## 5 Conclusions  
In this study, we used remote sensing classification images, meteorological monitoring data, soil surveys, and other data to analyze the impact of cultivated land changes on the GPP in Hunan Province. We found that the quantity balance can mitigate but cannot stop the decline in GPP caused by the spatial location change in cultivated land because the production conditions are different in different regions, and we found that the GPP per unit area in eastern, northern, and southern Hunan is higher than other regions. In addition, most of the newly added cultivated land was converted from forest land, water areas, and construction land, and its quality was generally lower than that of occupied land.  
Overall, the area and the GPP of cultivated land both declined in Hunan Province from 1990 to 2014, and the occupation of high-quality cultivated land in northern and eastern Hunan was the main reason for the decline in GPP. The net reduction in cultivated land area in Hunan Province was 276,481 ha, with a decrease of 4.43%. Among them, the paddy field area decreased by 120,184 ha, mainly in eastern and northern Hunan, which have good agricultural production conditions, accounting for 32.59% and 29.00%, respectively. The total GPP declined by 17.72 million tons, with a decrease of 17.21%, of which the paddy field production potential declined by 11.67 million tons, a decrease of 15.33%, mainly in northern and eastern Hunan, accounting for 55.42% and 16.91%, respectively, resulting in the GPP barycenter of paddy fields continuing to move southwest. Meanwhile, we found that all cities showed a declining trend in GPP, and the reduction in GPP exceeded 20% in Changde, Yiyang, Changsha, Chenzhou, Huaihua, Xiangxi, and Zhangjiajie. The sensitivity analysis results also show that the highly sensitive regions moved to the west, which means that the change in cultivated land in central and western Hunan caused the obvious change in GPP in these regions, suggesting the protection of cultivated land in central and western Hunan, where the economy is relatively underdeveloped, also requires more attention.  
Based on the analysis, we discussed the future development direction of cultivated land protection and proposed an integrated monitoring system that is more adaptive to global climate change. For stabilizing GPP, conserving resources, and protecting the environment, it is necessary to consider more factors such as quality and productivity to enrich the connotation of cultivated land balance and establish an integrated monitoring system, including the quantity, quality, productivity, and ecology of cultivated land. Meanwhile, the government should pay more attention to the phenomenon of cultivated land abandonment, standardize its definition, and conduct a survey, then they should develop a utilization plan, if possible, in order to prioritize the use of high-quality abandoned land as a backup land resource.  
Conceptualization, S.H. and D.Y.; data curation, D.Y. and C.X.; formal analysis, D.Y., L.T., and C.X.; funding acquisition, S.H.; methodology, D.Y., S.H., L.T., and C.X.; project administration, S.H.; software, D.Y. and C.X.; supervision, S.H. and L.T.; validation, D.Y., S.H., and L.T.; visualization, D.Y. and C.X.; writing--original draft, D.Y.; writing--review and editing, S.H., L.T., and D.Y. All authors have read and agreed to the published version of the manuscript. This research was funded by the major projects of National Social Science Fund (18 ZDA053), National Natural Science Foundation of China (41671518 and 41901236), the Evaluation Committee Project of Social Science Achievements of Hunan Province (XSP18 YBZ027), and the Natural Science Foundation of Hunan Province (2020)J4942). We are grateful to the handling editor and anonymous reviewers for their constructive comments. We would also thank [PERSON] at Central South Forestry University for providing survey statistics. The authors declare no conflict of interest.