Impact of Farmland Use Transition on Grain Carbon Sink Transfer in Karst Mountainous Areas

Farmland use transition (FUT) not only reshapes agricultural production systems but also significantly impacts cross-regional carbon sink transfers in the grain trade. However, comprehensive studies exploring connections between FUT and grain carbon sink transfer (GCST) are limited. We constructed an indicator system and transformation framework for FUT by considering dominant and recessive dimensions. Moreover, we estimate GCST based on grain supply–demand balance and fixed carbon coefficients. Fixed effects and threshold models are employed to identify both linear and nonlinear relationships between FUT and GCST. Results show that FUT significantly reshapes carbon sink flows. In terms of dominant FUT indicators, cultivation land rate (CLR) and grain planting area proportion (GPAP) positively drive GCST by expanding the carbon sink supply and exporting ecological services. Regarding recessive FUT indicators, both grain yield per unit area (GYield) and pesticide-fertilizer intensity (PFI) promote GCST, highlighting the role of efficiency and inputs, while rural per capita disposable income (RPCDI) suppresses GCST due to agricultural marginalization. A grain yield threshold of 2.092 t/ha is identified. Below this value, FUT exerts substantial positive effects on GCST. Above it, the effects weaken. This study explains the relationship between FUT and ecosystem carbon sinks, providing a scientific basis for advancing green agriculture in karst mountainous areas.