3.1 Land degradation
The land can undertake a variety of functions such as offering economic benefits, maintaining social stability, ensuring food security, and protecting ecological environment (Xiao et al., 2018). However, productive and constructive engineering always causes land destruction, which is mainly manifested in the excavation, and cover and occupation of the land. In the process of productive and constructive engineering, serious soil erosion always occurs in the construction area due to a lack of awareness of soil erosion protection and the weakness of protection measures. Especially in the ecological environment fragile areas, productive and constructive projects cause serious soil erosion, even soil desertification. As shown in Figures 2 and 3, soil desertification has been caused by the photovoltaic power generation project located in Zhongning County, Loess Plateau, Ningxia Province. This area is one of the ecologically fragile areas in China because the area is characterized by the low vegetation cover and highly erodible loess soil, and it is significantly impacted by climate change and human activity. The land-use type in the project construction area is grassland before the photovoltaic power project is constructed. However, land leveling of project construction area destroyed surface vegetation, resulting in that soil lost fine particle (silt and clay particle) and gradually became sandy, and eventually forming mobile dunes after the construction of the project (Figure 3). Zhang et al. (2006) found that desertification of the grassland and cultivated land were the most serious for all kinds of desertification of land, accounting for about 79.2% of the total. Besides, water erosion also occurred in the process of construction (Figure 4). If the constructive area is not treated promptly after excavation, rainfall directly causes water erosion and soil nutrients loss of exposed surface. The soil erosion and land desertification mentioned above are the most typical forms of land degradation. The land cover and occupation also occurred in productive and constructive projects (Figure 5). Cover and occupation of farmland not only narrow the area of farmland but also influence the growth and yield of the crop. Rapid industrialization has encroached a huge quantity of agricultural land and has caused a continuous reduction of farmland by an unprecedented degree, in which 60% of non-agricultural occupied land originates from farmland (Zhang et al., 2006; Liu et al., 2010). Although non-agricultural land use seems not to be land degradation, the destructive land-use change could lead to the loss of farmland. Also, non-agricultural land use may inevitably indirectly increase the input of chemical energy per unit area to guarantee food security due to the loss of farmland, ultimately resulting in land degradation and environmental pollution.