The low-carbon transition of cropland use can help to mitigate climate change to a certain extent. Based on carbon emissions accounting and sequestration of cropland use, this paper attempted to develop a theoretical framework for analyzing the low-carbon performance of cropland use. Then, the GB-US-S-SBM was applied to assess the performance in China's 30 provincial-level regions (hereafter provinces) from 2000 to 2019, and to examine the spatiotemporal characteristics. The study found that: (1) Overall, the intensities of carbon emissions, sequestration and net sequestration were 1.980 t∙hm^-2, 5.624 t∙hm^-2, and 3.644 t∙hm^-2, respectively, indicating that the cropland use system was a major carbon sink. (2) Nationwide, the low-carbon performance of cropland use went through four stages, namely, ups and downs, slow growth, fluctuation, and fast growth. According to the initial performance and growth rate, the provinces were classified into five types, namely, low performance with high potential (15 provinces such as Beijing), low performance with stable trend (4 provinces such as Jiangsu), high stable performance (3 provinces such as Chongqing), high performance with low potential (3 provinces such as Hubei), and high performance with high potential (5 provinces such as Jilin). At the early stage, provinces with high performance were clustered in the southwest, while those with low performance were agglomerated in the northwest. High-performance provinces were mainly located in the northwest and northeast, and low-performance provinces were distributed in the middle reaches of the Yangtze River. (3) σ convergence was not observed in the performance of the whole country, central region, or eastern region, while the performance of northeastern and western region strictly followed σ convergence. Both China and its four regions exhibited β convergence, indicating that provinces tended to chase each other in performance. This paper may provide a basis for the differentiated low-carbon transition of regional cropland use.

Studying the spatio-temporal patterns of net carbon sinks under conservation tillage is of great significance to the formulation of policies. Based on the analysis of the mechanism of carbon sink and emissions and the construction of its measurement method under conservation tillage, this paper analyzed the spatio-temporal pattern of net carbon sink and predicted its potential under conservation tillage from 2000 to 2019 in the provinces (autonomous regions and municipalities, hereafter provinces) of China. The results showed that: first, the carbon sink under conservation tillage is about twice the carbon emissions, soil carbon sequestration accounts for more than 2/3 of carbon sink, and biological carbon sink accounts for less than 1/3. Second, from 2000 to 2019, the net carbon sink under conservation tillage in China showed an increasing trend year by year, and the net carbon sink in North, Northwest and Southeast China increased significantly. The net carbon sink showed a trend of expansion-agglomeration-expansion in space, with the center of gravity shifting from north to south. Third, the net carbon sink under conservation tillage in the whole country had the characteristics of obvious spatial imbalance. In 2019, there was a "three-legged" pattern in North, Northwest and Southeast China. The seven provinces of Henan, Shandong, Inner Mongolia, Xinjiang, Anhui, Hubei and Jiangxi belong to high-carbon sink areas, Hebei, Jilin, Shaanxi, and Shanxi belong to low-carbon sink areas, and other provinces belong to carbon-neutrality areas. Fourth, the net carbon sink potential under conservation tillage from 2020 to 2030 will continue to increase, and the peak value will be between 57943800 t C and 79629300 t C in 2030.

Mountains are sensitive areas to global change and they play an important role in the ecological security and development of human society. The changes of mountain ecosystem services and the carrying capacity of the ecological environment have always been focusing on hotspots in geography and ecology. In this paper, the Qinling-Daba Mountains, the main body of the north-south transitional zone of China, is taken as the research object. The CA-Markov model and InVEST model are used to simulate and predict (2000-2040) the ecosystem carbon storage changes of the Qinling-Daba Mountains under different land-use scenarios. The spatial distribution difference of ecosystem carbon storage of the study area is discussed by using the hot spot analysis (Getis-Ord G_i^*). The results show that: (1) From 2000 to 2040, the main types of land use/land cover changes are farmland, forestland, grassland, and construction land. (2) From 2000 to 2020, the ecosystem carbon storage increased by 1.12×10⁷ t; under the natural growth scenario from 2020 to 2040, the carbon storage loss is severe, down by 50.24×10⁷ t; under the ecological protection scenario, there is less carbon loss, with a decrease of 29.52×10⁷ t, indicating that the adoption of ecological environment protection policies can effectively control the reduction of ecosystem carbon storage. (3) The changes in land use/land cover and ecosystem carbon storage show significant consistency. The change of land use quantity determined the quality and spatial distribution pattern of ecosystem carbon storage. (4) With the rise of the altitude, carbon stocks show a trend of "increasing first and then decreasing"; with the increase of the slope, carbon stocks show a "W"-shaped change trend. (5) The hot spot analysis results show that from 2000 to 2020, carbon storage hot spots and cold spots were scattered in the study area; under the natural growth scenario in 2040, the distribution range of carbon storage cold and hot spots will tend to increase gradually; compared with 2020, the ecological protection scenario in 2040 shows little change in the distribution range of the cold and hot spots of ecosystem carbon storage in the Qinling-Daba Mountains.

Land use/cover change (LUCC) is an important driving factor that affects the carbon storage of regional ecosystems. Studying the response and vulnerability of carbon storage to LUCC is significant to the realization of the "dual carbon" goal. Taking the nine districts of Chongqing municipality as an example, the paper deeply explores the response of carbon storage to land use transfer and uses the Potential Impact Index (PI) to assess the vulnerability of ecosystem carbon storage services. The results show that: (1) Between 2000 and 2020, the area of cultivated land in the main urban area decreased by 743.29 km², and the area of construction land surged by 773.48 km². The land transfer area accounted for 6.05% in the first 10 years and 13.98% in the next 10 years. The conversion of cultivated land to construction land is the main land transfer. (2) In the study period, the carbon storage in the main urban area decreased by 5.78 Tg. The encroachment of cultivated land by construction land is the leading factor in the rapid decline of carbon storage. The distribution of carbon storage presents a spatial pattern of "low in the middle, but high in the surroundings". (3) Over the 20 years, the main urban districts were carbon sources. The land use degree index increased by 14.73, and the PI index ranged from -2.50 Tg to -2.59 Tg, both having negative potential impacts on the main urban area, and the vulnerability continued to deteriorate from 2000 to 2015. In 2020, the vulnerability eased. The results can provide references for the sustainable development of regional ecology and the formulation of future land use management policies, and for other similar mountainous cities in western China.

In the context of the "dual-carbon" goal, the carbon sink of terrestrial vegetation ecosystems is an important way to achieve the carbon neutrality. In order to effectively identify the priority areas for improving the quality and efficiency of vegetation carbon sink service, the InVEST model was used to quantitatively evaluate the spatiotemporal evolution characteristics and distribution pattern of vegetation carbon storage in Shaanxi province. Based on this, we analyze the impact of land use/cover types on carbon storage changes, study the spatial differences in the growth of carbon sinks in forest-grass ecological construction, and determine the target areas for quality and efficiency improvement in ecological environment construction areas. The results show that: (1) The land use types in Shaanxi are mainly cultivated land, forest land and grassland, and the change of land use transfer mainly occurs in the mutual transformation among the three types. (2) From 1980 to 2020, the carbon storage of ecosystems in Shaanxi increased by 91.88×10⁶ t, an increase of 3.16%, showing the zonal distribution characteristics of "generally high in the south and low in the north, and obviously high or low in some areas". (3) The conversion of farmland to forest (grass) project has obvious effect on the improvement of carbon sink capacity, and there is a global spatial correlation, which is manifested as a certain spatial convergence and agglomeration phenomenon. (4) Northern Shaanxi is a high priority area and priority area for ecological protection and restoration projects, Southern Shaanxi is a medium priority area, and Guanzhong Plain is a general priority area. Based on the differences in the average annual growth rate of carbon sinks in ecosystems in different districts and counties, the priority ecological governance areas were determined to provide a reference for realizing the regional management of the leading mode of ecological restoration projects and improving the quality and efficiency of carbon sinks.