Synergistic Effects of Straw Return Rate and Subsurface Pipe Depth on Soil Water and Salt Transport in Coastal Saline Soils

This study investigates the effects of wheat straw incorporation and subsurface pipe depth on water–salt transport in coastal saline soil, providing technical references for saline–alkali land improvement. A soil column infiltration experiment with a straw interlayer was conducted, comparing four straw incorporation treatments—S0 (0 t·ha −1 ), S1 (5 t·ha −1 ), S2 (10 t·ha −1 ) and S3 (15 t·ha −1 )—under two subsurface pipe depths: H1 (0.8 m) and H2 (0.6 m). The results indicate that the straw layer affects water infiltration, initially inhibiting water infiltration during the early stage but promoting infiltration and enhancing the leaching efficiency of surface salts during the stable stage. The leaching effectiveness follows the order S3 → S2 → S1 → S0. Under the same straw incorporation rate, the drainage electrical conductivity of the H1 treatment is 154% higher than that of H2, suggesting that deeper drainage systems are more conducive to salt leaching. Considering technical effectiveness and practical feasibility, a synergistic regulation scheme is recommended: straw incorporation at 10 t·ha −1 (S2 level) combined with a subsurface pipe burial depth of 0.8 m (H1 level). This approach achieves the improvement goals of enhanced infiltration and efficient salt drainage.