Underlying Mechanisms for Growth Promotion by Low-Concentration Single Salt and Alkali Stresses and Growth Inhibition by Combined Salt-Alkali Stress in Quercus mongolica

Soil salinization is a global ecological issue that severely constrains forest tree growth and ecological restoration. The salt-alkali stress response mechanisms of Quercus mongolica, a key temperate forest species in China, remain unclear. A two-factor pot experiment was conducted using NaCl (0, 50, 100, 200 mmol·L^-1) and NaHCO₃:Na₂CO₃ (1:1; 0, 50, 100, 150 mmol·L^-1). Plant traits, soil properties, and enzyme activities were measured. Furthermore, high-throughput sequencing revealed that microbial responses enhanced network cooperation under 100 mmol·L^-1 salt stress and improved network stability under 50 mmol·L^-1 alkali stress. These responses also upregulated resistance genes and increased soil enzyme activities. This activation of seedling antioxidant and osmotic adjustment systems was directly associated with an increase in growth parameters. Under combined stress, however, soil environment deterioration and microbial network disruption, along with reduced key soil enzyme activities, resulted in an insufficient defense system to counteract reactive oxygen species (ROS) accumulation, thereby reducing growth parameters. The study found that low-concentration individual salt or alkali stress promoted Quercus mongolica seedling growth, while combined stress was associated with significant inhibition. This study refines the theoretical framework for non-salt-tolerant trees and establishes a basis for determining their survival thresholds in saline-alkali soils.