Spatial-temporal change of major reserve resources of cultivated land in China in recent 30 years

Abstract: Unused land, which is the major reserve resources of cultivated land in China, has changed rapidly over the past 30 years. Unused land is often used to supplement the cropland, is the largest variable of land utilization structural adjustment, and also is a kind of effective buffer space for rationally utilizing land resources during the process of ecological construction. In this paper, unused land pattern changes in China were analyzed based on the scale of 1:100,000 national land use database that represents the land use in the late 1980s, 1995, 2000, 2005, 2008 and 2010, and which was built by the Chinese Academy of Sciences. All data of land use was acquired by experts' interpretation from images, including Landsat TM, CBERS, HJ1, and Beijing-1. The unused land referred to in this paper means the still unused land, including those lands which are difficult to use. It includes sand land, Gobi desert, saline soil, marsh land, bare land, bare rock boulder, and other (includes alpine desert, tundra, etc). The aim of this paper is to improve the understanding of the major reserve resources change in order to assist the sustainable utilization and management of land resources in China. The mathematical methodology was developed for studying spatial pattern changes of unused land, which includes the dynamic degree index model of unused land, the spatial density model, the change intensity model of unused land, the gravity center model, and the land-use conversion matrix. Based on these models, the spatial-temporal process of unused land was calculated and analyzed, and the conclusions reached are as follows: 1) The total unused land area in China continues to decrease, but the reduced amount is trending slower. The areas of unused land all decreased during four consecutive time intervals except 1995 to 2000. 2) The dynamic degrees of unused land increased from 0.01 % in the period of the 1980s-2000, to 0.05% in 2008-2010, and reached its zenith in the 2005-2008. 3) The unused land changes in China showed strong spatial heterogeneity. Heilongjiang province incurred the greatest net decrease of unused land area among the 32 provinces in China during the entire study period, followed by Xinjiang Uygur Autonomous. Meanwhile, the unused land area in Heilongjiang province continued to decrease during the five monitoring stages, with its reduction values and its dynamic degree both reducing year by year. Inner Mongolia reveals the largest net increase of unused land area during the study period. In the first three stages before 2005, its unused land area net increased and then net decreased, leaving it with an overall net increase. 4) The dynamic change of unused land mainly embodied the acute mutual conversion between the unused land and the farmland, the grassland and the water bodies. The main reasons of unused land area decrease are grassland ecological restoration, reclamation of cropland, water area expansion, and increase in unused land mainly came from grassland degradation and farmland abandonment. Meanwhile, this mutual conversion showed different features in different monitoring stages. 5) The center of gravity of unused land in China was located in Gansu province during the study stage, but has shifted gradually to northwestward 10?25? and moved 168.57 km. 6) As the major reserve resources of cultivated land in China, the focus of cropland reclamation occupying the unused land areas has shifted from Heilongjiang province in the Northeast to Xinjiang Uygur Autonomous region, Gansu province in the Northwest, and Shandong province. In the most recent 30 years, the spatial pattern of unused land in China has dramatically changed. Some helped to improve the regional eco-environment quality, and some played a role reversely. So we should develop and utilize unused land according to local conditions, such as first evaluation, and then utilization or protection.