Characteristics Of Soil Freeze-thaw Process And Their Influences On Saline-sodic Soil Water And Salt Migrations In Different Landscapes Of The Western Songnen Plain,China

Understanding the characteristics of soil freeze-thaw process and their their influences on saline-sodic soil water and salt migrations is necessary to clarify the process and mechanism of soil salt accumulation in different landscapes in Songnen Plain of China.This study selected typical soil salinized landscape[farmland,(Leymus chinensis(Trin.)Tzvel.land(LT)),(Chloris virgata Swartz land(CS)),(Suaeda glauca land(SG))and alkali-spot land(AS)]to examine the spatiotemporal distributions of soil water,salt,electrical conductivity(EC),p H,sodium adsorption ratio(SAR)and exchangeable sodium percentage(ESP),and soil temperatures at0-200cm soil depths from July 2015 to September 2016.Simultaneously,air temperature,precipitation,groundwater tables and frost depth were also monitored.The results are as followings:(1)The soil moisture content(20-60cm)of SG and AS was higher than that of LT and CS in the study area.The soil salt content(0-50cm),SAR(0-30cm)and ESP(0-30cm)of SG and AS was also higher than that of LT and CS.Then,the soil moisture and salt content of farmland were lowest.(2)Compared with the summer of 2015,the soil moisture of 0-70cm in farmland,LT,CS,SG and AS increased in winter and summer of 2016,while the soil moisture of 0-30cm decreased significantly in spring.In spring and summer of 2016,the salt accumulation was obvious in the 0-50cm of LT,CS,SG and AS,among which CS and SG had the most significant salt accumulation.The salt accumulation in farmland was only obvious in winter,but not in the whole year.In the spring and summer of2016,the soil SAR and ESP(0-50cm)of CS,SG and AS increased significantly,while the soil SAR and ESP of farmland and LT did not change significantly with the seasons.(3)The freezing duration of farmland,LT,CS,SG and AS were 184,205,190,210and 210 d respectively,and the maximal frost depth was 148,154,122,182 and165.00 cm,respectively.During winter freezing,the freezing rate of SG and AS was greater than that of farmland,LT and CS.When soil thawed in spring,thawing rate of the upper soil layer is greater than that of the deeper soil layer.Farmland melted fastest,while AS thawed the slowest.(4)The effect of freezing caused the soil moisture of farmland,LT,CS,SG and AS accumulated in the frozen layer,and the soil moisture increased by 1.15,1.93,5.54,7.41 and 3.56%,respectively,while soil moisture(0-30cm)decreased in spring.The water migration of LT was the most significant,followed by CS,SG and AS,and then farmland in the freezing-thawing process.(5)In the process of freezing and thawing,the soil salt of farmland,LT,CS,SG and AS increased by 34.14,25.32,35.28,35.10 and 54.51%,respectively,with the salt accumulation rate of 0-10cm of the AS reaching 96.96%.The salinized ratio of the upper soil layer(0-50cm)was greater than that of the deeper soil layer,indicating that the salt is migrated upward during the freezing and thawing process.The effect of freezing-thawing also increased soil Na⁺,HCO₃^-,SAR and ESP,and the increase ratio of SAR and ESP of the upper soil layer(0-60cm)was also higher than that of the deeper soil layer.In addition,the salinized ratio of SG and AS was higher than that of farmland,LT and CS,so the effect of freeze-thaw salinization increased with the soil salt content.(6)Soil salt,SAR and ESP were positively correlated with water and frost depth,and negatively correlated with soil temperature and shallow groundwater levels in the freezing and thawing processes.This study further enriches the theory of soil salinization and contributes to a deeper understanding of the modern salt accumulation process,which provides a theoretical reference for the monitoring,prevention and control of soil salinization and the exploitation and utilization of agricultural resources in the western Songnen Plain.