## 4 Discussion
### Rural-Urban Migration and Peri-Urban Development  
Previous studies presented that rural-urban migration was the dominant driver of urbanization. Figure 11 shows that with the built-up land expansion, the proportion of rural settlements dramatically decreased from 76% of built-up land in 1990 to 47% in 2017, while the proportion of urban built-up land significantly increased over this entire time period. Previous research indicated that, before 2000, urbanization occupied less cropland compared with rural settlements (village areas), whereas, after 2000, this conversion trend reversed [25]. Rural-urban migration was the dominant factor for urbanization in China [55]. [PERSON] et al. [27] indicated that urban land in China will continue to expand at a rather rapid rate until 2030, so more than 50% of the population will be living in urban areas. [PERSON] et al. [55] stated that the income gap between rural and urban was the root cause for this migration. Figure 12 indicated that income gap between rural and urban has widened from 1994 to 2016 in ZXEB. Additionally, because of modernization and the shortage of agriculture in China, the surplus labor in the agricultural area moved to cities to seek better employment and higher income, resulting in migrant laborers (broadly defined) comprising 46% of the urban work force [56]. Concurrently, the relaxation of rural controls caused citizens to be active not only in traditional home industries, but in all types of commercial undertakings. The increase in urban employment reflected more quickly than that in rural employment, especially in Guangxi (Supplementary Figure S1B). All this activity accelerated rural-urban migration and urban land expansion [24; 57].  
Large migration has accelerated the urbanization process; most of the migration was concentrated in the peri-urban areas (in China, also called \"Chengzhongcun\") because of low-rental housing and a lack of space within Chinese cities [57; 58; 59]. The urban expansion has invaded cropland, especially in peri-urban areas. The farmers lost their land resources, but remain in housing plots [60; 61] in a bid to meet the huge demands of housing construction for migrants from rural areas. Previous studies show that urban expansion has led to more than 40 million farmers losing their important land resources, with this number increasing at a rate of 2 million per year in China [61]. Peri-urban  
Figure 11: Proportion of sub-class of built-up land change in 1990, 2000, and 2017.  
Figure 12: Urban and rural per capita net income.  
development resulted from the rapid urbanization and migration from rural, with its feedback driving urbanization and urban-rural migration [59; 62]. However, the peri-urban area has been facing many problems, like land-use efficiency, industrial structure, and environmental protection, for example [62; 63], and peri-urban areas also have a mix of urban and rural functions and are related closely to food provision for the urban lands [62; 64]. Additionally, the major effect of urbanization is the loss of cultivated land, especially in peri-urban areas [25; 62]. Most of this cropland was of high quality, and its loss threatens food security [17; 65]. Based on this, peri-urban development is crucial for the urbanization process. More specific-area polices for sustainable development in the peri-urban area, along with urban development, should be followed in the future.