Cropland: Surplus or Deficit? From the Perspective of Meeting People’s Grain Requirement

The quantity and quality of cropland plays an important role in ensuring food security. In order to explore spatiotemporal patterns of the extent to which cropland satisfies people’s grain need, we integrate multi-source heterogeneous data to investigate in which era, and in which region, the cultivated land can meet people’s food demands. It turns out that in the past 30 years, with the exception of the late 1980s, the amount of cropland could satisfy people’s grain needs at the nation scale. However, more than 10 provinces (municipality/autonomous region), mainly located in western China and southeast coastal areas, have been unable to meet the grain needs of local people. We projected the guarantee rate to the late 2020s. Our study concludes that the guarantee rate of cropland is estimated to be higher than 150% in China. Compared to 2019, except Beijing, Tianjin, Liaoning, Jilin, Ningxia, as well as Heilongjiang in the Sustainability scenario, and Shanghai in the Sustainability and the Equality scenarios, the guarantee rate of cultivated land will increase in every province (municipality/autonomous region) in 2030. This study has reference value for the study of China’s cultivated land protection system, as well as important significance for China’s sustainable development.


Introduction
Globally, factors such as population growth, higher incomes and urban lifestyles are driving changes in food demand and consumption, putting pressure on the quantity and quality of land resources [1,2]. Similarly, China now shares the same dilemma.
Since the reform and opening up in 1978, after 40 years of spectacular growth, China has become the world's second-largest economy. The population has increased by almost 40 percent, from 987 million in 1980 to 1.412 billion in 2020 [3]. Meanwhile, a marked increase of urbanization rate, 19.39 percent in 1980 and 63.89 percent in 2020, has been seen in China over the last several decades [3]. At the same time, diets have changed, resulting in a higher proportion of non-starchy foods [4], with per capita consumption of meat, aquatic products, and eggs increasing by 0.23, 1.04, and 1.12 times, respectively, between 1990 and 2020 [3,5]. Moreover, the demand for animal product is projected to increase further in China, and livestock production will nearly double in the next few decades [6,7], which means higher requirement for feed grain. In addition to the change of diet structure, the reduction of cultivated land was along with the urbanization process [8], which brought great pressure to China's food security.
A great deal of effort has been made in China to safeguard domestic food security. Grain output in China has increased steadily under a series of polices and measures implemented by the Chinese government and farmers, such as the delimitation of farmland protection red line and the construction of well-facilitated cropland, etc [9]. Grain output This study includes three steps for the analysis of the past and the future. First, we calculated people's grain needs based on the population and per capita grain requirements. Then, by taking gain crops sown area, farm crops sown area, grain yield per unit area and cropland area into account, we obtain the cropland requirement. Finally, according to the cropland requirements, cropland guarantee degree was calculated (Figure 1). The following paragraphs illustrate the details of the calculation.

Scenario Description
Projections of the level of cropland satisfying people's grain need in the future are critical to enable a better understanding and anticipation of cropland's bearing capacity. The climate projections and scenarios assessed by the IPCC (the Intergovernmental Panel on Climate Change, IPCC) based on SSP (the Shared Socioeconomic Pathway, SSP)-RCP (the Representative Concentration Pathway, RCP) framework, have furnished an exhaustive grasp of the restrictions and opportunities for policy action [27].
The Representative Concentration Pathway 2.6 represents scenarios that lead to quite low greenhouse gas concentrations. This is a scene of "peak and fall". Its radiative forcing level first reached about 3.1 W/m 2 by the middle of this century, and then returned to 2.6 W/m 2 by 2100. In order to achieve such a level of radiative forcing, greenhouse gas emissions (as well as indirect emissions of air pollutants) should gradually decrease over time [28]. Under RCP 4.5, the total radiation forcing tends to be stable soon after 2100 and does not exceed the target level of long-term radiation forcing [29]. The Representative Concentration Pathway 3.4 denotes an intermediate mitigation effort pathway that lies between RCP 2.6 and RCP 4.5 [30].
The Shared Socioeconomic Pathway describes possible changes in various aspects of society in the 21st century, such as population, economy, technology, society, governance

Scenario Description
Projections of the level of cropland satisfying people's grain need in the future are critical to enable a better understanding and anticipation of cropland's bearing capacity. The climate projections and scenarios assessed by the IPCC (the Intergovernmental Panel on Climate Change, IPCC) based on SSP (the Shared Socioeconomic Pathway, SSP)-RCP (the Representative Concentration Pathway, RCP) framework, have furnished an exhaustive grasp of the restrictions and opportunities for policy action [27].
The Representative Concentration Pathway 2.6 represents scenarios that lead to quite low greenhouse gas concentrations. This is a scene of "peak and fall". Its radiative forcing level first reached about 3.1 W/m 2 by the middle of this century, and then returned to 2.6 W/m 2 by 2100. In order to achieve such a level of radiative forcing, greenhouse gas emissions (as well as indirect emissions of air pollutants) should gradually decrease over time [28]. Under RCP 4.5, the total radiation forcing tends to be stable soon after 2100 and does not exceed the target level of long-term radiation forcing [29]. The Representative Concentration Pathway 3.4 denotes an intermediate mitigation effort pathway that lies between RCP 2.6 and RCP 4.5 [30].
The Shared Socioeconomic Pathway describes possible changes in various aspects of society in the 21st century, such as population, economy, technology, society, governance and environmental factors. The purpose is to promote a comprehensive analysis of future climate influence, vulnerability, adaptation and mitigation [31,32]. In SSP1, the world is gradually moving towards a more sustainable path, with a focus on development that respects environmental boundaries. SSP2 implies that the world is on an intermediate path, where social, economic and technological trends have not deviated significantly from the historical pattern. In SSP4, highly unequal social investment in human capital, coupled with growing inequality between economic opportunities and political power, has led to increasing inequality and stratification between and within countries. In this study, three climate models from CMIP6 (the Coupled Model Intercomparison Project Phase 6, CMIP6) were considered under the three SSPs in 2030, namely the Sustainability (SSP1-RCP2.6), "Middle of the Road" (SSP2-RCP4.5), and the Inequality (SSP4-RCP 3.4). These three scenarios describe the possible future world and represent different combinations of mitigation and adaptation challenges [30].

Calculation of Grain Demand
In this study, people's grain consumption was considered as the grain demand by people. Ration and feed grain were two parts that people needed, and the calculation of grain demand is: where Grain requirement is the grain demand; Ration per capita and Feed grain per capita are ration and feed grain needed of each person, respectively; Pop is the population.
(1) Ration The calculation of ration consumption in rural and urban areas is the same with previous study [9], and it can be described as follows: Feed grain all = Pork grain + Beef grain + Mutton grain + Poultry meat grain + Egg grain +Milk grain + Aquatic product grain where Ration all means the total amount of ration consumption in each province; Ration urban_per and Ration rural_per stand for per capita ration consumption in urban and rural areas, respectively; Pop urban and Pop rural mean the population in urban and rural areas. Details about population data can be seen in Section 2.2. The descriptions of per capita ration consumption are illustrated in Section 2.2.2.
(2) Feed Grain The method of calculation of feed grain in rural and urban areas is similar to the previous study [9], which can be written as: Feed grain all = Pork grain + Bee f grain + Mutton grain + Poultry meat grain + Egg grain +Milk grain + Aquatic product grain where Feed grain all represents total amount of feed grain needed, and Pork grain , Bee f grain , Mutton grain , Chicken grain , Egg grain , Milk grain and Aquatic product grain mean the amount of grain needed in the people's consumption of pork, beef, mutton chicken, egg, milk and aquatic product, respectively. Here, we take the calculation of Pork grain as an example to illustrate the process: where Pork grain means the amount of grain needed in the people's consumption of pork; Pork grain_urban and Pork grain_rural mean the amount of grain needed in the people's consumption of pork in urban and rural areas, respectively.
Pork grain_urban = (Pork per_urban × Pop urban ) × δ × Rice pork + Wheat pork + Maize pork +Soybean pork + Tuber pork where Pork per_urban represents the amount of pork consumption for each person in urban areas; Pop urban represents the population in urban areas; δ represents forage required per unit of pork, which can be referred to in Table 1; Rice pork , Wheat pork , Maize pork , Soybean pork and Tuber pork represent the propotion of rice, wheat, maize, soybean and tuber in the forage, respectively, which can be referred to in Table 2. The calculation of the amount of grain needed in the people's consumption of pork in rural areas is the same as the calculation in urban areas.
However, there are some differences: First, the change in feeding structure has been taken into account when calculating feed grain. The details are as follows: Forage required per unit of product in 1989 and 1999 is based on the research results of Wang Minli and other researchers of the Institute of Agricultural Economy and Development, Chinese Academy of Agricultural Sciences [33]. With social and economic development, the livestock breeding structure in China has changed greatly, and breeding has gradually become large-scale. In the study of Xie [34], the feed required under the large-scale feeding for per unit product was illustrated. Therefore, the feed required per unit in the study of Xie et al. [34] was adopted to determine the forage required per unit of product in 2009, 2019 and 2030 in this study. The final results are shown in Table 1.
Second, the proportion of edible parts of meat, aquatic products, milk and eggs and the loss of production and circulation are considered. The proportion of edible parts was 65.9%, 55.6%, 100% and 85% respectively, and the loss proportion was 15%, 34%, 6% and 10% respectively [34].

Calculation of Cropland Requirement
The definition of cropland requirement is as below: where Cropland requirement is the amount of cropland needed; Grain requirement is the amount of grain needed by the people; m is the grain yield per unit area; n is the percentage of the area sown by grain crops in the area of cultivated land; k is multiple-crop index, which means the ratio of crop sown area to cultivated area. Data from 1989 to 2019 used in this part can be seen and calculated from Table 3. As for 2030, the data can be obtained from the following paragraphs [35]: m: Due to the bottleneck of per unit yield potential of cultivated land, when the per unit yield level continues to increase and approaches the maximum per unit yield potential, the potential for per unit yield increase will gradually decrease. The function curve of exponential decay model can better reflect this change trend. This paper assumes that the external environment disturbance variable of crop growth is constant, that is, without considering crop improvement factors, we can use the grain yield data over the years (1997-2020) to build regression analysis models of every province, and the formula is as follow: where Y p is the potential of average grain yield per unit area, which is 11,349.21 kg/hm 2 ; Y t is the grain yield per unit area in year t.
In this study, the values of n and k in 2030 originated from previous studies, which were 0.68 and 1.2 respectively.

Definition of Guarantee Level of Cropland
Guarantee level of cropland is defined as: where Rate cropland is the guarantee rate of cropland; Cropland supply is the quantity of cropland available; Cropland required is the quantity of cropland which needed by people.

Data Source
Six categories of data were applied in this study, including population, per capita grain requirement, cropland, per unit area grain yield, sown areas of farm crops, and sown areas of grain crops (Table 3), the details of which are presented in the following sections.  [3,5,36,37]. By reason of the lack of rural and urban population in 1989 and 1999, we calculated the urban and rural population in 1989 and 1999 based on the proportion of urban population in 1990 and 2000 [5,36].
Population data in 2030 under three scenarios were accessed from the high resolution data set for global future population developed with RCP (the Representative Concentration Pathway, RCP) and SSP (the Shared Socioeconomic Pathway, SSP) scenarios (https:// dataguru.lu.se/app#worldpop, accessed on 2 September 2022) [38].
Population of each province from 1989 to 2030 can be seen from Table 4.

Per Capita Grain Requirement
People's needs for grain include ration and feed grain. Therefore, per capita food consumption is necessary in the calculation of ration and feed grain. The data for per capita food consumption in rural and urban area in 1989,1999,2009 Table 5.  [3,5,36,37].
Based on the average increasing rate of per unit area grain yield from 1989 to 2019, we calculated the grain yield per unit area of 2030.

Nationwide
During the past 30-year period from 1989 to 2019, the amount of grain needed by people rose from 3.43 × 10 11 kg to 4.11 × 10 11 kg. Compared with 2019, the amount of grain needed by people is projected to increase, which would reach 4.14 × 10 11 kg, 4.25 × 10 11 kg and 4.12 × 10 11 kg in 2030 under the Sustainability, the Middle Road and the Inequality scenarios (Figure 1).
Under the three scenarios in 2030, the guarantee rate of cropland is projected to increase, with the Middle Road being the highest and the Sustainbility is expected to be the lowest (Figure 2). Inequality) of 2030 is 1.92 × 10 ha, 1.32 × 10 ha, 1.26 × 10 ha, 1.53 × 10 ha, 1.62 × 10 ha, 1.64 × 10 ha and 1.61 × 10 harespectively. However, the amount of cropland supplied is 1.76 × 10 ha , 1.79 × 10 ha , 1.78 × 10 ha , 1.78 × 10 ha , 2.55 × 10 ha , 2.73 × 10 ha and 2.69 × 10 ha, respectively. This implies that the amount of cropland could satisfy people's grain need except 1989. In addition, it can be seen that the ability of cropland to satisfy people's grain needs increased from 1989 to 2009, while it decreased from 2009 to 2019. Under the three scenarios in 2030, the guarantee rate of cropland is projected to increase, with the Middle Road being the highest and the Sustainbility is expected to be the lowest (Figure 2).

Amount of Grain Needed by People
To capture the regional heterogeneity of grain needed by people, we disaggregate grain consumption into five categories from low to high: Lowest (<3); Medium low (3≥ and <9); Low (9≥ and <15); Medium high (15≥ and <21); Highest (≥21). There are distinct patterns of the amount of grain needed by people ( Figure 3): for example, relatively large shares of grain needed by people in Shandong, Henan, Sichuan, Jiangsu, Hunan and Guangdong.

Amount of Grain Needed by People
To capture the regional heterogeneity of grain needed by people, we disaggregate grain consumption into five categories from low to high: Lowest (<3); Medium low (3≥ and <9); Low (9≥ and <15); Medium high (15≥ and <21); Highest (≥21). There are distinct patterns of the amount of grain needed by people ( Figure 3): for example, relatively large shares of grain needed by people in Shandong, Henan, Sichuan, Jiangsu, Hunan and Guangdong.
A look at demand patterns over time provides insight into the amount of grain needed by people (Figure 3). Over the past thirty years, Jilin, Heilongjiang, Hubei, Shaanxi and Qinghai have seen a declining trend in people's grain demand, while the opposite has been true in other provinces. Under the Sustainability and the Inequality scenarios in 2030s, people's grain demand in Hebei, Zhejiang, Anhui, Fujian, Jiangxi, Hunan, Guangdong, Guangxi, Hainan, Chongqing, Sichuan, Yunnan and Tibet is expected to decrease compared to the late 2010s, while the other provinces (municipality/autonomous region) show the opposite. With the exception of Hebei, the development of the Middle Road is the same as the above scenarios (Figure 3).

Cropland Needed by People
Delineating cropland by group is critical to understanding cropland demanding trends. Provinces in the western part of China led in cropland demand from 1989 to 2009. In 2019 and 2030, it can be seen that the southeastern coastal areas, from Huang Huai Hai Plain to Sichuan Basin areas, and northwest areas play major role in the demand for cropland (Figure 4). the same as the above scenarios (Figure 3).

Cropland Needed by People
Delineating cropland by group is critical to understanding cropland demanding trends. Provinces in the western part of China led in cropland demand from 1989 to 2009. In 2019 and 2030, it can be seen that the southeastern coastal areas, from Huang Huai Hai Plain to Sichuan Basin areas, and northwest areas play major role in the demand for cropland (Figure 4).  In the past thirty years, the guarantee rate of cultivated land in the other provinces (municipality/autonomous region) has declined, with the exception of Beijing, Tianjin, Shanghai, Zhejiang, Fujian, Guangdong, Guangxi, Hainan, Sichuan, Tibet, and Qinghai ( Figure 5).

Guarantee Rate of Cropland in the Past Few Years
In the past thirty years, the guarantee rate of cultivated land in the other provinces (municipality/autonomous region) has declined, with the exception of Beijing, Tianjin, Shanghai, Zhejiang, Fujian, Guangdong, Guangxi, Hainan, Sichuan, Tibet, and Qinghai ( Figure 5). In 1989, cropland in Tianjin, Shanxi, Inner Mongolia, Liaoning, Jilin, Heilongjiang, Shanghai, Hainan, Guizhou, Yunnan, Tibet, Shaanxi, Gansu, Qinghai, Ningxia, and Xinjiang could not meet people's grain needs. Cropland in Sichuan, Jiangsu and Fujian met people's grain needs to a great extent. While the cropland of the remaining provinces (municipality/autonomous region) was able to satisfy people's grain need basically.
In 1999, the cropland in Beijing, Tianjin, Shanxi, Liaoning, Shanghai, Yunnan, Tibet, Shaanxi, Gansu, Qinghai and Xinjiang was unable to meet people's grain need. Cropland in Heilongjiang, Jilin, Hebei, Shandong, Henan, Hubei, Anhui, Jiangsu, Hunan, Jiangxi, Zhejiang and Guangxi met people's grain need to a great extent. While the cropland in other provinces (municipality/autonomous region) was able to satisfy people's grain need basically.
In 2009, cropland in Beijing, Tianjin, Shanxi, Liaoning, Shanghai, Zhejiang, Fu-jian, Guangdong, Hainan, Tibet, Shaanxi, Gansu and Qinghai could not meet people's grain needs. Cropland in Inner Mongolia, Ningxia, Heilongjiang, Jilin, Hebei, Henan, Shandong, Jiangsu, Anhui, Hubei, Hunan, Jiangxi and Guizhou met people's grain need to a great extent, while the cropland of the remaining provinces (municipality/autonomous region) was able to satisfy people's grain need basically. In 1989, cropland in Tianjin, Shanxi, Inner Mongolia, Liaoning, Jilin, Heilongjiang, Shanghai, Hainan, Guizhou, Yunnan, Tibet, Shaanxi, Gansu, Qinghai, Ningxia, and Xinjiang could not meet people's grain needs. Cropland in Sichuan, Jiangsu and Fujian met people's grain needs to a great extent. While the cropland of the remaining provinces (municipality/autonomous region) was able to satisfy people's grain need basically.
In 1999, the cropland in Beijing, Tianjin, Shanxi, Liaoning, Shanghai, Yunnan, Tibet, Shaanxi, Gansu, Qinghai and Xinjiang was unable to meet people's grain need. Cropland in Heilongjiang, Jilin, Hebei, Shandong, Henan, Hubei, Anhui, Jiangsu, Hunan, Jiangxi, Zhejiang and Guangxi met people's grain need to a great extent. While the cropland in other provinces (municipality/autonomous region) was able to satisfy people's grain need basically.
In 2019, the cropland in Beijing, Tianjin, Shanghai, Zhejiang, Fujian, Guangdong, Guangxi, Hainan, Tibet, Shaanxi and Qinghai was unable to meet people's grain need. Cropland in Xinjiang, Inner Mongolia, Ningxia, Heilongjiang, Jilin, Hebei, Henan, Shandong, Jiangsu, Anhui, Hubei, Hunan, Jiangxi, Guizhou and Yunnan met people's grain need to a great extent, while the cropland of the remaining provinces (municipality/autonomous region) was able to satisfy people's grain need basically.

Guarantee Rate of Cropland in the Future
Under the Sustainability, the Middle Road and the Inequality scenarios, apart from Beijing, Tianjin, Shanghai, Jiangsu, Fujian, Guangdong, as well as Hainan in the Sustainability scenario, all provinces (municipality/autonomous region) are projected to achieve guarantee rate of cropland greater than 100% ( Figure 6). Cropland in Xinjiang, Inner Mongolia, Ningxia, Heilongjiang, Jilin, Hebei, Henan, Shandong, Jiangsu, Anhui, Hubei, Hunan, Jiangxi, Guizhou and Yunnan met people's grain need to a great extent, while the cropland of the remaining provinces (municipality/autonomous region) was able to satisfy people's grain need basically.

Guarantee Rate of Cropland in the Future
Under the Sustainability, the Middle Road and the Inequality scenarios, apart from Beijing, Tianjin, Shanghai, Jiangsu, Fujian, Guangdong, as well as Hainan in the Sustainability scenario, all provinces (municipality/autonomous region) are projected to achieve guarantee rate of cropland greater than 100% (Figure 6).
. With the exception of Beijing, Tianjin, Liaoning, Jilin, Ningxia as well as Heilongjiang in the sustainability scenario and Shanghai in the sustainability and equality scenario, the guaranteed rate pf cropland is expected to be higher in all provinces (municipality/autonomous region) compared to 2019 ( Figure 6). With the exception of Beijing, Tianjin, Liaoning, Jilin, Ningxia as well as Heilongjiang in the sustainability scenario and Shanghai in the sustainability and equality scenario, the guaranteed rate pf cropland is expected to be higher in all provinces (municipality/autonomous region) compared to 2019 ( Figure 6).

Conclusions
In this study, we analyzed whether or not cropland in China could satisfy people's grain needs across geographic and temporal scales. The following conclusions can be drawn: (1) With the exception of 1989, the amount of cropland could meet people's grain need. Moreover, the ability of cropland to satisfy people's grain need increased from 1989 to 2009, while decreased from 2009 to 2019. Under the three scenarios in 2030, the guarantee rate of cropland is estimated to be higher than 150%.
(2) From 1989 to 2019, the guarantee rate of cultivated land in other provinces (municipalities/autonomous regions) decreased except Beijing, Tianjin, Shanghai, Zhejiang, Fujian, Guangdong, Guangxi, Hainan, Sichuan, Tibet and Qinghai. Furthermore, more than 10 provinces (municipality/autonomous region), which are mainly located in western China and southeast coastal areas, were unable to satisfy the grain demand of local people. Compared to 2019, all provinces (municipalities/autonomous regions) except Beijing, Tianjin, Liaoning, Jilin, Ningxia, and Heilongjiang in the Sustainability scenario, and Shanghai in the Sustainability and the Equality scenarios, are projected to have a higher guarantee rate of cropland in 2030 than in 2019.

Discussion
The demand for cultivated land is the key content of this study. By comparing with the existing research, the demand for cultivated land calculated in 2030 (161 million hectares to 164 million hectares) is slightly higher than the previous research results (about 150 million hectares) [35], which is mainly related to the determination of the future population and the calculation method of the food required for nutritional needs.
From 1989 to 2009, under the comprehensive influence of population and per capita grain ration and feed grain consumption changes, the grain consumption demand of the Chinese population changed little, but due to technological progress, the unit yield of cultivated land increased, and the amount of cultivated land required decreased. As a result of urbanization and the project of returning farmland to forests, a large amount of cultivated land in China has been occupied [8,40]. However, the implementation of the policy of balancing the occupation and compensation of cultivated land has made the occupied cultivated land replenished [41,42], so the degree of China's cultivated land to meet people's food needs has been increasing. From 2009 to 2019, due to population growth, urbanization process and changes in the unit yield of cultivated land, the area of cultivated land required by Chinese residents for food consumption increased, while the actual existing cultivated land area changed little, and the final cultivated land satisfaction rate decreased. Under the three scenarios, China's cultivated land satisfaction rate in 2030 is between 158% and 169%, showing a slightly tight balance. In addition, the future scenario is set on the basis of a reasonable dietary structure. If the current dietary structure is followed, the pressure on cultivated land in the future may be greater.
China is a vast country with large regional differences. The differences in natural conditions, diet structure, population development and urbanization process in different regions have led to differences in the cultivated land satisfaction rate in different regions of China. The cultivated land resources in western China are poor, and the problems of soil erosion, desertification and soil salinization are serious [43][44][45]. The local food production in these areas is facing serious challenges. Due to the rapid economic development in the eastern coastal areas, a large area of cultivated land has been occupied and food production has been hindered.
China now is in a new era of food security. The decline in the area of cultivated land and low utilization efficiency have a very negative impact on the potential productivity of China's cultivated land. Although the results of this study suggest that the amount of cultivated land in China will be more than 1.5 times of that needed to meet people's grain needs by 2030, the marginalization of cultivated land in the process of urbanization, the non-agricultural transformation of cultivated land, and the transformation of agricultural cropping patterns to achieve economic benefits have potential impacts on China's food production. Hence, under the background of food security, we need to reasonably use and control the cultivated land, strictly monitor the quantity of cultivated land, and improve the productivity of cultivated land. In addition, due to the geographical differences in population distribution, cultivated land quantity and cultivated land production potential, cultivated land protection in different regions of China faces various pressures. Therefore, under the strategic background of national food security, each region should actively explore and formulate cultivated land protection policies and agricultural production plans, that are suitable for itself and promote the sustainable development of China's food production.