Threshold effects of cultivated land expansion in agro-pastoral ecotones: non-linear amplification of drought vulnerability and composite disaster risk

As climate change accelerates, ecologically fragile regions are increasingly exposed to compound extremes. Agricultural intensification may perturb land–atmosphere feedback processes and exacerbate drought–heatwave coupling, yet the land-use thresholds governing non-linear risk amplification remain poorly constrained. This study focuses on a representative area in the agro-pastoral ecotone of northern China, the Horqin Sandy Land Ecological Conservation Area, to quantify cultivated-land-proportion thresholds for compound drought and heatwave onset. A space-for-time substitution was applied to 50 grid cells across five land-use patterns (cultivated land: 6.4%–70.62%). Surface temperature was the primary determinant of the surface energy balance (first-order sensitivity index = 0.54), with evapotranspiration exerting a stronger biophysical control than albedo. Month-specific thresholds were identified as 16.74% (June), 22.29% (July), and 19.48% (August). Partial least squares discriminant analysis indicated that soil conservation, climate regulation, and waste treatment were most responsive to threshold-based categorization. When the cultivated land proportion exceeded 40%, drought vulnerability exhibited a marked non-linear shift: the temperature–vegetation drought index increased by 82% (bootstrap 95% confidence interval: 61–103%), and drought–heatwave co-occurrence increased by 375% (bootstrap 95% confidence interval: 189–561%). These results provide a threshold-based quantification framework for climate-driven composite hazard risk, offering evidence-based early-warning benchmarks and land-use policy targets for drought-prone agricultural systems worldwide.