Saline–alkaline soil is a severe threat to Sustainable Development Goals (SDGs), but it can also be a precious land resource if properly utilized according to its properties. This research takes the Songnen Plain as the study area. The aim is to figure out the saline–alkaline status and mechanisms for its scientific utilization. Sentinel-2 multispectral imagery is used, and a 3D spectral space optimization method is proposed according to the restrictive relationships among the surface soil salinity index (SSSI), vegetation index (VI), and surface soil wetness index (SSWI) to construct a surface soil salinization–alkalization index (SSSAI) for estimation of the surface soil salinity (SSS). It is testified that SSS can be precisely estimated using the SSSAI (R² = 0.74) with field verification of 50 surface salinized soil samples. Surface water and groundwater investigations, as well as deep soil exploration, indicate that the salt ions come from groundwater, and alkalinization is a primary problem in the deep soils. Fine-textured clay soils act as interrupted aquifers to prevent salt ions from penetrating and diluting downward with water, which is the cause of the salinization–alkalization problem in the study area. Finally, a sustainable solution for the saline–alkaline land resource is proposed according to the deep soil properties.

Desertification is destabilizing the Agro-Pastoral Ecotone of Northern China (APENC) as a critical ecological barrier, making its monitoring and management a central challenge for dryland sustainability. We tracked desertification dynamics across the APENC (2000–2024) using the desertification difference index (DDI) and, by integrating multi-source remote sensing data with an interpretable machine learning framework, systematically deciphered its drivers. The primary results were as follows: (1) Desertification underwent a net reversal, with 60.09% of the region showing significant improvement; extremely severe and severe desertification declined by approximately 40% and 35%, respectively, despite a brief degradation pulse during 2005–2010. (2) Land surface temperature (LST), precipitation (PRE), and soil moisture (SM) were identified as the primary drivers of desertification outweighing human activities, exhibiting nonlinear threshold behaviors with critical tipping points at 17.93 °C (LST), 416.44 mm (PRE), 6.91 mm (SM), and 0.75 kPa (VPD) that govern ecological degradation–recovery transitions. (3) 2D partial dependence plots (PDPs) reveal that under compound dry-heat stress (LST > 20 °C and PRE < 300 mm), wind speed reduction alone fails to reverse degradation due to collapsed ecosystem resilience. The results reveal elevation-dependent pathways of desertification reversal. Strong hydrothermal coupling in lowland areas enables water supplementation to rapidly promote vegetation recovery and desertification reversal, whereas weakened hydrothermal coupling in highland areas requires coordinated management of moisture limitation, thermal stress, and wind erosion. These findings underscore the nonlinear dynamics and critical thresholds of dryland restoration, offering a practical framework for adaptive, spatially explicit land management in the APENC and similar dryland ecotones globally.