forces of the spatial-temporal evolution characteristics, Geodetector was applied in the research. The results analyzed by Geodetector are shown in Section 3.3. Based on the single factor detection and dual-factor detection results of Geodetector, driving forces of the evolution of the multifunctional coupling coordination degree of cultivated land is identified in Section 3.2, further, the influence mechanism of the driving forces is analyzed in Section 3.3.3. Following are the research results.

3. Results

3.1. Spatial-Temporal Evolution Characteristics of Multifunctional Cultivated Land

3.1.1. Production Function of Cultivated Land

The spatial-temporal evolution of the production function of cultivated land in the study areas is shown in Figure 2. It can be seen that the production function of cultivated land in the study areas increases year by year: the production function of cultivated land in Heilongjiang shows a trend of generally increasing, the production function improvement of cultivated land in Jilin is relatively weak, and the production function of cultivated land in some cities in Liaoning is improving. The cities (districts) with a high value for the evaluation of the production function of cultivated land are mainly distributed in Heilongjiang and the central and eastern areas of Liaoning, which boast a high land reclamation rate, high grain yield, relatively complete agricultural infrastructure, and relatively good agricultural mechanization level. As a result, the cultivated land output is high. The evaluation value of the production function of cultivated land in the Daxing’anling area of northern Heilongjiang was at a low level for a long time, mainly due to the cold climate, low cultivated land reclamation rate, and low grain yield, which led to the low land output in this area. However, on the whole, it can be seen that the production function of cultivated land in Heilongjiang is quite high. As an important national grain base, Heilongjiang has a good agricultural production foundation and natural resource endowment advantages. Its cultivated land production capacity is increasing year by year; thus, this area effectively ensures national food security.

3.1.2. Social Function of Cultivated Land

The spatial-temporal evolution of the social function of cultivated land in the study areas is shown in Figure 3, where it can be seen that the social function of cultivated land in the study areas is increasing year by year. In 2005, nearly half of the cities (districts) in the study areas were at the middle level in terms of the land’s social function. By 2020, only nine cities (districts) in the study areas were at a low level or relatively low level, and nearly one-third of the cities (districts) achieved a high level in terms of the land social function; these were mainly distributed in Heilongjiang and Liaoning. In cities with a high social function level, the production function of cultivated land was also quite high. On the basis of fully developing and utilizing cultivated land resources, farmers constantly improve their grain production capacity and grain self-sufficiency. As a result, the agricultural output value and household income of farmers in these areas are also high, with a large rural agricultural labor force because cultivated land features a large agricultural labor force, demonstrating its prominent social security function. With the national cultivated land protection policy favoring the major grain-producing areas in northeast China and further rural revitalization strategies being implemented, agriculture has become the dominant industry in the major grain-producing areas in northeast China. This has effectively solved the employment problem faced by the local agricultural labor force, and the per capita disposable income of farmers has increased year by year, providing strong social security for farmers in the study areas. Over time, the social function of cultivated land in the study areas has gradually strengthened, with these areas gaining increasing prominence in terms of social security.

3.1.3. Ecological Function of Cultivated Land

The spatial-temporal evolution of the ecological function of cultivated land in the study areas is shown in Figure 4, where it can be seen that the ecological function of cultivated land in the study areas has been at a high level since 2005. From the temporal perspective, it can be seen that the ecological function of cultivated land in the study areas has not undergone any significant change in pattern. Generally speaking, Heilongjiang is the area with the highest ecological function of cultivated land, while Liaoning has the lowest ecological function among the three provinces. However, from 2005 to 2020, Liaoning saw an improvement in terms of its ecological function of cultivated land after previously undergoing a decline. Northeast China is the main grain-producing area in China, with a good climate, beneficial hydrothermal conditions, and a large proportion of paddy fields, which are mainly distributed along the Shenyang–Changchun–Harbin line of the three northeast plains and the coastal areas of major rivers. The Heilongjiang paddy fields are mainly distributed across Sanjiang Plain, Songnen Plain, Harbin, and Suihua, while the Liaoning and Jilin paddy fields are mainly distributed in the central areas of various provinces [43,44]. In recent years, the provinces and cities in the study areas have vigorously promoted the development of farmland water conservation facilities and increased the effective irrigation area of the cultivated land, thus significantly improving the biodiversity of paddy fields. In addition to rice, corn, wheat, millet, sorghum, beans, potatoes, and other crops cover large planting areas in the study areas, effectively improving the diversity of the farmland ecosystem. Due to the national cultivated land protection policy, the reduction in fertilizer application has significantly reduced the fertilizer load of cultivated land in the study areas, thus improving the ecological function of cultivated land in these areas. Through the implementation of important measures such as the triple protection policy for cultivated land and the ecological restoration of land space, the ecological value of cultivated land in these areas has been effectively improved, while the ecological function of cultivated land has also been significantly enhanced.

3.2. Spatial-Temporal Evolution of Multifunctional Coupling Coordination Degree of Cultivated Land

The production, social, and ecological functions of cultivated land are interrelated and influence one another. In the process of cultivated land utilization, the change and development of any function causes a change in the degree of multifunctional coupling coordination of cultivated land. The spatial-temporal evolution of the multifunctional coupling coordination degree of cultivated land in the study areas is shown in Figure 5.

From Figure 5 and Table 4, it can be seen that the coupling coordination degree of the production, social, and ecological functions of cultivated land in different years differs significantly in different regions. From 2005 to 2020, the multifunctional coupling coordination degree of cultivated land in all provinces and cities in the study areas showed a trend of improvement, but the number of cities (districts) with serious imbalance remained basically unchanged. From 2005 to 2010, the multifunctional coupling coordination degree of cultivated land in the study areas was largely barely coordinated, and there were no cities (districts) with a coupling coordination degree greater than 0.6. In 2015, there were three cities in Liaoning whose multifunctional cultivated land coupling coordination degrees were between 0.6 and 0.8; these were largely coordinated and were distributed in the central part of Liaoning. At this time, there were no mostly coordinated cities (districts) in Heilongjiang and Jilin. By 2020, there were seven cities (districts) in Heilongjiang and three cities (districts) in Liaoning whose multifunctional cultivated land coupling coordination degrees were largely coordinated; these were mainly distributed in the south of Heilongjiang and the middle of Liaoning. The degree of multifunctional coupling coordination of cultivated land in Jilin was largely barely coordinated, showing that there was no significant change during the study period.

The cultivated land resources in the study areas are abundant and the quality of the cultivated land there is good. With the modernization and intensification of agriculture, the intensive and large-scale utilization of cultivated land, and the improvement of the social and economic levels in these areas, the multifunctionality of cultivated land in most cities (districts) in the study areas is gradually developing in an orderly manner. However, the mode of land operation dominated by traditional agricultural production, to a certain extent, leads to farmers having a high dependence on the production functions of cultivated land while neglecting the orderly development of the social and ecological functions of cultivated land, thus limiting the coordinated development of the multifunctional coupling of cultivated land. Therefore, from the temporal perspective, the degree of the multifunctional coupling and coordinated of cultivated land in the study areas has been improved, but this improvement is not significant, and the degree of multifunctional coupling and coordinated of cultivated land in some cities (districts) shows a serious imbalance.

3.3. Driving Forces behind the Multifunctional Coupling Coordination Degree of Cultivated Land

3.3.1. Single Factor Detection

To reveal the driving forces behind the spatial-temporal evolution of the multifunctional coupling coordination degree of cultivated land in different years in the study areas, the differentiation and factor detection module of Geodetector was used for analysis. The differences in the driving forces and the results of factor detection for each year are shown in

Table 5. Results of single factor detection of the spatial-temporal evolution of the multifunctional coupling coordination degree of cultivated land in the study areas in 2005, 2010, 2015, and 2020.

2005			2010			2015			2020
Ranking	q Statistic	p Value	Ranking	q Statistic	p Value	Ranking	q Statistic	p Value	Ranking	q Statistic	p Value
X1	0.2986 **	0.0166	X1	0.3279 **	0.0114	X6	0.2114 **	0.0216	X7	0.2706 **	0.0165
X10	0.2265 **	0.0467	X6	0.2215 **	0.0362	X10	0.1809 **	0.0257	X6	0.1859 **	0.0411
X4	0.1920 *	0.0921	X4	0.1782	0.4243	X7	0.1267 **	0.0480	X4	0.1317 **	0.0362
X8	0.1587	0.2102	X10	0.1373	0.4056	X1	0.1204	0.4848	X1	0.0887	0.2969
X3	0.1363	0.2099	X7	0.1654	0.5118	X4	0.1175	0.6644	X2	0.0806	0.6780
X2	0.1058	0.5346	X8	0.1615	0.1965	X3	0.0908	0.6252	X8	0.0448	0.9114
X9	0.0856	0.4660	X2	0.1212	0.6708	X2	0.0649	0.8626	X3	0.0371	0.5930
X6	0.0853	0.8518	X9	0.0794	0.8596	X9	0.0608	0.9053	X10	0.0258	0.9198
X5	0.0790	0.8319	X5	0.0722	0.8470	X8	0.0407	0.8204	X5	0.0251	0.9486
X7	0.0614	0.7363	X3	0.0329	0.9882	X5	0.0053	0.7302	X9	0.0128	0.9430

Note: * and ** represent statistically significance at 10% and 5%, respectively.

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Generally speaking, the factors that have a significant impact on the spatial-temporal evolution of the multifunctional coupling coordination degree of cultivated land in the study areas include the slope of cultivated land (X1), the contribution of primary industry to GDP (X4), the rural per capita disposable income level (X6), the effective irrigation rate of cultivated land (X7), and the urbanization level (X10).

From 2005 to 2020, the q-value ranking of the rural per capita disposable income and the effective irrigation rate of cultivated land increased, indicating that the construction of modern irrigation and water conservancy facilities and the improvement of the agricultural income level are conducive to promoting the orderly development of the multifunctional coupling coordination degree of cultivated land. The q-value ranking of the primary industry’s contribution to GDP remained basically unchanged and only passed the significance test in 2005 and 2010, indicating that the pulling effect of the primary industry on economic development had a limited effect on the multifunctional coupling coordination degree of cultivated land. The q-value ranking of the slope and urbanization level declined, illustrating that with the development of science and technology, the influence of the natural local conditions of cultivated land and the number of agricultural populations on cultivated land utilization declined. In 2020, both of these failed to pass the significance test and their q-value was small, indicating that these factors have basically no influence on the ordered development of cultivated land used for multiple functions.

3.3.2. Dual-Factor Detection

The factor interaction detection module of Geographical Detector was used in our analysis to explore the effect of the factor interaction on the spatial-temporal evolution of the coupling coordination degree of cultivated land in the study areas. The results obtained for the dual-factor interaction of the spatial-temporal evolution of the multifunctional coupling coordination degree of cultivated land in the study areas are shown in Figure 6.

According to the results obtained for the factor interaction, the interaction among factors has shown dual-factor enhancement and nonlinear reinforcement, indicating that the multifunctional coupling coordination degree of cultivated land in the study areas is influenced by multiple factors. Figure 6 shows that the interaction between slope factor (X1) and other factors is strong, with the strong interaction between the four factors representing the level of agricultural development. It is worth noting that there is an interaction between the altitude factor (X2) and the other factors, but it can be seen that this interaction tends to weaken over time. The interaction between the rural per capita disposable income level (X6), the effective irrigation rate of cultivated land (X7), and other factors relating to the agricultural development level is strong, and it can be seen that this interaction tends to increase over time. The interaction between the average salary level (X5) of those working in agriculture, stockbreeding, forestry, and fishery and other factors is also strong, but the influence of a single factor is not significant. The results obtained for the interaction among factors further verify that the agricultural development and cultivated land resource endowment are the key driving factors that influence the spatial-temporal evolution of the multifunctional coupling coordination degree of cultivated land in the study areas, demonstrating the complexity of the spatial-temporal evolution of the multifunctional coupling coordination degree of cultivated land.

3.3.3. Influence Mechanism

Among the natural resource endowments, the slope is the only key factor that influences the evolution of the spatial-temporal pattern of the multifunctional coupling coordination degree of cultivated land in the study areas. The slope of cultivated land reflects the topographic conditions of the surface units to which the cultivated land belongs and is an important indicator for grading the quality of cultivated land. Under the appropriate slope conditions, cultivated land resources can be fully and effectively utilized, ensuring the grain yield and agricultural income, enriching the crop variety, and attracting agricultural labor to engage in agricultural production, thus improving the production, social, and ecological functions of cultivated land and promoting the multifunctional coupling coordination degree of cultivated land. However, the q-value ranking of the influence of altitude, average annual precipitation, and other indicators on the multifunctional coupling coordination degree of cultivated land each year is in the middle level and has not passed the significance test, indicating those two indicators have limited effects on the coordinated development of cultivated land utilization and multifunctional coupling. If one wants to improve the multifunctional coupling coordination degree of cultivated land, one should start by upgrading the natural conditions of cultivated land, improving its quality, and ensuring the suitability of cultivated land.

The agricultural development level strongly explains the spatial-temporal pattern of the multifunctional coupling coordination degree of cultivated land in the study areas. Over time, the effect of the rural per capita disposable income level and effective irrigation rate of cultivated land on the spatial-temporal pattern of the multifunctional coupling coordination degree of cultivated land gradually strengthens. The increase in farmers’ disposable income can effectively improve agricultural production factors and can lead to the allocation of more funds for agricultural production, land transfer, the employment of labor force, the use of modern agricultural machinery, etc. The improvement of agricultural development is conducive to realizing the large-scale and intensive utilization of cultivated land, thus enhancing the production function of cultivated land. The construction of farmland water conservancy facilities can further improve the mechanization level of agricultural production, increasing farmland biodiversity and thus promoting the orderly development of the production, social, and ecological functions of farmland, moving towards coupling and a coordinated direction.

The influence of social factors on the spatial-temporal evolution of the multifunctional coupling coordination degree of cultivated land in the study areas is generally weak. Among the three factors, only urbanization had a significant influence on the multifunctional coupling coordination degree of cultivated land in 2005 and 2015, indicating that with the improvement of the urbanization level, the rural population gradually declined, resulting in a reduction in agricultural labor force input to a certain extent, which was not conducive to the development of the production function and social function of cultivated land or to its coupling and coordinated development. Therefore, in 2020, this factor only had a very weak effect on the multifunctional coupling coordination development of cultivated land.

4. Discussion

4.1. Policy Implications for Utilization and Management of Multifunctional Cultivated Land

Achieving the coordinated development of cultivated land for multiple functions is a long-term goal. In this study, we calculated and evaluated the values of each function of cultivated land and the multifunctional coupling coordination degree of cultivated land. Visualizing their spatial-temporal pattern can help us in understanding the evolution of the use of cultivated land for multiple functions. In addition, revealing the key driving factors behind the spatial-temporal evolution of the multifunctional coupling coordination degree of cultivated land can help us to clearly define the main factors influencing the coordinated development of various functions of cultivated land and allow us to put forward suitable and feasible development paths and plans for the orderly utilization of cultivated land for multiple functions. The policy implications are as follows:

Guaranteeing the comparative benefits of agricultural production and steadily increasing the level of disposable income of rural residents: The research results show that the disposable income level of rural residents is an important factor affecting the evolution of the multifunctional coupling coordination degree of cultivated land. Therefore, through policy regulation, the government should stabilize the price of agricultural production materials, increase the market price of grain crops, and raise the subsidy standard of grain cultivation to ensure the comparative benefits of agricultural production. By continuously improving the comparative income level of agriculture, steadily increasing the level of disposable income of rural residents, and continuously improving the economic and production functions of arable land, the multifunctional utilization and management of cultivated land can be improved.

Enhancing the construction of agricultural production infrastructure to improve the level of agricultural development: According to the research results, the effective irrigation rate of cultivated land is an important factor affecting the development of the multifunctional coupling coordination of cultivated land. This shows that the construction of agricultural production infrastructure should be further improved in order to provide good production conditions for agriculture. Especially in the main grain-producing areas, agricultural production is the basis of all social and economic activities. It is therefore necessary to continuously improve the construction of agricultural infrastructure, create a good production environment for agricultural production, and effectively promote the coordinated development of multifunctional cultivated land based on the improvement of its production function.

Strictly implementing the farmland protection policy and continuously improving farmland productivity: Among the natural resource endowment conditions, the slope of cultivated land is a key factor affecting the evolution of the multifunctional coupling coordination degree of cultivated land. Slope represents the basic condition of cultivated land resources, and a suitable arable land slope is the foundation of agricultural production, ensuring a good grain yield, enriching the diversity of agricultural crops, and improving farmers’ enthusiasm for farming. Therefore, the farmland protection policy must be strictly implemented and the use of farmland with suitable slopes for non-food and non-agricultural production must be strictly prohibited. Through the implementation of policies and measures such as “storing grain in the land” and “storing grain in technology” to protect the cultivated land effectively, we can improve the quality of cultivated land and the production capacity of cultivated land. The coordinated development of multifunctional cultivated land is promoted through the synergistic improvement of the production, social, and ecological functions of cultivated land.

4.2. Contribution to Research, Limitations, and Future Perspectives

During the rapid process of rural development, ensuring the success of multifunctional cultivated land in China has become a critical objective in achieving the sustainable use of cultivated land. The existing studies in this area have provided us a cultivated land multifunction utilization level through the improved TOPSIS model [3], but the results of empirical analysis can only tell us the distance from the current utilization level to the optimal utilization level of multifunctional cultivated land. This does not allow us to clearly evaluate the actual use of multifunctional cultivated land. Thus, in this study we combined the Comprehensive Index Model and the Coupling Coordination Degree Model to calculate the exact coupling coordination degree of multifunctional cultivated land, which provided much clearer results relating to the multiple functions of cultivated land. Furthermore, the existing studies in this area mostly focus on the factors influencing cultivated land use [26,38,45]. However, cultivated land is a complex system and the interaction of different factors can have an impact on multifunctional cultivated land. Detailed analyses of cultivated land used for multiple functions and the impact of its interaction are rare. Thus, we used Geodetector to explore the effect of the interaction of multiple factors on the multifunctional coupling coordination degree of cultivated land, providing a deeper understanding of the spatial-temporal patterns and driving forces behind the use of cultivated land for multiple functions.

However, in this study we only considered the production, social, and ecological functions of cultivated land, ignoring other factors such as landscape functions, cultural functions, reserve functions, etc. In a future follow-up study, our analysis of the use of cultivated land for multiple functions will be further extended to allow for the establishment of an index system for the evaluation of multifunctional cultivated land. This will be supplemented with microdata to allow us to further analyze the spatial distribution of the multifunctional coupling coordination degree of cultivated land, to explore the trend of the spatial-temporal evolution of the multifunctional coupling coordination degree of cultivated land more scientifically, and to provide scientific reference for the management and utilization of multifunctional cultivated land.

5. Conclusions

In this paper, we analyzed the production, social, and ecological functions as well as the spatial-temporal evolution of the multifunctional coupling coordination degree of cultivated land in major grain-producing areas of northeast China in 2005, 2010, 2015, and 2020, revealing the driving forces. The research showed that:

From 2005 to 2020, the production, social, and ecological functions of cultivated land in the study areas showed significant regional differences, and the multifunctional coupling coordination degree of cultivated land in the study areas showed a trend of gradual improvement, but did not reach a level of good coordination. As time passed, the number of cities (districts) in the study areas where the degree of multifunctional coupling coordination of cultivated land was seriously out of balance remained unchanged.

As for the driving forces, the spatial-temporal evolution of the multifunctional coupling coordination degree of cultivated land in the study areas was mainly influenced by the level of agricultural development. According to the results obtained from the analysis of the factor detection module of Geographical Detector, the level of per capita disposable income in rural areas and the effective irrigation rate of cultivated land representing the level of agricultural development were ranked highest in almost every year, showing a strong explanatory power for the spatial-temporal pattern of the multifunctional coupling coordination degree of cultivated land. In addition, the slope factor in the natural resource endowment category and the urbanization factor in the socioeconomic category also exerted an important influence on the spatial-temporal pattern of the multifunctional coupling coordination degree of cultivated land in the study areas.