Understanding the multidimensional morphological characteristics of urban idle land: Stage, subject, and spatial heterogeneity

Urban idle land is a manifestation of urban system degradation in the process of rapid urbanisation. Scientifically and effectively identifying its multidimensional morphology and regional distribution rules is not only the core content of future urban sustainable development research but is also practically needed to carry out urban renewal and urban land renovation. From the 'space-society-time' perspective, this study constructed a theoretical framework for urban idle land research and used '3S' (RS, GPS, and GIS) spatial analysis technology, sample surveys, extensive correspondence, and other methods to obtain the spatial locations and basic attributes of 12,966 urban idle land plots in Shandong Province, China. Then, this study quantitatively determined the stage, subject, and spatial heterogeneity of urban idle land and analysed its spatial differentiation characteristics and formation mechanism using a spatial quantitative analysis. The results showed that the total area of urban idle land in Shandong Province is vast, the plot area has the characteristic of outlier 'fat tails' at the small scale, and the proportion of idle land is approximately 85 %. The types of urban idle land are dominated by the AIL type (idle land approved but not supplied) with a dense spatial distribution, which is highly related to the level of regional urbanisation and economic development. The behavioural subject of urban idle land mainly concerns the balance between government departments and land use units, which has obvious periodic differences. The AIL type is greatly influenced by the government's behaviour, while the SIL type (idle land supplied but unused) is greatly affected by the land use units. Moran's I index of the types of urban idle land exceeds 0.5, and there is a significant low-value agglomeration state between different county administrative regions, with 'one core and two zones' characterising the hotspots and cold spots throughout the area. The spatial differentiation of urban idle land results from the positive or negative response to land use behaviours for the interests given by government departments and land use units on different scales of urban interior space. Moreover, the differentiation is influenced by the restriction or promotion of urban external environmental factors, with different degrees of influence. In general, a greater advantage of the urban internal system elements results in the weaker constraint of the external system control elements. Moreover, when the interests of the government departments and land use units tend to reverse, the urban idle land scale tends to be large, and vice versa.