1. Introduction
Cultivated land resources are the material basis for human survival and development. The report of the CPC’s 19th National Congress pointed out that “we must Ensure national food security and secure the livelihood of our people”. The basic requirement of protecting food security is to ensure that the quantity and quality of cultivated land resources are not reduced. The Black Soil Region is fertile and is one of the most suitable areas for farming. With the acceleration of the new urbanization process and the rapid economic and social development, land degradation in the Black Soil Region, such as soil erosion, secondary salinization of soil, and thinning of the thickness of the black soil layer, has become a severe problem, which considerably affects the grain production capacity. It is of great practical significance for the study of food security and sustainable development of regional cultivated land to analyze the temporal variation pattern and spatial trend evolution of cultivated land resources in Heilongjiang Province, a Black Soil Region of Northeast China [
1].
At present, the research on the characteristics of the change of cultivated land resources at home and abroad mainly focuses on aspects including the temporal and spatial variation of cultivated land [
2,
3,
4], its driving mechanism [
5,
6], and the influence of such change on grain production capacity [
7,
8]. Some scholars have analyzed the characteristics of cultivated land using conversion from the perspective of the dominant form according to the connotation of land-use transformation theory [
9,
10]. The temporal and spatial variation characteristics in terms of research scale include national, regional [
11,
12], city, and county [
13]. Only a few studies focus on the Black Soil Region as the research area. In such studies, remote sensing is often used for analysis, and econometric models, such as transfer matrix [
14], kernel density, and spatial autocorrelation [
15], have also been gradually introduced. They focus primarily on aspects such as ecological effects [
16] and soil quality evaluation of the region [
17,
18,
19,
20]. Further studies are required given this situation. In addition, some scholars have also proven the impact of changes in cultivated land resources on grain production capacity. Cultivated land is an important carrier of grain production, and scientific assessment of cultivated land productivity is of great significance to ensure food security [
21]. The potential agricultural productivity was developed at a pixel level in mainland China [
22].
In this paper, the temporal and spatial variation pattern of cultivated land resources in Heilongjiang Province from 1995 to 2015 is analyzed. The spatial distribution of cultivated land resources was analyzed by using kernel density to explore the law of temporal variation and the trend in spatial evolution of cultivated land resources in the study area, and to provide an important theoretical basis for ensuring the food security and the protection of cultivated land in the Black Soil Region.
Heilongjiang Province is an important grain production area in China. It is located in Northeast China, 43°26′~53°33′ north latitude, 121°11′~135°05′ east longitude. It is adjacent to Russia, with Wusuli River and Heilongjiang River as the boundary in the east and north. It is bordered by the Inner Mongolia Autonomous Region in the west, and connects Jilin Province in the south. Its western part belongs to Songnen Plain, and the northeast belongs to Sanjiang Plain. The cultivated land area accounts for 35.1% of the total land area, 4525.4 × 104 hm2. Heilongjiang Province consists of 12 prefecture-level cities and 1 prefecture, including Harbin City, Qiqihar City, Mudanjiang City, Jiamusi City, Qitaihe City, Daqing City, Heihe City, Suihua City, Yichun City, Hegang City, Shuangyashan City, Jixi City, and Da Hinggan Ling Prefecture.
Heilongjiang Province has a temperate continental monsoon climate. The annual average temperature is between −5 °C and 5 °C. The annual average precipitation between 400 mm and 650 mm, with the most precipitation in the middle mountain area, then in the east, and the least in the west and north. The terrain is generally high in the northwest, north, and southeast, and low in the northeast and southwest. The Black Soil Region of Northeast China is one of the only “three major Black Soil Regions” in the world. It is mainly distributed in Heilongjiang Province, Jilin Province, Liaoning Province, and Inner Mongolia Autonomous Region, and concentrated in the central and western Heilongjiang Province, including Heihe City, Qiqihar City, Suihua City, Harbin City, Jiamusi City, Qitaihe City, and Shuangyashan City. The Black Soil Region of Heilongjiang Province is about 65% of the area of the Black Soil Region of Northeast China, and its cultivated land area accounts for about 83% of the area of the Black Soil Region of Heilongjiang Province. The cultivated land area in Qiqihar City, Harbin City, Suihua City, Heihe City, and Jiamusi City is large, while the cultivated land area in Yichun City and Daqing City is the smallest.
In 2016, Heilongjiang Province became the “No. 1 of Grain Production” in the whole country. The total grain production of Heilongjiang Province accounted for 1/10 of the national total grain production. However, at the same time, problems of soil erosion and thinning of the thickness of the black soil layer remain the main limiting factors for the utilization of cultivated land resources in Heilongjiang Province. Therefore, it is of great significance for the sustainable utilization of cultivated land resources and the improvement of grain production capacity to study the temporal and spatial variation characteristics of cultivated land resources by taking the Black Soil Region of Heilongjiang Province as the study area (
Figure 1).
3. Results
3.1. Analysis of the Temporal Variation of Cultivated Land Resources in the Study Area
According to the calculation of the dynamic rate of the change of cultivated land resources (
Table 1,
Figure 3) during the two decades from 1995 to 2015, great changes took place to the cultivated land resources in the Black Soil Region of Heilongjiang Province, as the total quantity of cultivated land increased only slightly, but there were certain internal conversions of cultivated land. In different periods, the rate of change of cultivated land resources varied. For example, from 1995 to 2005, the total area of dryland increased by 106,007 hm
2, the total area of paddy field decreased by 22,887 hm
2, the total quantity of cultivated land increased by 83,120 hm
2, and the dynamic rate of dryland and paddy field was 0.35% and 1.26%, respectively. Compared with the other land types, the dynamic rate of dryland was only higher than that of construction land, while the paddy field ranked third, indicating that the change rate was not particularly obvious in this period. Spatially, the increase of dryland area was mainly concentrated in Harbin City, Heihe City, and Qiqihar City, which was 53,265 hm
2, 33,649 hm
2, and 25,306 hm
2, respectively; the decrease of dryland was mainly seen in Shuangyashan City and Jiamusi City, which was 7563 hm
2 and 4216 hm
2, respectively. The decrease of paddy field was mainly in Harbin City, Qiqihar City, and Suihua City, which was 22,855 hm
2, 22,055 hm
2, and 2413 hm
2, respectively; the increase of paddy field mainly took place in Shuangyashan City and Jiamusi City, which was 14,879 hm
2 and 8592 hm
2, respectively. Except for Hegang City and Suihua City, the total quantity of cultivated land in other cities increased.
From 2005 to 2015, the total area of dryland decreased by 37,129 hm2, the total area of paddy field increased by 28,905 hm2, the total quantity of cultivated land decreased by 8224 hm2, and the dynamic rate of dryland and paddy field was 0.12% and 1.82%, respectively. At this time, the dynamic rate of dryland was only higher than that of woodland, while the rate of paddy field was the highest of all land types, indicating that the range of change of cultivated land was small, but its internal conversion was very obvious. Spatially, the decrease of dryland area was mainly found in Suihua City, Jiamusi City, and Qiqihar City, which was 13,956 hm2, 13,079 hm2, and 4252 hm2, respectively; the increase of dryland area mainly happened in Qitaihe City, which was 1732 hm2. The increase of paddy field was mainly concentrated in Jiamusi City and Suihua City, which was 14,310 hm2 and 11,625 hm2, respectively; the decrease of paddy fields was mainly concentrated in Qiqihar City, Harbin City, and Suihua City, which was 3732 hm2, 2818 hm2, and 2331 hm2, respectively. The cultivated land area of Daqing City, Jiamusi City, Qitaihe City, Shuangyashan City, and Yichun City increased by 1826 hm2.
3.2. Analysis and Change Characteristics of the Cultivated Land Kernel Density
The current maps of cultivated land use in the Black Soil Region of Heilongjiang Province in 1995, 2005, and 2015 were converted into a 1-km grid. The analysis of kernel density was carried out according to the kernel density estimation to reflect the spatial distribution of cultivated land use in the study area. The cultivated land density areas were divided by Natural Breaks. To make the kernel density over the three periods more comparable, a slight adjustment was made in the boundary value of Natural Breaks; that is, the cultivated land was divided into five types of cultivated land area (
Figure 4), including high-density cultivated land area (0.61–0.95), medium–high-density cultivated land area (0.41–0.60), medium-density cultivated land area (0.24–0.40), medium–low-density cultivated land area (0.09–0.23), and low-density cultivated land area (0–0.08). It can be seen that the high-density cultivated land area was concentrated in Qiqihar City, Suihua City, and Harbin City, and the kernel density of other cities was at the moderate level. From 1995 to 2015, the cultivated land kernel density of each city varied, and the range of high-density cultivated land area of Heihe City and Qitaihe City increased the most.
3.3. Analysis of the Spatial Variation of Cultivated Land Resources in the Study Area
The spatial distribution characteristics of cultivated land use are the basis for analyzing the driving mechanism of cultivated land conversion characteristics, and are also an important part of a LUCC study. In this paper, the original vector data were converted into plane data, and a superposition analysis was carried out by using ArcGIS. By the calculation method of the transfer matrix, the spatial variation characteristics of cultivated land resources from 1995 to 2005 and from 2005 to 2015 were obtained, respectively.
3.3.1. Characteristics of the Internal Conversion of Cultivated Land
The secondary classification of cultivated land was dryland, paddy field, and irrigated field. The cultivated land in the Black Soil Region of Heilongjiang Province was mainly divided into dryland and paddy field. Therefore, the analysis of internal conversion of cultivated land mainly focused on the conversion between dryland and paddy field. From 1995 to 2005 (
Figure 5,
Table 2), the conversion area of dryland into paddy field was 66,161.67 hm
2, with a conversion ratio of 9.46%, which was mainly concentrated in Harbin City and Suihua City in the south of Heilongjiang Province as well as Shuangyashan City and Jiamusi City in the east of Heilongjiang Province. The conversion area of paddy field into dryland was 79,974.95 hm
2, with a conversion ratio of 11.43%. Harbin City ranked first, with a conversion ratio of 51.26%, followed by Qiqihar, with a conversion ratio of 23.46%. From 2005 to 2015, the conversion area of dryland into paddy field was 34,221.57 hm
2, with a conversion ratio of 47.30%, which was mainly concentrated in Jiamusi City, Suihua City, and Harbin City. The conversion area of paddy field into dryland was 10,363.59 hm
2, with a conversion ratio of 14.32%, which was mainly concentrated in Harbin City, Jiamusi City, and Suihua City.
During the two decades from 1995 to 2015, great changes took place within the cultivated land. The number of spots in the transfer map can reflect the level of the internal conversion of cultivated land. The number of spots in the transfer map from 1995 to 2005 was more than the number of spots in the transfer map from 2005 to 2015; that is, the internal conversion of cultivated land in the first period was more obvious than that in the second period.
3.3.2. Characteristics of Cultivated Land Transfer-In
The characteristics of cultivated land transfer-in mainly indicated the ways in which the quantity of cultivated land increased. Based on the analysis of the characteristics of cultivated land conversion in the Black Soil Region of Heilongjiang Province (
Table 3,
Figure 4,
Figure 6), it can be seen that, from 1995 to 2005, the conversion of cultivated land was mainly the conversion of other land types into dryland. The conversion ratios of construction land, woodland, grassland, and unused land into dryland were large, which were 15.28%, 13.01%, 7.59%, and 6.51%, respectively. Spatially, the largest conversion of construction land into dryland was mainly concentrated in Qigihar City, Suihua City, and Harbin City; the conversion of woodland into dryland was mainly in Hehe City, Harbin City, and Qiqihar City; the conversion of grassland into dryland was mainly found in Harbin City, Qiqihar City, and Heihe City; and the conversion of unused land into dryland was mainly in Heihe City, Suihua City, and Qiqihar City. From 2005 to 2015, the characteristics of cultivated land transfer-in changed in both dryland and paddy field, and the transfer-in was mainly the conversion between construction land, woodland, and dryland, and unused land and paddy field, with a conversion ratio of 3.93%, 3.91%, and 2.42%, respectively. Spatially, the conversion of construction land into dryland was mainly concentrated in Harbin City, Jiamusi City, and Suihua City; the conversion of woodland into dryland was mainly in Harbin City, Heihe City, and Qitaihe City; and the conversion of unused land into dryland was mainly in Jiamusi City, Suihua City, and Harbin City.
The transfer-in of cultivated land was mainly the conversion between dryland and other land types, while the number of conversions of other land types into paddy field was small. The number of spots in the transfer map from 1995 to 2005 was larger than the number of spots in the transfer map from 2005 to 2015. Spatially, the conversion of cultivated land was mainly concentrated in Harbin City, Heihe City, and Suihua City in the center and west of Heilongjiang Province, and also in Qiqihar City in the east of Heilongjiang Province.
3.3.3. Characteristics of Cultivated Land Transfer-Out
The characteristics of cultivated land transfer-out were the direct reflection of the decrease of cultivated land quantity. From 1995 to 2015, the characteristics of cultivated land transfer-out changed obviously, and the transfer-in was mainly the conversion between construction land, woodland, and dryland. From 1995 to 2005 (
Table 4,
Figure 4,
Figure 7), the transfer-out of cultivated land was mainly from dryland into other land types. The conversion ratios of dryland into construction land, woodland, and unused land were large, which were 14.69%, 7.06%, and 4.51%, respectively, while the number of conversions of paddy field into other land types was small. Spatially, the conversion of dryland into construction land was mainly concentrated in Suihua City, Qiqihar City, and Harbin City; the conversion of dryland into woodland was mainly in Heihe City, Qiqihar City, and Harbin City; and among the conversions of dryland into unused land, Heihe city ranked first, with a conversion ratio of 63.44%. From 2005 to 2015, the conversion ratio of dryland into construction land was the highest, which was 11.88%, followed by the conversion ratio of dryland into water area and woodland, which was 4.55% and 4.40%, respectively. Spatially, the conversion of dryland into construction land was mainly concentrated in Harbin City and Suihua City; among the conversions of dryland into water area, Qiqihar City ranked first, with a conversion ratio of 78.59%; and the conversion of dryland into wood land was mainly concentrated in Heihe City, Jiamusi City, and Harbin City. There was basically no conversion of cultivated land in Daqing City and Yichun City because of their small area of cultivated land. By analyzing the characteristics of cultivated land transfer-out, it was found that the transfer-out ratio of cultivated land transfer was lower than the transfer-in ratio, indicating that the decrease of cultivated land was well-controlled, which was achieved by the conversion between dryland and other land types. The range of change from 1995 to 2005 was higher than that from 2005 to 2015. Spatially, the conversion of cultivated land was mainly concentrated in Suihua City, Harbin City, and Heihe City in the center and west of Heilongjiang Province, and also in Qiqihar City in the east of Heilongjiang Province.
5. Conclusions
This paper takes the Black Soil Region of Heilongjiang Province as the study area, and analyzed the temporal and spatial variation characteristics of cultivated land resources in the periods 1995–2005 and 2005–2015, from which the following conclusions were drawn:
During the two decades from 1995 to 2015, great changes took place in the cultivated land resources in the Black Soil Region of Heilongjiang Province, as the total quantity of cultivated land increased slightly. However, there were certain internal conversions of cultivated land. The characteristics of the change of cultivated land resources from 1995 to 2005 and from 2005 to 2015 were different. In the first time period, the total quantity of cultivated land increased, but the dynamic rate of cultivated land was not obvious. In the second one, the total quantity of cultivated land decreased, and the dynamic rate of paddy field was the highest of all land types. Spatially, the conversion of cultivated land was mainly concentrated in Harbin City, Suihua City, Jiamusi City, and Qiqihar City.
In the analysis of cultivated land kernel density, the high-density cultivated land area was mainly in Qiqihar City, Suihua City, and Harbin City, and the cultivated land kernel density of other cities was at the moderate level. The range of high-density cultivated land area of Heihe City and Qitaihe City increased the most.
Through the superposition analysis by ArcGIS and the calculation method of the transfer matrix, it was found that a conversion between each land type and cultivated land took place. The internal conversion was between dryland and paddy field. The transfer-out of cultivated land was mainly the from dryland into construction land, woodland, and unused land, and the transfer-in was mainly from woodland, unused land, and construction land into dryland, while the number of conversions of other land types into paddy field was small. Spatially, the conversion of cultivated land was mainly concentrated in Suihua City, Harbin City, and Heihe City in the center and west of Heilongjiang Province, and also in Qiqihar City in the east of Heilongjiang Province.