## 4 Discussion
In terms of the impact of cultivated land changes on the GPP, we first analyzed the impact of changes in the area and the structure of cultivated lands on the total GPP and found that the impact of changes in paddy fields on the GPP was much greater than that of dryland. Then, we analyzed the impact of cultivated land location change on the GPP from the perspective of the difference in GPP per unit area and found that the GPP per unit area of supplementary cultivated land was generally higher than that of occupied cultivated land, e.g., the land decrease occurred in developed regions such as northern, southern, and eastern Hunan with better production conditions, but supplementary cultivated land occurred in other regions, further proving that the spatial transfer of cultivated landgreatly reduced the regional GPP. When analyzing the gravity center of arable land area and the gravity center of grain yields, it was found that the gravity center of grain yield in paddy fields and drylands moved far more than their gravity center (Figure 10), and the GPP gravity center of paddy fields continued to shift to the southwest, indicating that more efforts are needed to protect high-quality paddy fields in eastern and northern Hunan. In addition, the sensitivity of GPP to changes in cultivated land was analyzed, and we found that highly sensitive areas tended to migrate from east to west, which means that investment in the protection of arable land in central and western Hunan should be increased in the future. For example, more funds should be invested to accelerate the implementation of dryland changed into paddy field projects and high-standard farmland construction projects in areas with higher GPP to enhance the productivity of cultivated land. It is crucial to strengthen the comprehensive monitoring of the quantity, structure, and quality of cultivated land to reduce the risk of managers making incorrect decisions about cultivated land protection due to information bias and to guide the development of cultivated land resources. It is necessary to establish a new mechanism of cultivated land balance based on quantity, structure, and quality, with balanced productivity as the core in the future practice of cultivated land protection, which is more consistent with the request of equal quality between supplementary cultivated land and occupational cultivated land, as clearly stated in the Land Administration Law of the People's Republic of China (2019 amended version) [70]. Therefore, the production potential of cultivated land can be used as an important indicator to distinguish ecological or productive land when we perform spatial planning. In general, it is feasible to use remote sensing images, meteorological monitoring, soil surveys, and other data to systematically analyze the impact of cultivated land changes on GPP from the perspectives of light, temperature, water, and soil differences before and after the spatial transfer of cultivated land, which has certain reference significance for exploring the establishment of a dynamic, all-process, all-around monitoring system for cultivated land protection. Compared with previous studies [32; 33; 35; 49; 50; 51; 56; 62], our study is more detailed concerning land types and spatial resolution. Further, our study can guide the practice of cultivated land protection at the provincial scale. Of course, there are many insufficiencies in this paper, e.g., first, limited by the resolution of remote sensing images, the land classification data are bound to have some differences from reality. Second, the ecological and biodiversity protection of cultivated land has not been studied. Third, our research results may not be directly applicable to actual land management because the data we used were not obtained from their database. Fourth, some scholars have raised whether potatoes can add to China's food self-sufficiency [71], but we did not assess potato productivity in our study. Fifth, compared with the GAEZ model, the GPP evaluation method in this article may only apply to China.