Subsurface drainage alleviates salt-affected soil constraints and improves soil fertility through altering physical and hydraulic properties in the Hetao Irrigation District, China

Subsurface drainage (SD) a representative drainage water management measure that mitigates agricultural soils salinization. Over recent decades, its demonstrated effectiveness in salt leaching and soil health preservation has driven extensive investigation into the underlying mechanisms. Nevertheless, most studies, particularly those conducted in China’s Hetao Irrigation District, remain isolated and often overlook the coupled and feedback processes among soil structure, nutrients, and microbial communities. To explore the integrated‌ mechanism of SD and address the knowledge gap in Hetao Irrigation District, a field experiment was conducted in sunflower cropping systems. This study examined the influence of SD on soil physicochemical properties, nutrient utilization and biological processes. By comparing variations in soil structure, salt distribution, and microbial community between SD and non-drainage (ND) treatments at different depths within the tillage layer, SD was found to increase silt content from 30% to 37% and macro-aggregates by 49%. These structural changes, in turn, facilitated water-salt featuring notable increases in Ca²⁺/Mg²⁺ and decreases in CO₃²⁻/HCO₃⁻). Consequently, the pH decreased, whereas total carbon (C), nitrogen (N), and phosphorus (P) contents increased. Moreover, high-throughput sequencing showed that SD restructured bacterial communities toward oligotrophic taxa (e.g., Acidobacteriota), while fungal communities remained stable. It also significantly increased the absolute abundance of N/P cycling genes (e.g., amoA, phoD) and relevant enzyme activities (e.g., alkaline phosphatase, urease and nitrate reductase). In general, these findings underscore that SD sustains agricultural productivity via integrated physicochemical and biological mechanisms, offering a scientific basis for the integrated remediation of salt-affected soils.