Influence of Clay Layer on the Migration of Soil Water and Solutes under Strong Evaporation

Soil salinization constitutes a major limiting factor in agricultural production and ecological environments worldwide. In nature, medium texture and structure play important roles in the transport of water and solutes in soil under evaporation. To characterize the influence of clay layer on the migration of soil water and solutes under strong evaporation, indoor one-dimensional soil columns experiment which lasted 133 days was conducted. There were four soil columns with different thickness clay layers (0, 5, 10 and 15 cm, labeled A, B, C and D, respectively) in this experiment, and the water tables were kept at a uniform depth of 25 cm and all recharged by NaCl solution with a concentration of 100 g/L. Results showed that the thinner the clay layer, the smaller the cumulative bottom water flux in the layered columns (B < C < D). The thicker the clay layers stored more soil water for its larger porosity, and on the other hand, the thicker clay layer may make the soil water migrate upward easier for its higher matric potential. The homogeneous sand column A had the largest cumulative bottom water flux. Whereas for the salt transport, the appearance showed that the layered columns (B, C and D) had a prominent salt accumulation at the depth around 1 cm, while the salt accumulation in the homogeneous sand column (A) was at the surface (even out of the surface). According to the final profile soluble salt measurements, salt contents in the clay layers were all higher than the nearby sand and had sudden changes at the interfaces of the two mediums in the three layered-columns (B, C and D). The depths where salt accumulated the most were 0 cm, 1－1.5 cm, 0.5－1 cm and 0.5－1 cm respectively in the four columns. The column with thinnest clay layer (B) finally acquired the least salt content (155.22 g) from the recharge solution. The clay layer inhibited the upward migration of soil water, which resulted in the lower final total salt content of the layered columns B and C (210.64g) than the homogenous column A (240.92 g). On the other hand, due to the larger porosity, the thicker clay layer stored more solution than sand, which induced the highest final total salt content in columns D (257.34g). The results indicated that appropriate thickness and position of clay layer in sand could help to alleviate soil salinization and secondary salinization.