Optimal Subsurface Drip Irrigation Depth (25 cm) for High-Yield Cultivation of Dry Alkali Wheat: Insights from Soil-Microbe-Crop Interactions in Saline-Alkali Soil

To explore the directional regulatory effects of drip irrigation burial depths on the growth and rhizosphere microenvironment of dry alkali wheat in saline-alkali soil, three treatments were set up in this study: control (CK, no irrigation), shallow-buried drip irrigation (T5, 5 cm), and deep-buried drip irrigation (T25, 25 cm). The differences in soil physicochemical properties, microbial community structure and functions, as well as crop yield were systematically analyzed. The results showed that drip irrigation treatments significantly improved the soil environment and production performance of saline-alkali soil. The T5 treatment reduced the electrical conductivity (EC) of the 0–60 cm soil layer (15.4% lower than CK), increased microbial species richness (OTU number, ACE index, etc., were significantly higher than those of other groups), and achieved a yield of 4830.96 kg/hm² (40.0% higher than CK), but it had the disadvantages of large water evaporation and low water-fertilizer use efficiency. The T25 treatment exhibited more prominent comprehensive advantages: its desalination effect was significantly better than that of T5 (EC of 0–60 cm soil layer was 61.5% lower than CK), soil organic matter content was 53.8% higher than CK, and total nitrogen and total potassium contents were slightly lower than those of T5. However, T25 could directionally enrich functional groups such as Acidobacteria and Sphingomonas, enhance nitrogen cycling functions including nitrite_respiration, and improve nutrient availability. Meanwhile, it directly delivered water and fertilizer to the root dense zone, reducing losses and promoting absorption. The final yield reached 5347.1 kg/hm² (55.0% higher than CK and 10.7% higher than T5), and the drip irrigation tapes had a longer service life and were suitable for large-scale cultivation. In conclusion, drip irrigation burial depths improved dry alkali wheat yield through a chain mechanism of "soil microenvironment improvement—microbial community optimization—crop growth promotion". Among them, the 25 cm deep-buried drip irrigation synergistically optimized soil water-salt status, nutrient availability and microbial functions, which balanced ecological and production benefits, and provided theoretical basis and technical support for efficient water and fertilizer management in dry alkali wheat drip irrigation cultivation in saline-alkali soil.