Background Super absorbent polymer (SAP) is a kind of polymer with high hydrophilicity and repeated suction function. In arid and semi-arid areas, it is an effective measure to add SAP to sandy soil in order to restore ecological environment and improve physical and chemical properties of soil. For SAP applied in arid and semi-arid sandy soil, its durability is related to the long-term effect of drought resistance and moisture preservation. Drying-rewetting alternation is a common phenomenon in arid and semi-arid areas. SAP added to the soil will absorb water and expand when climate is wet, and slowly lose water when climate is dry, and go through the process of repeated water absorption and water loss. Although the existing studies have mentioned the possible impact of this process on the water retention performance of the SAP, there are few experimental verification and analysis of the impact mechanism.

Methods In order to study the effect of drying-rewetting alternation on sand soil with SAP, sand soil in Lingwu city of Ningxia Hui Autonomous Region was selected as the research object. Different cycles of drying-rewetting alternation, different types and amounts of SAP were selected as experimental variables to complete the simulated soil column experiment. The soil physical and chemical properties, soil cumulative evaporation, soil water characteristic curves and soil moisture availability were selected as the test indexes.

Results 1) The saturated water content and total porosity of sandy soil with SAP increased, the Gardner model parameter a of sand soil with SAP increased, and the cumulative evaporation decreased. The more the amount of SAP was, the greater the impact was. The water-holding capacity of sandy soil containing 6 g/kg acrylamide acrylate copolymer cross-linker was the best. 2) After several drying-rewetting alternations, the effect of increasing the soil water-holding capacity was reduced, the saturated water content and total porosity of sandy soil decreased, and the cumulative evaporation of sandy soil increased. After several drying-rewetting alternations, the water-holding capacity of sand soil with high dosage of SAP weakened more than that with low dosage of SAP. 3) The soil available water content of the sand soil with SAP increased after 1 to 3 cycles of drying-rewetting alternation and it decreased after 3 to 6 cycles of drying-rewetting alternation. After 10 drying-rewetting alternations, the soil water-holding capacity of sand soil with SAP was still better than that of sandy soil without SAP.

Conclusions The drying-rewetting alternation may weaken the water-holding capacity of sand soil with SAP by reducing the saturated water content and total porosity. However, the drying-rewetting alternation increases the soil available water content of the sand soil with SAP and then decreases, thus SAP still has a strong application value.