Urbanization in China drives farmland uphill under the constraint of the requisition–compensation balance

The slope is an important objective attribute of farmland that changes with the evolution of its spatial pattern. A growing area of plain farmland is being occupied by built-up land owing to rapid urbanization, while the newly added are sloping and terrace farmland under the constraint of the requisition-compensation balance. Researchers have focused on the horizontal spatial redistribution of farmland quantity while ignoring vertical variations in its slope, which is critical for its overall quality. Based on data on land use classification in China from 1990 to 2019, this study uses land use change trajectory as well as trend and driver analyses to identify the impact of urbanization on change in the slope of unstable farmland. The results show the following: (1) The area of unstable farmland accounted for ~20% of all farmland, with its slope increasing from 5.77° in 1990 to 6.25° in 2019 due to conversion in land use. (2) Variation in the slope of unstable farmland had significant heterogeneity, with regions undergoing a significant increase concentrated in the east and those undergoing monotonous decline not spatially clustered. (3) Farmland development and built-up land occupation have driven increases in the slope of unstable farmland with a relatively balanced effect, whereas the trend of increasing has been mainly suppressed by farmland marginalization. (4) The area of urban land expanded by 158,446.70km2 during 1990-2019, 24.15% of which was due to encroachment on farmland with a slope of 1.31°. Farmland development with a slope of 6.98° helped replenish 90.30% of the occupied area. This combined process has led to unstable farmland uphill under the constraint of the requisition-compensation balance. The results here can provide a reference for the protection and sustainable utilization of farmland.