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Journal of Geographical Sciences >

2024 , Vol. 34 >Issue 4: 699 - 721

DOI: https://doi.org/10.1007/s11442-024-2224-0

Research Articles

Uphill cropland and stability assessment of gained cropland in China over the preceding 30 years

  • HE Tingting , 1 ,
  • LI Jianhua 2 ,
  • ZHANG Maoxin , 1, * ,
  • ZHAI Ge 1 ,
  • LU Youpeng 3 ,
  • WANG Yanlin 4 ,
  • GUO Andong 1 ,
  • WU Cifang 1
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  • 1. School of Public Affairs, Zhejiang University, Hangzhou 310058, China
  • 2. College of Water Conservancy, Yunnan Agricultural University, Kunming 650201, China
  • 3. School of Architecture and Urban Planning, Huazhong University of Science and Technology, Wuhan 430074, China
  • 4. Institute of Land and National Development, Zhejiang University, Hangzhou 310058, China
* Zhang Maoxin (1983-), PhD, specialized in cropland use change effect and optimal control. E-mail:

He Tingting (1986-), PhD, specialized in remote sensing of cropland. E-mail:

Received date: 2023-03-13

  Accepted date: 2023-11-09

  Online published: 2024-04-24

Supported by

Zhejiang Provincial Philosophy and Social Science Planning Project(24JCXK04YB)

Natural Science Foundation of Zhejiang Province(Q22D018818)

Zhejiang Provincial Postdoctoral Research Foundation(ZJ2023051)

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Abstract

In recent years, cropland development in high-slope regions in China has alleviated the contradiction between total cropland balance and insufficient development space. However, this change in cropland slope brings risks to sustainable resource utilization. This study explored the slope characteristics of cropland in China from 1990 to 2020 and assessed the gained cropland stability. The results showed that (1) From 1990 to 2020, the lost cropland area was greater than the gained area, and the distribution showed an uphill trend. (2) The areas with a significant upslope change in cropland were mainly concentrated in the southern plain, containing the central grain-producing regions at its core and other well-developed eastern coastal areas. (3) The areas with distinct downslope cropland changes were mainly concentrated in the upper reaches of the Yangtze and Yellow rivers and the ecologically fragile areas of Inner Mongolia and the Loess Plateau. (4) The gained cropland stability was unsatisfactory; about one-third of the gained cropland was unstable, which had the highest abandonment rate within 5 years, and the gained cropland stability decreased with the slope. In addition, this study explored cropland conversion types at different slopes in different regions and discussed the reasons for slope cropland changes and gained cropland instability in different regions. Finally, optimization policies were proposed to protect and control newly gained cropland.

Key words： cropland conversion; uphill; slope; stability; China

Cite this article

HE Tingting , LI Jianhua , ZHANG Maoxin , ZHAI Ge , LU Youpeng , WANG Yanlin , GUO Andong , WU Cifang . Uphill cropland and stability assessment of gained cropland in China over the preceding 30 years[J]. Journal of Geographical Sciences, 2024 , 34(4) : 699 -721 . DOI: 10.1007/s11442-024-2224-0

1 Introduction

Land is a valuable resource for human survival, and agriculture is one of the most crucial land uses to meet human needs (Ramankutty et al., 2006). From 1982 to 2011, the world’s arable land increased by 52.876 million hectares, an average annual increase of 0.792 million hectares. Under the dual requirements of food security and urbanization development, most countries tend to expand newly cultivated land and reduce existing cultivated land, and the cultivated land tends to expand to high-latitude areas (Yao et al., 2017). This spatial transfer of cultivated land area leads to an imbalance in the ecological environment and directly affects the production capacity of cultivated lands, the social economy, and the ecological environment (Li et al., 2018b; Wang et al., 2018a; Liu et al., 2019b; Lark et al., 2020; Yang et al., 2023).
This phenomenon is particularly pronounced in traditionally large agricultural countries such as China. In terms of quantitative cropland development, the cropland area in China has been on a fluctuating but increasing trend over the past 2000 years. From early 500 million mu to 2 billion mu in 1980 (Feng et al., 2005b), it remained relatively stable until the late 20th century. Based on the perspective of the cognition of cropland use, driven by the ideas of different periods, land-use methods, values, and functions are also constantly changing (Ma et al., 2020). From its beginnings as a means of production to ensure food security, land has acquired commodity attributes due to land scarcity and the theory of land’s commodity value. Social development has now entered the ecological civilization stage, and the ecological function of land is highly valued. As a result, China’s cropland protection policy is also changing, from the cropland requisition-compensation balancing policy, which focused only on quantitative development, to the current trinity cropland protection strategy of quantity, quality, and ecology. Regarding cropland spatial development, most ancient Chinese capital sites were located in plains, producing life in areas with relatively flat terrain (Sun et al., 2013; Yue et al., 2013). Along with an increase in population and economic activity, land resources on these plains are becoming scarce. As the amount of cropland declines, farmers cultivate sloping land to grow food through deforestation. This newly added cropland usually occurs at higher altitudes, away from water sources and cities, which is why the phenomenon of uphill cropland occurs (Li et al., 2021b) (Figure 1).
Figure 1 Theoretical analysis of cropland up the hill
The main reason for the current change in cropland distribution in China is the rapid urbanization and population explosion (Liu et al., 2019a; Kuang et al., 2022). China’s urbanization rate has surged from 17.6% to 63.89% in the past four decades. A large amount of cropland has been converted into construction land, resulting in an average annual decrease of 630,700 hectares in high-quality cropland resources (Shi et al., 2013). Although China has implemented a strict policy of requisition-compensation to balance cropland conversion, the phenomenon of “occupying the best and replenishing the worst” still occurs (Zhang et al., 2011). Rapid urbanization has resulted in the occupation of most of the high-quality, flat-topped farmland around cities. In Chongqing, for example, between 1980 and 2015, a total reduction of 2142.41 km2 of cropland occurred, of which 71.0% was converted into construction land (Li et al., 2021c). The shortage of cropland reserves and the drive for economic efficiency have caused additional cropland to be found in ecologically fragile areas on steep slopes (Liang et al., 2015; Jin et al., 2016). When cropland goes uphill, slope farming can cause severe negative impacts on the ecological environment, such as soil erosion (Wu et al., 2018), geological hazards (Yan et al., 2009), landslides (Bruun et al., 2009), and water quality degradation (Sidle et al., 2006). Disturbed soils on higher slopes are highly susceptible to water erosion, runoff-induced nutrient losses, and reduced soil quality (Bouraima et al., 2016; Reza et al., 2016; Wang et al., 2019). Coupled with the household contracting system and a high degree of cropland fragmentation (Ji et al., 2016), large-scale agricultural production cannot be achieved, and grain yields are low (Chen et al., 2018). In addition, topographical and socio-human factors make mountainous areas highly susceptible to cropland abandonment (Li et al., 2018a; Han and Song, 2020), posing a potential threat to food security. Therefore, cropland’s slope is a natural attribute of cropland’s spatial distribution and an intuitive indicator of cropland quality and productivity (Li et al., 2021a).
However, there are few studies on the effects of uphill cropland. First, there is a lack of accurate descriptions of cropland characteristics on a national scale. At the research scale, the research area for sloping cropland is mainly concentrated in southwestern and northwestern China, as well as in several agricultural provinces, with very little research conducted in the more economically developed eastern coastal areas (Yin, 2018). Current studies are mainly based on statistical data describing the spatial distribution of uphill cropland. At a national scale, most studies focus on changes in cropland quantity and spatial distribution, and less attention is paid to changes in the slopes of cropland distribution. On the other hand, based on the perspective of spatial distribution, comprehensive regionalization or geomorphic units are subdivided into statistical units. For example, Li et al. (2021a) analyzed the slope characteristics of cropland conversion in China from 2000 to 2015. That study quantified the gain and loss of cropland at different slopes within comprehensive regionalization units. To some extent, it directly reflects the characteristics of location change. However, China’s terrain is complex, and such comprehensive zoning ignores topographical differences, which is not conducive to accurately reflecting the change in intensity of uphill cropland. By comparison, the statistical analysis of cropland distribution based on topographic and geomorphic zoning can better reveal slope change characteristics (Bruun et al., 2009; Gao et al., 2019). Altitude and slope influence not only climatic factors such as temperature and precipitation but also govern the ease of cultivation. Thus, topography determines the spatiotemporal distribution of cropland and strongly influences the quantity and quality of cropland (Wang et al., 2018b). However, most research conducted on topographical effects on cropland changes was at the city, county, and watershed scales, with relatively little research performed at the provincial and national levels. Many studies are based on unique landscape features in a study area (Fu et al., 2011; Zhao et al., 2013). Due to the specific and homogeneous topography, it is difficult to study the impact of topography on changes in cropland. At the national scale, Gao et al. (2019) analyzed cropland losses and gains from 1990 to 2015 using landscape zoning but focusing only on the quantitative changes and cropland distribution in different landscape zones, lacking a picture of spatial heterogeneity and changes on different slopes.
Second, there is a lack of evaluation of gained cropland stability at different slopes. In the past, under the balancing policy of requisition-compensation gained cropland was needed to balance quantity, quality, and ecology. The quality of gained cropland was usually assessed based on the balance of cropland quantity and the grading standard of agricultural cropland. Some scholars applied an indicator method to investigate the differences in quality, potential production, and ecosystem, which can enhance the performance of local governments in assessing the quality of gained cropland (Cai et al., 2021; Wang et al., 2021; Zhang et al., 2022a). By constructing different indicators and parameters, these studies reveal the gained cropland quality in China to a certain extent based on various aspects in different periods. However, it does not reflect the stable utilization of gained cropland in mountainous areas, such as cropland abandonment (Peña-Angulo et al., 2019), and a large amount of gained cropland that has not been used (Xin and Li, 2018). Driven by the policy of balancing requisition-compensation over a long-term period, the gained cropland, whether continuously cultivated, is the most direct reflection of this effect. From the slope perspective, there is a lack of intuitive understanding of gained cropland stability. Therefore, the assessment of gained cropland stability in different regions and slopes based on the perspective of abandonment needs to be studied.
Considering the problems in studying uphill cropland, it is necessary to portray the spatiotemporal changes in China’s cropland in terms of topographical regions based on a slope perspective. This study proposes to use the newly released land cover dataset based on 30 m remote sensing data extracted from the Landsat satellite and, through the GEE platform, detect spatiotemporal changes in cropland from the perspective of geomorphic regions and slopes. A new assessment method is proposed to reveal gained cropland stability in different regions and slopes. We further examine the cropland conversion types at different slopes in different regions to explore the driving factors of uphill cropland. We also analyze the impacts of China’s cropland protection policies to provide basic data for managing cropland resources and sustainable development. The specific purposes include the following: (1) outline the characteristics of cropland conversion in China over the past three decades coupled with quantity, distribution, and slope attributes; (2) explore the gained cropland stability and cropland conversion types at different slopes; (3) discuss the policy impact on the dynamics of cropland conversion in China and the potential risks of uphill cropland.

2 Materials and methodology

2.1 Study area

China is located in East Asia on the west coast of the Pacific Ocean, with a land area of about 9.6 million km2. According to the Third National Land Census, the existing cropland area is 1,288,600 km2, ranking third globally. However, the per capita cropland area is small, less than half the world level. China’s cropland is mainly concentrated in the eastern plain and central plateau regions (Figure 2). To ensure food security, in 2006, China declared that “1.8 billion mu (1.2 million km2) of cropland is a red line for agricultural development.” Although the government implements a land expropriation compensation balancing policy to protect the cropland trinity of quantity, quality, and ecology, it aims to promote sustainable cropland resource utilization and ensure national food security, ecological security, and social stability (Sun et al., 2014). However, a phenomenon of new cropland in China appearing on slopes has occurred, which endangers cultivated land stability and the ecological environment.
Figure 2 Cropland distribution in China (2020)

2.2 Data sources

Land cover data: We adopt China Land Cover Year Dataset (CLCD) including 5-year cropland data for 1990, 1995, 2000, 2005, 2010, 2015 and 2020 published by Wuhan University (Yang and Huang, 2021). The dataset contains nine central land-cover classification systems: cropland, forest, shrub, grassland, water, snow, barren, impervious and wetland. The product pixel is 30 m, and the accuracy rate of reaches CLCD was 79.31%, which is widely used in land change detection (Zhang et al., 2022b). The dataset is freely available at https://zenodo.org/record/8176941.
Topographic and geomorphological zoning: The topography of China is characterised by a distinct three-stage gradient, influenced by global plate tectonics and regional geological formations. Based on the dominant factors of regional differences in landforms, this study adopted the widely accepted topographic landscape zoning method (Wang et al., 2020a). In this paper, China is divided into six major first-order regions, including Eastern China Plains-Low Mountains-Hills Region I, Southeastern China Low Mountains-Hills Plains Region II, Northern China-Eastern Inner Mongolia Middle Mountains-Plateaus Region III, Northwestern China High and Middle Mountains-Basins-Plateaus Region IV, Southwestern China Middle and Low Mountains-Plateaus-Basins Region V, And the Tibetan Plateau High and Extremely High Mountains-Basins-Valleys Region VI.
Slope classification and DEM data: To accurately describe the distribution characteristics of China’s cropland on different slopes from 1990 to 2020, the slope classification criteria from the Third National Land Survey were used. The slope of cropland was divided into five classes, namely [0°, 2°], (2°, 6°], (6°, 15°], (15°, 25°], and (25°, 90°]. This slope classification is based on thresholds for soil erosion, irrigation, agricultural machinery and land use, and is widely used to analyse the characteristics of China’s cropland resources and as an important basis for the formulation and implementation of policies related to cropland in China (Feng et al., 2005a; Wang et al., 2013). And DEM data have a 30-m resolution from the National Aeronautics and Space Administration (http://reverb.echo.nasa.gov/reverb/).

2.3 Methodology

2.3.1 Constructing indices

The following indexes are used to quantify the slope fluctuation of cropland: the mean slope index (MS), slope index (SI), slope index of newly gained cropland (GSI), slope index of lost cropland (LSI), and slope index of accumulated in regions (ωSI).
$GSIn=(MSgc-MSic)/MSic$
where formula 1 calculates the slope change amplitude of the newly gained cropland. GSIn is used to describe the difference between the average slope of newly gained cropland in a certain region n in a certain period and the average slope of cropland in the initial year of the whole region. The higher the GSIn value, the more obvious the gained cropland is on the hill, and vice versa. MSgc is the mean slope of gained cropland, and MSic is the mean slope of regional cropland in the initial year. Here the newly gained cropland slope with 30 m pixels extracted is compared with the average slope of all cropland in the initial year of the region. Then calculate the overall change amplitude of the region according to the slope change of each pixel of newly gained cropland.
LSIn is used to describe the difference between the average slope of lost cropland in a certain region in a certain period and the average slope of cropland in the initial year of the whole region.
$LSIn=(MSl\text{c}-MSic)/MSic$
where MSlc is the mean slope of lost cropland, and MSic is the mean slope of regional cropland in the initial year. The higher the LSIn value, the higher the slope of lost cropland, and vice versa. The formula calculates the slope change amplitude of the lost cropland, that is, the slope of the lost cropland extracted 30 m pixels is compared with the average slope of all cropland in the initial year of the region. Finally, calculate the overall change amplitude of the region according to the slope change of each pixel of lost cropland.
SIn describes the difference between the changed cropland in Region (n) in a certain period and the average slope of cropland in the initial year of Region (n). The formula for SIn can be expressed as:
$SIn=GSIn-LSIn$
A positive SIn value indicates an increase in slope due to cropland changes in the region. The higher the SIn value, the more pronounced the upward slope of the cropland. A negative SIn value indicates a decrease in the overall slope of the cropland in the region. The smaller the SIn value, the more pronounced the downward slope of the cropland.
The ωSIn reveals the conversion of regional croplands. The index quantitatively reflects the significant degree of slope changes caused by regional cropland changes. The formula is:
$\omega SIn=f(SIn\times TAn)$
where f is the normalized function. TAn is the total cropland area change (increase or decrease) in a specific region in a certain period

2.3.2 Stability assessment of gained cropland

Slope cropland is easily abandoned because of its small arable land area and rough terrain, which makes it hard to apply mechanization (Han and Song, 2019). Therefore, it is necessary to assess gained cropland stability. We refer to Fan’s conception of stability assessment (Fan et al., 2020). In this study, based on identifying cropland changes at different slopes, we further tested the sustainable use years of newly added cropland to reflect the risk of cropland after moving uphill. A value was assigned to newly gained cropland according to the land class in the CLCD dataset. Among them, cropland is denoted as 1, other is denoted as 0, and impervious is denoted as 2.
The United Nations Food and Agriculture Organization defines cropland not used for agricultural production for 5 years as abandoned (Yin et al., 2020). This study tested newly gained cropland stability on a 5-year cycle, consistent with the concept of cropland abandonment. Therefore, the non-sustainable use of newly added cropland in this study could be considered cropland abandonment.
(i) The type of the gained cropland in the last year of the testing period (2020) is detected. If it is impervious (value = 2), this pixel is excluded; if it is cropland (value = 1), it is regarded as stable cropland; (ii) If it is anything else (value = 0), it means it has been abandoned. At this time, we further detect the pixels in each period (Figure 3):
Figure 3 Stability assessment process and grading
If a pixel changes from 1 to 0 and is always 0 until 2020, the year when the pixel first became gained cropland, it is marked as C, and the year with the first 0 value is defined as the year of abandonment is marked as A; the period of continuous use of gained cropland is marked as U, U=A-C. The gained cropland stability is marked as S. The six levels include: Excellent (S1): Did not occur due to abandonment, each period of the pixel’s value is 1; Better quality (S2): U = 25; The gained cropland was abandoned within 21-25 years; Good quality (S3): U = 20; The gained cropland was abandoned within 16-20 years; Medium quality (S4): U = 15; The gained cropland was abandoned within 11-15 years; Poor quality (S5): U = 10; The gained cropland abandoned within 6-10 years; Low quality (S6): U = 5. The gained cropland was abandoned within 1-5 years.

3 Results

3.1 General characteristics of the slope distribution of cropland at the national scale

3.1.1 Slope trend of cropland distribution

Seven periods of data from 1990, 1995, 2000, 2005, 2010, 2015 and 2020 were extracted using the Landsat data product of Wuhan University to examine the changes in China’s cropland from 1990 to 2020. From 1990 to 2020, China’s cropland area goes from 1973897.98 km2 to 1886712.64 km2. We illustrate the distribution of cropland slope through the proportion of each period. As shown in Figure 4, every five years is divided into a stage: 1990, 1995, 2000, 2005, 2010, 2015, and 2020, respectively. Cropland in China is divided into five slope ranges, [0°, 2°], (2°, 6°], (6°, 15°], (15°, 25°], (25°, 90°], with the percentage of each slope range being 33.81%-34.10%, 40.95%-41.63%, 16.42%-17.05%, 6.10%- 6.47%, 1.67%-1.85%, respectively. More than 90 per cent of the cropland was distributed between 0° and 15°, with the greatest variation in slopes of (2°, 6°] and (6°, 15°]. The cropland in (2°, 6°] decreases significantly with each year, while the cropland in (6°, 15°] increases significantly with each year. The above shows that the distribution of cropland in China presents an upward trend.
Figure 4 Proportion of cropland distributed on different slopes in China (1990-2020)

3.1.2 Slope characteristics of cropland conversion

From the perspective of different slopes, the area and proportion of the cropland gain and loss in China are shown in Table 1. From 1990 to 2020, the area of cropland lost was more significant than the area of cropland gained, and the area of cropland gained and lost was 298,410.86 km2 and 385,596.20 km2, respectively. Furthermore, the gain and loss of cropland showed a trend of increased at first and then decreased later. In addition, below 6°, there is a tendency for an increase followed by a decrease. Above 6°, the opposite is true. At higher slopes, the proportion of gain is higher than the proportion of loss. The proportion of the gain is higher than the loss in the higher slope.
Table 1 The gain and loss of cropland on different slopes
Slope The change of cropland
area (km2)		 The gain The loss
Area (km2) Ratio (%) Area (km2) Ratio (%)
[0°, 2°] -28,580.01 68,251.43 22.87 96,831.44 25.11
(2°, 6°] -48,387.86 86,835.97 29.10 135,223.82 35.07
(6°, 15°] -2312.25 81,538.20 27.32 83,850.46 21.75
(15°, 25°] -3539.20 46,006.60 15.42 49,545.80 12.85
(25°, 90°] -4366.02 15,778.66 5.29 20,144.68 5.22

3.1.3 The slope fluctuation of gain and loss cropland

Divided into phases every five years, calculate the GSI, LSI and SI values of cropland changes in 1990-1995, 1995-2000, 2000-2005, 2005-2010, 2010-2015, and 2015-2020. GSI is significantly higher than LSI in 1995-2005 and 2015-2020. SI is positive in 1995-2000, 2000-2005, 2015-2020, with a significantly increase in the slope of cropland. The SI is negative in the other periods, and the GSI is slightly lower than the LSI.
In terms of spatial distribution, the SI values and average slope of cropland were calculated separately for each longitude and latitude, using 1° as the segmentation unit. As shown in Figure 5, there are significant differences in the spatial distribution of SI in China. In terms of longitude, SI is higher in eastern China, where the cropland’s average slope is lower. Positive values of SI are concentrated at 111°-126°E, between 0.36 and 1.25. Negative values of SI occur sporadically, mainly at 93°-95°E and 105°-108°E, and range from -0.09 to -0.66. In terms of latitude, SI is higher in southern China, where the cropland’s average slope is higher. The positive value of SI is concentrated at 18°-36°N between 0.06 and 1.82. The negative value of SI is concentrated at 37°-41° and ranges from -0.19 to -0.53.
Figure 5 Slope fluctuation of gained and lost cropland from 1990 to 2020, and the average slope of cropland at each degree

3.2 Slope fluctuation characteristics of conversion cropland at the regional scale

As shown in Table 2, in terms of the proportion of cropland area in different topographic-landscape regions to the national cropland area, Northwestern China High and Middle Mountains-Basins-Plateaus Region (IV) increased significantly with the increase years between 1990 and 2020, with an increase of 50.43% compared to the arable area in 1990. The proportions of Eastern China Plains-Low Mountains-Hills Region (I), Northern China-Eastern Inner Mongolia Middle Mountains-Plateaus Region (III), And Southwestern China Middle and Low Mountains-Plateaus-Basins Region (V) decreased slightly with increasing years. Among them, the area of cropland loss in Region I is up to 124,169.73 km2. The cropland change in Region III is the most prominent, 11.97%. The newly increased cropland area in Region V is up to 72,387.77 km2. The proportion of cropland in other regions in the country did not change much.
Table 2 Area and proportional of cropland in different landscape regions (1990-2020)
Region 1990 1995 2000 2005 2010 2015 2020
Area
(104 km2)		 Ratio
(%)		 Area
(104 km2)		 Ratio
(%)		 Area
(104 km2)		 Ratio
(%)		 Area
(104 km2)		 Ratio
(%)		 Area
(104 km2)		 Ratio
(%)		 Area
(104 km2)		 Ratio
(%)		 Area
(104 km2)		 Ratio
(%)
I 88.06 44.61 88.08 44.83 86.83 44.51 84.76 44.12 83.51 44.11 83.47 43.91 82.52 43.74
II 38.49 19.50 37.73 19.20 37.25 19.10 37.21 19.37 36.02 19.03 36.08 18.98 36.63 19.42
III 25.96 13.15 25.68 13.07 25.50 13.07 24.12 12.56 23.33 12.32 22.99 12.10 22.86 12.12
IV 6.53 3.31 6.69 3.41 7.07 3.63 7.64 3.98 8.76 4.63 9.85 5.18 9.82 5.21
V 36.92 18.70 36.93 18.79 37.01 18.97 37.02 19.27 36.34 19.19 36.38 19.14 35.55 18.84
VI 1.43 0.72 1.36 0.69 1.43 0.73 1.35 0.70 1.36 0.72 1.31 0.69 1.29 0.68
As shown in Figure 6, based on the slope perspective, the proportion of cropland in each topographic region to the national cropland and the slope fluctuation characteristics of the gain and loss changes were observed. In the seven periods of 1990, 1995, 2000, 2005, 2010, 2015, and 2020, the proportion of cropland in Eastern China Plains-Low Mountains-Hills Region I and Southeastern China Low Mountains-Hills-Plains Region II distributed in [0°, 2°] and (2°, 6°] decreased significantly with the increase of the year, and in (6°, 15°], (15°, 25°], (25°, 90°] increased significantly with the increase of the year. The GSI values range from 0.43 to 1.14 and LSI values from 0.30 to 0.68. The SI values range from 0.20 to 0.74 for all periods except Region 1, with an SI of -0.11 from 1990 to 1995. The slope of the cropland shows a significant increase in the three-time periods 1995-2000, 2000-2005 and 2015-2020.
Figure 6 Slope characterisios of cropland distribution in iffrent rgions and slope fluctuation charaterisis of onvered copland(a.Pacenage of copand ara on difiratslopes;b.The fluctuation of cropland in different stages)
In Northern China-Eastern Inner Mongolia Middle Mountains-Plateaus Region III, the proportion of cropland in [0°, 2°] and (2°, 6°] increased significantly over the increase years, and the proportion of cropland on the slope decreased significantly with the increase of years in (6°, 15°], (15°, 25°], (25°, 90°]. GSI and LSI are between 0.24 and 0.54. Except for the two time periods of 2000-2005 and 2015-2020, SI is positive, and the SI of other periods is negative.
In the Northwestern China High and Middle Mountains-Basins-Plateaus Region IV, the proportion of cropland with slopes of (2°, 6°], (6°, 15°], and (15°, 25°] increased slightly with the year and decreased slightly for the other slopes. Values of GSI and LSI ranged from 0.02 to 0.11. The value of SI tended to zero at all times, and the slope of the region’s variable cropland had slight fluctuation.
In the Southwestern China Middle and Low Mountains-Plateaus-Basins Region V, the proportion of cropland at (6°, 15°] and (15°, 25°] slope increases slightly with the years and decreases slightly for other slopes. In the Tibetan Plateau High and Extremely High Mountains-Basins-Valleys Region VI, the proportion of cropland on each slope is almost unchanged. GSI and LSI are between 0.1-0.56. Both the gain and loss of cropland have slight slope fluctuations. SI during 1990-1995 and 2010-2015 was negative, and the rest of the years were positive.

3.3 Spatial variation in slope intensity of cropland

From the three-level topographic and geomorphological divisions, the degree of slope variation of cropland in China from 1990 to 2020 was observed. As shown in Figure 7, the SI is divided into five grades according to the natural breakpoint method and the concept of upslope and downslope, which are significant upslope, upslope, slight upslope, downslope, and distinct downslope. The areas with significantly upslope changes in cropland are mainly concentrated in Huang-Huai-Hai alluvial plain small-region (IG4), Piedmont of Taihang Mts diluvial-alluvial plains small-region (IG5), Lower reaches of Liaohe in Region I of the main grain-producing area River alluvial-marine plains small-region (IE2), Central Shandong middle and low mountains and hills small-region (IC3), Jiaolai alluvial plains small-region (IC2), Jiaodong low mountains and hills small-region (IC1), Zhejiang and Fujian coastal low mountains, hills and plains small-region (IIA2), Nanyang Basin low mountains, hills, platforms and plains small-region (IIB2), and other topographical areas sporadic areas. These areas in Region I have negative LSI and positive GSI. Due to the region’s flat topography and the slight average slope of the cropland, the data further reveals the degree of upslope. Region II as a whole is on an uphill trend. These regions have positive LSI values in the higher slope areas, but the additional cropland has a higher slope and higher GSI values. The slope of the cropland in these regions increases further.
Figure 7 Extent of spatial variation in slope change of cropland in China (1990-2020)
The cropland areas showing a significant downward slope are mainly concentrated in Southern Qilian Mt high mountains, valleys and basins small-region (VIA2), Altun Mt high and extremely high mountains small-region (VIA3), Northern Qilian Mt high mountains and valleys small-region (VIA1), Western segment of central Kunlun Mt high mountains and lake basins small-region (VIC3), Sources of Yangtze, Yellow and Lancang rivers hilly mountains and plateaus small-region (VIE1), Hoh Xil plateau hilly mountains and plateaus and lake basins small-region (VIG1), Nyainqentanglha and Gangdise Mts high and extremely high mountains small-region (VIH1), Himalayan extremely high and high mountains small-region (VIH3) in Region VI. And Northwestern Hebei middle and low mountains small-region (IIIA3), Northern Shanxi middle and low mountains and basins small-region (IIIB1), Helan Mt subalpine mountains small-region (IIID4), Yinshan Mt middle and low mountains small-region (IIID1), Liupan Mt middle and low mountains, hills and valleys small-region (IIIE3), Northern Shaanxi loessic ridges, tablelands and mounds small-region (IIIE1) in Region III. These regions are mainly located in ecologically fragile areas, such as the upper reaches of the Yangtze River, the upper reaches of the Yellow River, the Loess Plateau area in the middle reaches, and the interlocking agricultural and pastoral regions in Inner Mongolia. Most of them are areas where ecological projects of returning farmland to forests and grassland have been implemented (Liu et al., 2014a; 2014b; Tao et al., 2022). In addition, most of the other areas showed a slight increase in the slope of the cropland.
Based on SI, ωSI was further calculated using the scale of regional cropland changes to reveal the significant degree of regional changes in cropland slope undulation from a quantitative perspective. The ωSI is divided into five grades according to the natural breakpoint method and the concept of upslope and downslope, which are significant upslope, upslope, slight upslope, downslope, and distinct downslope. Significant upslope areas are concentrated in IG4, mainly due to its high SI value and the fact that it is the main grain-producing region in China. The cultivated land area is the largest among the three-level subregions. The changed arable land area is up to 29,561.82 km2.
The regions showing an upward cropland slope are IC3, IIA2, IIB2, IIC2, IG3, IIA1, and IID4, mainly concentrated in the lower Yangtze River region and the southeastern coast. The regions with a significant degree of downward cropland slope are IIIA3, IIIB1, IIIE3, and IIIE1, mainly concentrated in the northern agro-pastoral intersection.
As shown in Figure 8, to further observe the specific distribution of gained cropland slope changes in each region over 30 years, a 5 × 5 km grid was used to calculate the average SI. It was found that the slopes of changed cropland in Regions A and D decreased, while the slopes of Regions B, C, and E increased. The slope of the changed cropland in Region E, the southwestern mountainous region, increased and then decreased significantly. The gained cropland was returned to forests in this region, so there is a clear upward slope.
Figure 8 Spatial disturbance of SI (left) and ωSI (right) in China from 1990 to 2020. The cropland change areas were summarised as 5 km × 5 km grid.
Using the 5 × 5 km grid to calculate the ωSI, the influence degree of slope change can be analyzed based on the perspective of regional cropland amount. Based on Figure 8 (right), it can be seen that the regional slope fluctuates significantly due to the large amount of cropland changes in Regions A and B. Due to the significant local variation of croplands in Regions C and F, the areas with higher ωSI values are concentrated in areas that underwent rapid urban development and experienced a greater demand for cropland occupation. Region D has a small cropland change amount, and the downward slope has reduced significance. Region E has more variation in cropland, so the significance of the downhill and uphill slopes increased. This indicates that this area is undergoing massive ecological conservation while a large-scale increase in cropland is moving uphill, which is a significant potential hazard.

3.4 Gained cropland stability in undulating areas

We further assess gained cropland stability at different slopes in different regions. Figure 9 shows the results. From 1990 to 2020, the increase in the cropland area was 453,565 km2, and the abandoned area was 156,682.9 km2, with an abandonment rate of 34.54%. Excellent (S1), Better quality (S2), Good quality (S3), Medium quality (S4), Poor quality (S5), and Low quality (S6), respectively, account for 65.45%, 2.12%, 3.56%, 4.76%, 7.46%, 16.45%, while Low quality (S6) accounts for the highest proportion, indicating that a large rate of abandonment occurred in a short period of 1-5 years. The probability of abandonment over 10 years is relatively small, accounting for 10.44%. The average time to abandonment of gained cropland is 6.26-10.26 years. This indicates that except for stable land, the gained cropland stability in China is roughly at an S5 level.
Figure 9 The stability levels and abandoned ratio of gained cropland on different slopes (a. The national scale; b. Region and slope-based perspective)
Based on the regional perspective, the gained cropland stability in different regions differs. In Regions I-VI, S1 stability accounted for 71.39%, 65.48%, 54.69%, 83.23%, 61.81%, and 56.54%, respectively. The gained cropland stability in IV is the highest, and the abandoned area in each period is small. The stability in Regions III and V are relatively the worst, and S6 has many abandoned areas, with 21,434.78 km2 and 20,256.37 km2, accounting for 22.73% and 17.29%, respectively; meanwhile, in II and VI, Low quality (S6) at 16.82%- 18.55% accounts for a very high proportion. From the perspective of the average abandonment time in Regions I-VI, it is 6.50-10.50 years, 5.88-9.88 years, 6.03-10.03 years, 6.11-10.11 years, 6.59-10.59 years, and 6.80-10.80 years, respectively. This indicates that except for the stable cropland, the stability of other regions is at the S5 level, and Region II is the worst.
From the slope perspective, the gained cropland stability increases with the added slope. The proportion of Excellent (S1) decreases from 72.52% with a slope of [0°, 2°] to 49.65% with a slope greater than 25°, while the proportion of Low quality (S6) increases significantly, from 13.51% with a slope of [0°, 2°] to 24.37% with a slope greater than 25°. Among them, abandoned cropland areas with a 6°-15° slope and 1-5 years are the largest, reaching 20,962.35 km2. As the slope increased, the average abandonment years of the gained cropland were 6.19-10.19, 6.38-10.38, 6.36-10.36, 6.21-10.21, and 5.93-9.93, respectively. This indicates that except for stable cropland, the gained cropland stability is mainly at the S5 at each slope level, and regions with a slope greater than 25° have the worst stability.

4 Discussion

4.1 Conversion types regarding the gain and loss of cropland

Figure 10a shows the gain and loss of cropland in different geomorphic regions at different slopes from 1990 to 2020, respectively. Overall, the amount of gained cropland is less than the lost land, which varies in different regions and slopes. Except for Region IV, the gain in cropland exceeds the loss, and other regions are reduced, especially cropland with a less than 6° slope in Region I. Therefore, we calculated the proportion of cropland converted to other land types at different slopes in different regions to illustrate the cropland conversion trend.
Figure 10 Conversion between the gain and loss of cropland with other land types (a. The gain and loss of cropland areas in different regions, respectively; b. Proportion of conversion types from lost and gain of cropland on different slopes)
As shown in Figure 10b, about 82.72%-99.87% of the lost cropland was converted to impervious surfaces (ISs), forests, and grasslands at different slopes and regions. Therefore, we mainly focus on the conversion ratio of cropland to ISs, forests, and grasslands. The lost cropland in Regions I, II, and V at slopes of [0°, 2°], (2°, 6°] was mainly converted to an IS, while the lost cropland above a 6° slope was mainly converted to forests. The cropland in Region III was mainly converted to grassland, accounting for 60.55%-89.59%, and the conversion rate increased with increasing slope. Regions IV and VI were converted to forest and grassland at (25°, 90°], and over 70% of the lost cropland at other slopes was converted to grassland, indicating that construction land in the east and southwest has encroached on cropland at lower slopes. In the central and western regions, cropland is mainly occupied by forests and grasslands due to the extensive ecological work carried out.
The gain in cropland is mainly due to grassland reclamation and deforestation (Liu et al., 2005). In the Changbai Mountains of northeastern China, the proportion of forest areas declined from 91.28% in 1977 to 84.02% in 2018, with a large amount of deforestation converted to cropland (Zhang et al., 2020). Driven by forestry activities and agricultural expansion, forest loss in southwestern China was 3,752,700 hectares from 2001 to 2019, mainly in areas with slopes less than 40° and gradually shifting toward lower elevations with gentler slopes. The increase in cropland from 1990 to 2015 in the southeastern watersheds was mainly due to the conversion of forest land (Zhang and Zang, 2019). Over the last 40 years, China’s Three-North Shelter Forest Program has increased cropland, construction, and sandy lands. The primary land types reduced are grasslands, meadows, and bare soils (Hu et al., 2021). This study has systematically examined increased cropland sources in China over the last three decades, and the results show consistent but more comprehensive explanations than the above literature. In terms of vegetation distribution, grasslands are mainly found in the arid and semi-arid northwestern interior, where precipitation is less than 400 mm. Thus, increased croplands in Regions III, IV, and VI are mainly due to grassland conversion. In Region I, cropland is mainly converted from forests and grasslands, while in Regions II and V it is mainly converted from forest to cropland.

4.2 The possible reasons affecting newly gained cropland stability

It can be seen from the gained cropland stability assessment in undulating areas that cropland stability decreases with an increase in slope. This phenomenon is particularly obvious in the southwestern mountainous areas of Regions V and VI. The main reason is that areas with large slopes are usually located in mountainous areas, subject to complex terrain, land fragmentation, and other factors (Wang et al., 2022), and a lack of mechanized cropland production in mountainous areas, resulting in small-scale and low-intensity agricultural systems (Han and Song, 2019). On the other hand, because of low comparative agricultural benefits and rural labor precipitation, the cropland in these mountainous areas is more prone to abandonment (Yan et al., 2016). However, it is worth noting that gained cropland instability in Region III is also very high, which is an essential region of China’s project to return croplands to forests, so the overall cropland slope is reduced (Yin et al., 2018). However, this region is located in the agricultural and pastoral ecotone, and the ecological environment is fragile; the value of the scarce population, backward planting conditions, and other reasons are straightforward to abandon (Wuyun et al., 2022; Zhang et al., 2022b). In addition, there is also a considerable amount of new instability in Regions I and II, and the croplands in these two regions have increased, and SI is generally increased. The main reason is the rapid development of urbanization in the eastern region, which has occupied a large amount of high-quality cropland (Lin and Zhu, 2021). The poor stability and utilization of new cropland may be because Region I, including the provinces in northeastern China and Shandong, has been the main region of population loss in the past two decades. The lack of labor force may be the main reason for cropland abandonment (Deng et al., 2018; Sun and Wang, 2021), while Region II, mainly affected by the low specific benefit of agriculture, has a severe phenomenon of non-grain cultivation in these regions, which is also a sign of cropland instability (Sun et al., 2021). The main reason for the high stability of new croplands in Region IV is the LSI and GSI in this region have the smallest variation among the six regions, ranging from 0 to 0.1, so the change of SI is minimal, that is, the cropland planting conditions in this region have not changed significantly, which is conducive to stable cropland utilization.

4.3 A new way to assess the effect of gained croplands

To prevent the encroachment of urbanization on cropland resources, China implemented the most stringent balancing policy between the occupation and compensation of cropland, requiring a balance of cropland quantity, quality, and ecology (Liu et al., 2019b). In the implementation and supervision of supplementary cropland, mainly using the plot scale as a unit, the cropland quality was evaluated using the soil index. At the same time, the quantity balance was ensured, or the regional gained cropland quality was evaluated using the cropland grading standard. This kind of assessment of gained cropland has node timeliness, reflecting the effect of gained cropland in the requisition-compensation balance at that time. In terms of the long-term effects, a large amount of gained cropland is abandoned or converted to other land types due to a lack of long-term management (Xin and Li, 2018; Wang et al., 2020b). However, there is a shortage of long-term supervision and assessment of gained cropland at the national scale. Although (Chen et al., 2022) paid attention to the problem of uphill cropland, the slope distribution, and factors driving changes in slope, the stable utilization degree of newly cultivated cropland after the mountain was ignored.
The core requirement of requisition-compensation balancing is quantity balance. Therefore, abandonment and the number of years are used as the core indicators to assess the gained cropland stability, and the gained cropland stability is divided into six levels according to the continuous utilization period, which can reflect the stability level at different slopes and regions at the national scale, and the macro effect of the long-term requisition-compensation balancing policy. The assessment result shows that overall cropland stability is not satisfactory. About a third of gained cropland is abandoned, 16.45% of it is abandoned within 1-5 years, and the proportion of gained cropland with a stability level of S3-S6 increases significantly with the added slope, indicating that a high slope is prone to abandonment. From a regional perspective, the area and proportion of abandoned cropland in Regions III and V are very high, mainly because Region III may be converted to a forest by the balancing policy. The climate in Region III is arid, and the soil is generally poor, which is not conducive to farming. Region V is abandoned because of topography and landforms because of so many mountains. These results are consistent with previous studies, which is beneficial to improving cropland policy.

4.4 Policy implications

Combined with the evaluation results of slope change and cropland stability nationwide in this study, suggestions are presented as follows. First, improve policies for the management of newly gained cropland. China’s cropland is moving uphill and is mainly concentrated in Shandong, the southeastern coast, and parts of the southwestern mountainous regions, which undoubtedly impacts food security. Therefore, in newly developed cropland, it is necessary to strengthen the restrictions on the slope to prevent the cropland from moving further uphill. Moreover, these regions in the southwestern mountainous region face ecological regulations, due to the fragile regional ecology, such as newly gained cropland slope controls, but also to demonstrate geological hazards, ecological impacts, and improve the development of related policies. Second, formulate a policy to regulate the utilization of newly gained cropland stability in response to the problem of newly gained cropland’s low stability. Of which, one-third is eventually abandoned, more than half of which occurs in areas at slopes greater than 25°, and has the highest probability occurring in a short period of within 1-5 years. Accordingly, the protection and regulation of newly gained cropland should be prioritized, including potential utilization assessments carried out before targeted development. Especially in the first 5 years of new cropland utilization, cropland functional settings and stability management should be strengthened. Third, carry out the restoration and management of newly gained cropland instability. Newly gained cropland left fallow within 5 years, which accounts for a large proportion of the cropland that has been left fallow for a short period, is the most easily restored potential management region. Each region should comprehensively assess the possibility of recovery based on natural conditions, human resources, and other factors. For example, cropland in the eastern plains is in good condition and should be used as a critical restoration and treatment area. In contrast, the southwestern mountainous area has a higher slope, and the western arid area and other ecologically fragile areas with inferior cultivating conditions should be rehabilitated or returned to grassland or forests. Fourth, establish a systematic supervision and operation mechanism to protect and utilize new cropland. In the practice of cropland management, the development and utilization of cropland is coordinated at multiple levels and departments. For example, the agriculture and rural sector is responsible for cropland quality, while the natural resources sector is responsible for the planning and upgrading cropland. Therefore, to manage uphill cropland, it is necessary to strengthen the top-level design, form a unified management standard, fully implement the cropland supervision task, and form a multi-department collaborative operation mechanism. In addition, incentive policies and special funds should be introduced to guide social capital and farmers to resume cultivating land.

4.5 Limitations

This study reveals the status of croplands in China over the preceding 30 years and evaluates gained cropland stability. It fills the research gap to a certain extent, but there are also shortcomings. On the one hand, the quality of remote sensing data, including spatial resolution, collection time, and length, will affect the results (Guo and Song, 2019; Yin et al., 2020). We used CLCD to detect the slope changes in croplands, and the overall accuracy of this dataset was 79.31%, although these data were of high accuracy and wide application. However, inevitably, there were classification errors, which can affect our results. On the other hand, as for the cropland stability evaluation method, these data were selected based on a 5-year cycle, which cannot be accurate for each year, and the discontinuity of these data may affect the stability evaluation effect for gained cropland. Future research studies need to study land use classification. Multi-source remote sensing images should be integrated to establish long-time series data and more accurately detect the cropland conversion types at different slopes. In cropland stability evaluation, the abandonment type should be discussed in addition to the precise abandonment year of gained cropland. At the same time, the driving effect of physical geography and social economy on cultivated land transformation was discussed through multi-scale and multi-methods to put forward targeted cultivated land protection policies.

5 Conclusions

Cropland cultivation in high-slope areas in China has alleviated the contradiction between the limited total amount of cropland and the lack of development space; however, there are risks coupled to the phenomenon of cropland moving uphill. Therefore, this study examines the cropland conversion in China over the past three decades from the slope perspective and assesses the gained cropland stability. The results show that: (1) The croplands at different slope ranges in China show an upslope trend from 1990 to 2020. (2) The croplands in Regions I and II present a phenomenon of moving uphill in different years. The slope of croplands in Region III shows a downslope. Regions IV, V, and VI all change slightly. (3) In view of this spatial divergence, the areas with significant upslope gain and loss of cropland are mainly concentrated in the main grain-producing areas, such as IG4 and IG5 in the southern part of Region I, and the well-developed eastern coastal areas, such as IIA2 and IIB2 in Region II. The areas with obvious downslope movement of cropland are mainly concentrated in ecologically fragile areas such as the upper reaches of the Yangtze and Yellow Rivers, the Loess Plateau in the middle reaches, and the agro-pastoral cross belt in Inner Mongolia. (4) Overall cropland stability is unsatisfactory. About two-thirds of the gained cropland is S1, and 16.45% is abandoned within 1-5 years. Except for stable croplands, the stability is mainly S5 at each slope level, and the region with a slope greater than 25° shows the worst stability. Based on the results, we discuss the cropland conversion types in different regions and slopes and suggest cropland protection policies. Although China has implemented a strict policy of balance between the occupation and compensation of croplands, the quantity is out of balance, the slope of croplands fluctuates significantly, and gained cropland stability has not been high over the past three decades. In the future, this policy should be further improved to prevent croplands from moving further uphill, and the supervision and management of sloping croplands should be strengthened to maintain sustainable use. Cropland change dynamics are a complex process with multiple characteristics, which need to be complemented using quantitative and directional analyses to understand the process. Future studies can further investigate the spatial correlates of slope change in croplands. By combining socio-economic data with quantitative policy tools and landscape patterns, a multi-factor and multi-faceted analysis can be carried out to provide a basis for more profound systematic control and policy formulation for cropland conservation

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