| Super absorbent polymer(SAP)is a kind of polymer with high hydrophilicity and repeated suction function.Super absorbent polymer can release water slowly for plants to absorb and use.In arid and semi-arid areas,it is an effective measure to add super absorbent polymer to sandy soil in order to restore ecological environment and improve physical and chemical properties of soil.Adding proper amount of super absorbent polymer to sand can effectively improve soil three-phase ratio,increase soil porosity,reduce soil bulk density,enhance soil moisture retention ability,reduce water evaporation and infiltration,and alleviate water stress on vegetation.For super absorbent polymer applied in arid and semi-arid sandy soil,its durability is related to the long-term effect of drought resistance and moisture preservation.It is of great significance to study the durability of water retention agents for improving water stress in this area.Drying-rewetting alternation is a common phenomenon in arid and semi-arid areas.Super absorbent polymer added to the soil will absorb water and expand when it is wet,and slowly lose water when it 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 super absorbent polymer,there are few experimental verification and analysis of the weakening mechanism.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.In this experiment,different frequency of drying-rewetting alternation,different types and amounts of SAP were selected as experimental variables to complete the simulated soil column experiment.In this paper,we selected the soil physical and chemical properties,soil water infiltration rate,soil cumulative evaporation,soil water characteristic curves,soil moisture availability and the ability ofsoil to maintain available nitrogen,available phosphorus,available potassium,available sodium fertilizer as the test index.The results show that:(1)The saturated water content,total porosity,soil water-stable aggregates of sandy soil with SAP will increase.The soil water-stable aggregates of sandy soil with SAP will decrease.The organic matter,soil p H value,soil electric conductivity of sandy soil with SAP will increase.The stable soil water infiltration rate,soil available water content,water-holding capacity of sandy soil with SAP will increase.The cumulative evaporation of sandy soil with SAP will decrease.The volume of drenching solution,available nitrogen content,available potassium,available sodium of sandy soil with SAP will decrease.The more the amount of SAP was,the greater the impact was.The SAP added in sand soil can improve the physical and chemical properties of sand soil,and play the role of water and fertilizer conservation,but it will aggravate the salinization of sand soil.In this experiment,adding 6 g/kg of acrylamide acrylate copolymer crosslinker to the sand soil has the best performance of inhibiting evaporation,reducing infiltration and water retention,and the best effect of keeping nitrogen fertilizer;adding 6 g/kg of polyacrylic acid / attapulgite composite water retaining agent to the sand soil has the best effect of keeping potassium and sodium fertilizer.(2)After several drying-rewetting alternation,the effect of increasing the soil water-holding capacity will be reduced,the saturated water content and total porosity of sandy soil will decrease,the cumulative evaporation of sandy soil will increase.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.After 10drying-rewetting alternations,the soil water retention indicators of sand soil with SAP is still better than that of sandy soil without SAP.The soil available water content of the sand soil with SAP will increase after 1 to 3 times of drying-rewetting alternation and it will decrease after 3 to 6 times of drying-rewetting alternation.The function of SAP to slow down water stress of plants will not be weakened immediately because of the increase of drying-rewetting alternation.Drying-rewetting alternation will weaken the retention effect of SAP on nitrogen,potassium and sodium fertilizers in sand soil.After10 drying-rewetting alternations,the fertility holding capacity of sand soil with SAP is still better than that of sandy soil without SAP.(3)The drying-rewetting alternation will destroy the structure of SAP.It will weaken the improvement effect on water-holding and fertility holding capacity of sandy soil.Considering the effect of drying-rewetting alternation on SAP,the service life of three kinds of water retaining agents in this experiment is two years,which can increase the water-holding and fertility holding capacity of sand soil within the service life,and has a strong use value. |
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