Geomorphic dependence and temporal instability: Soil conservation effects of cropland abandonment are not monotonically positive in Southwest China karst

Cropland abandonment has become increasingly significant in mountainous regions, with its ecological effects attracting extensive attention. According to previous studies, cropland abandonment improves vegetation recovery and certain ecosystem service functions. However, spatial heterogeneity driven by variations in natural environmental conditions may lead to substantial differences in its ecological effects across regions. Despite this, the understanding of long-term dynamics of soil conservation under complex geomorphic conditions remains scarce. Taking the karst region of Southwest China as the study area, this study used the 2000–2023 China Land Cover Dataset (CLCD) to identify cropland-abandonment onset during 2000–2020. The land-use trajectory-tracking method was employed in this study to identify cropland abandonment processes, and the InVEST model was used to simulate soil retention functions. Subsequently, comparisons were made between abandoned and non-abandoned cropland, so as to evaluate spatial heterogeneity and temporal instability in soil conservation under different geomorphic backgrounds. The results show that: (1) From 2000 to 2020, the annual newly abandoned cropland area fluctuated between 273,227.49 and 2,584,205.28 ha, while the annual abandonment rate ranged from 0.889% to 8.415%; both reached their minima in 2018 and maxima in 2020. Spatially, abandonment is concentrated in the mountainous and hilly areas of Guizhou, eastern Yunnan, and northwestern Guangxi; (2) During the study period, the regional average soil retention decreased from 11.48 to 8.32 t·km -2 ·a -1 , with an overall decline of about 27.5%. The area with the lowest values increased from 9.4% to 26.9%; (3) Between 2001 and 2018, soil conservation in abandoned cropland was generally higher than in non-abandoned cropland, with values ranging from 3.66 to 15.47 t·km -2 ·a -1 . However, in 2019–2020, this trend reversed, with negative values (−1.20 and −0.33 t·km -2 ·a -1 , respectively); (4) There are significant variations in the effects of abandonment depending on geomorphic contexts. In low-altitude alluvial-proluvial land (LAAP), was the highest (6.83 t·km -2 ·a -1 ), whereas mid-high altitude high-relief terrain (MHHRT) exhibited negative effects (−1.10 t·km -2 ·a -1 ). These findings indicate that the soil conservation effects of cropland abandonment in the Southwest China karst regions are highly dependent on geomorphic conditions and are temporally unstable, and the increase in abandoned cropland does not necessarily align with increased ecological benefits. This study contributes to a deeper understanding of the variability of ecological effects across complex geomorphic settings and provides scientific references for ecological restoration and land-use optimization in karst regions.