It is essential to clarify the resource list of unstable cultivated land for maintaining ecological balance, ensuring regional food security and promoting the sustainable development of economy and society in the arid areas of Northwest China. At present, the definition and classification of unstable cultivated land in China is relatively general, and the resource endowment of different regions is quite different, so it is difficult to highlight the characteristics of unstable cultivated land in the arid areas of Northwest China with a national standard classification system. In this paper, the connotation and concept of the unstable cultivated land in the arid areas of Northwest China were expounded, and the classification system was constructed according to the unstable factors. Taking Changji Prefecture as a typical case, this paper analyzed the quantity, quality and spatial distribution of unstable cultivated land. The main results are as follows. (1) The definition of unstable cultivated land in the arid areas of Northwest China is as follows: limited by ecological factors (desertification, soil salinization, etc.), or production factors (water, soil, etc.), or socio-economic factors (location, farmland infrastructure, etc.), it is difficult to stabilize production, and continuous cultivation will threaten ecological security of cultivated land ecosystem and surrounding environment, and there is uncertainty in the follow-up land use types. Unstable cultivated land can be divided into four types: productive unstable type, ecological unstable type, socio-economic unstable type and mixed unstable type. (2) The area of unstable cultivated land was 14.56 ten thousand hm2 in Changji Prefecture, accounting for 22.37% of the total cultivated land area. Ecological unstable type, productive unstable type, mixed unstable type, and socio-economic unstable type accounted for 36.54%, 34.03%, 17.94% and 11.49% of the total area of unstable cultivated land, respectively. (3) The unstable cultivated land was mainly at low-grade level in Changji Prefecture, and the fourth to sixth grade cultivated land accounted for 79.96% of the total area of unstable cultivated land. Our results are of great significance for the deep understanding of the unstable cultivated land in the arid areas of Northwest China, and provide scientific support for the effective management of cultivated land resources in the case area.
The large-scale expansion of cultivated lands in arid region has caused water overload and a series of ecological problems. Therefore, conducting the scientific assessment of the scale of suitable cultivated land in arid areas can provide reference for sustainable development of agricultural and ecological environment. In this research, five years, 2000, 2005, 2010, 2015, and 2018 were selected as study periods. Water resources were considered as the limiting factor, and a living-production-ecology water consumption structure was built in this research, which is suitable for local socioeconomic development and eco-environment characteristics. The water and heat balance method was improved coupling physical factors like planting structure and crop coefficient in different growth periods, and management factors like irrigation guarantee rate and mulching. Changji Hui Autonomous Prefecture was selected as a case study. The results show that the available water amount of the scenario of critical arable land area for planting decreased from 3.175 billion m3 in 2000 to 2.483 billion m3 in 2018, while the available water amount of the scenario of optimal arable land area for planting decreased from 2.763 billion m3 in 2000 to 2.110 billion m3 in 2018. The total planting area proportion of cotton and wheat increased from 43.58% in 2000 to 59.08% in 2018. Among the main crops, cotton and wheat had lower water requirements after being compensated by precipitation, while the sown area proportion of both increased, indicating that the planting structure is evolving toward the aspect helpful to expansion suitable cultivated land. In the five years, the optimal arable land area which can ensure adequate ecological water consumption is between 28.93 ten thousand hectare and 48.97 ten thousand hectare, while the critical arable land area which can ensure minimal ecological water consumption is between 42.63 ten thousand hectare and 67.83 ten thousand hectare. Available water for plant industry declined with fluctuations. Although the factors affecting the scale of suitable cultivated land fluctuated, under the scenarios of critical and optimal scales of arable land, the overload degree of cultivated land scale showed an upward trend, which varied from -55.15% and -39.41%, at no overload level, in 2000, to 19.92%, a level close to severe overload, and 71.37%, a level of far exceeding severe overload, respectively. The scale of local cultivated land has presented an overload development level.
The large-scale expansion of cultivated land in the arid region has caused water overload and frequent ecological problems, so it is urgent to reduce the area of cultivated land to save irrigation water. Therefore, exploring the spatial optimized scheme of ecological cropland conversion under the constraints of water resources can not only contribute to resolving the current ecological problem of arid areas, but also help implement land use planning and decision-making. Until now, the research on quantitative analyses of the multi-benefit contradiction of the cropland-conversion plan is still insufficient. Therefore, a spatial optimization allocation and trade-off analysis model for ecological cropland conversion is built in this study. Taking Qitai County as a case study, we built a solution group of ecological cropland conversion under the constraint of water resources. Two scenarios, priority for maintaining cultivated land scale and priority for ecological environment, were designed in this research to realize the spatial simulation and optimization of ecological cropland conversion by Ant Colony Optimization Algorithm. Results show that the cultivated land area of Qitai County is 99.39 thousand hectares in priority for maintaining cultivated land scale scenario, and 69.55 thousand hectares in priority for ecological environment scenario. In the cropland-conversion process, the proportion of ecological water consumption in the river channel accounted for 10% of the total water consumption in the cultivated land retain priority scenario and 30% in the ecological protection scenario; and the amount of windbreak and sand fixation increased from 7.13 million tons to 8.17 million tons, while the economic benefit decreased from 3.49 billion yuan to 2.48 billion yuan. Through comparing the reduction of unit economic benefit producing the increasing proportion of unit ecological benefit, the scenario that cultivated land area decreased to 83.49 thousand hectares was chosen as the optimal one considering the trade-off between economic and ecological benefit. The cropland-conversion areas were mainly located in the northeast, northwest and southwest edges of the farmland concentration area in Qitai County, which will benefit the prevention and control of desertification in the north and the protection of water resources in the south. This research realizes the spatial simulation and optimization of the ecological-conversion plan under the constraints of water resources in arid areas, which provide significant reference to agricultural management and ecological protection.
Soil organic matter (SOM) plays an important role in improving soil fertility and crop productivity, and studying its spatial variability can provide a scientific basis for targeted fertilization and sustainable land use and management. This paper selected Changji Prefecture of Xinjiang as the study area, and used three-stage SOM data (the second soil survey, 2010 and 2018) to calculate the generalized dimension spectrum D(q), multifractal singularity index ɑ(q) and the multifractal spectrum function f(ɑ(q)) based on the multifractal method to explore the dynamic changes of SOM spatial variability. The results showed that: (1) The average SOM contents in the three periods were 10.46, 16.69 and 18.16 g/kg, respectively, showing an upward trend. However, the SOM contents of all counties were lower than the national average level (24.30 g/kg) in 2018. (2) The spatial distribution of SOM in Changji Prefecture had non-uniform multifractal characteristics from the second soil survey to 2018. The distribution range of SOM value gradually became narrowed, and the spatial distribution variability decreased, tending to be uniform. The degree of SOM spatial variability in Fukang city, Jimusaer county, and Mulei Kazakh autonomous county increased slightly from 2010 to 2018. (3) In 2018, the spatial distribution range of SOM widened from west to east, with an increasing variation degree, and internal differences became larger, tending to be non-uniform. (4) In the three periods, all the low-value data had a greater variability, with the variation degree being higher than that of the high-value data. Fukang and Mulei were dominated by the high-value data in 2018. Long-term farming had increased the SOM content, and the spatial distribution gradually tended to be uniform. However, there was obvious spatial heterogeneity of SOM. It is necessary to adopt targeted approaches to improve overall soil fertility in different parts of the study area. It is recommended to promote targeted fertilization and to improve irrigation and farming measures in the places with high spatial variability of organic matter, such as Fukang, Jimusaer and Mulei.
It is meaningful for guiding regional agricultural fertilization management measures and improving farmland soil fertility to quantitatively describe the impact of cultivated land reclamation tracks on soil fertility changes at a regional scale. This study identified the cultivated land reclamation tracks by superimposing multiple periods of land use data based on a large number of sample data in 1980 and 2018, to analyze the effects of initial land use and soil types and cultivation years on soil fertility changes in Changji Prefecture, Xinjiang. The result were as follows: (1) Among the current cultivated land in 2018, 90.12% of the original land type was grassland, and 9.88% of the original land type was unused land. The content of other soil fertility indexes increased except for the decrease of available nitrogen (AN) content. Except for soil organic matter (SOM), the spatial variability of other indicators increased. (2) The SOM content reduced because of the reclamation of high-coverage grassland. The reclamation of grassland and unused land would reduce AN content. (3) With the increase of cultivation years, SOM showed a slow increase and gradually stabilizes; while the available nutrient content increased in a short period of time, and stabilized or began to decrease after reaching a certain number of years because of the threshold effect. (4) The loss of SOM caused by grassland reclamation gradually recovered after the cultivation period reached the medium-long term. The SOM content of current cultivated land that the original land type was unused land was always positive. The loss of AN caused by reclamation of grassland and unused land had not been recovered after 38 years of cultivation. (5) The plots with the initial soil types of meadow soil and shrub desert soil had higher nutrient content, and brown calcium soil and gray desert soil had lower nutrient content. The SOM content of current cultivated land that the original soil type was meadow soil decreased. Therefore, it is recommended that the application ratio of fertilizer should be reasonably adjusted, and the potassium fertilizer should be increased. In addition, nitrogen fertilizer should be applied in low intensity and high frequency.