## 5 Discussion
### Land-Use Changes (LUCs)  
During the two-decade span from 2000 to 2020, there was a consistent trend in Putila City of other land types being converted into built-up land. Over these 20 years, a total of 226.45 km\({}^{2}\) was transformed into built-up areas, predominantly in the economically vibrant regions surrounding the city centers and county seats, with Xiuyu District being particularly affected. The dominant patterns of land-use transition observed during the periods of 2000-2010 and 2010-2020 were \"cultivated land to built-up land\" and \"cultivated land to grassland to built-up land\". Over the course of the past 20 years, there was a net transfer of 297.32 km\({}^{2}\) of cultivated land out, with only 39.43 km\({}^{2}\) being transferred in, indicating a considerable disparity in the shifts of cultivated land. Especially noteworthy was the rate of expansion of built-up land in Xiuyu District, which outpaced all other districts and counties with a 275% increase, amounting to an expansion of 109.75 km\({}^{2}\) over the two-decade period. Given the scarcity of land resources, the ongoing expansion of urban land is bound to encroach upon other types of land, particularly those used for agricultural production and ecological preservation. This situation underscores the need for careful land-use planning and management to balance urban development with the conservation of vital agricultural and ecological lands.  
In addition, regions throughout Putila City have been progressively forsaken to different extents as a consequence of issues such as adverse agricultural conditions (e.g., soil with high salinity and alkalinity), low crop yields, and the migration of rural labor, leading to the emergence of deserted fields that often turn into grasslands. This situation contributes to the wasteful use of scarce land resources. As a key grain-producing region within Fujian Province, Putila City is heavily dependent on external sources for its grain needs. The city's essential farmland is mainly located in Xiuyu District and Xianyou County, which presents substantial difficulties in safeguarding food security and preventing the conversion of arable land to non-agricultural and non-grain uses. In light of these challenges, it is imperative for the government to take proactive measures to tackle this issue. To meet these challenges head-on, it is essential to improve the spatial arrangement of urban, rural, industrial, and mining lands, and to revitalize abandoned areas. Such actions can help in the efficient use of land resources, support local agricultural productivity, and ensure the preservation of land for food production, thereby maintaining a sustainable balance between urban development and agricultural needs. Furthermore, it is crucial to improve the efficiency of land resource utilization. For example, the research team discovered that the fields in Xiuyu District are predominantly saline and alkaline, which have low fertility. This soil condition restricts the types of crops that can be grown, with sweet potatoes and soybeans being the mainstays. The prevalence of monoculture farming practices has led to ongoing challenges in cultivation, which in turn has caused a decline in both the quantity and quality of the harvest. Enhancing the use of such lands would involve adopting strategies such as crop rotation, the use of soil amendments to improve fertility, and the introduction of more salt-tolerant crop varieties to maintain and increase productivity on these lands [77; 78]. Additionally, the research findings suggest that geometric mapping of information offers significant benefits when it comes to examining LUCs. This approach can effectively illustrate the spatial arrangement of each category, present multifaceted spatial details, and represent an innovative technique for investigating LUCs. Other research results have the same conclusions as ours [60; 63; 79; 80].