Transport Characteristics And Influence Mechanism Of Soil Water And Salt On Shell Island In The Yellow River Delta

Shell dyke is formed by the shells of shellfish and their debris accumulated in the intertidal zone at the high tide line.Shell Island is located on the coast of the Yellow River Delta?YRD?in China,which is known as one of the three largest ancient shell islands in the world and plays an important role in the study of Quaternary geology and coastal geomorphology.The water and salt condition is the key factor to restrict the growth and distribution of vegetation in the shell dike,and it is the key link and necessary premise in the process of vegetation restoration and ecological restoration construction to clarify the distribution characteristics and migration process of soil water and salt.Vegetation type,seawater intrusion,soil particle composition,groundwater salinity,temperature change,evaporation and so on are the main factors affecting the water and salt transport process of shell sand.The study site is located in the YRD region of China,within the buffering zone of Binzhou Shell Island and Wetland National Nature Reserve.Based on the theory and method of soil water and salt transport and multifractal of soil particles,the characteristics and fractal mechanism of shell sand soil water storage under different vegetation types were studied.Through the indoor control simulation experiment,we carry out research on the process of soil water and salt transport with the ratio of shell sand to fluvo-aquic soil,the effects of rainfall leaching and seawater intrusion on soil water and salt transport,and the response characteristics of soil moisture to temperature changes.We expected to explore the influence of salt on the natural evaporation of shell sand soil,clarify the movement process of soil moisture and salt separators in shell sand,clarify the response relationship and its mechanism of water and salt migration of shell sand soil to impact factors,and determine the appropriate proportion of shell sand and fluvo-aquic soil.The reseach results should provide a reference for the water and salt regulation theory required for vegetation restoration in the special habitat of shell dikes.The main results are as follows.?1?Influence of different vegetation types on water storage characteristics of shell sandAccording to American soil texture classification standard,the soil of shrub-grass community?Ziziphus jujuba var.spinosa and Astragalus adsurgens Pall.?is loamy sand soil,the soil of shrub community?Periploca sepium Bunge?,grass community?Messerschmidia sibirica?and bare land is sandy soil.It showed that under the long-term effects of plants,the soil texture changes.The shrub-grass community can change the soil texture from coarse to fine.The soil bulk densities were significantly lower and the total porosity was significantly higher than that under bare land.The soil water retention capacity was shrub-grass community>shrub community>grass community>bare land.Compared with bare land,the three different shrub-grass community soil bulk density desceased by 24.79%,17.39%and12.08%;silt and clay increased by 27.12%,3.91%and 2.78%;soil water storage increased by16.61%,15.52%and 1.56%;residual water content increased by 63.79%?32.76%?12.07%.In terms of the vertical structure,the soil water retention in the 0-20 cm soil layer was higher than that in the 20-40 cm layer under all vegetation types.Correlation analysis and factor analysis had been conducted,it was found that the characteristic factors of soil water storage and particle size were closely related to the soil water retention capacity and could be used as indicators to evaluate the water storage capacity of shell sand.However,there was had no significant correlation between the multifractal dimension and soil particle content with soil water storage indexes.van Genuchten model fit the soil water characteristic curve of shell sand.?2?Change characteristics of soil water and salt under different proportion of shell sand to Fluvo-aquic soilThe change of water and salt ions in the different proportion of shell sand to fluvo-aquic soil was generally followed by the change of salt water,and the salt ions appeared differentiation during the water and salt transport process.The distribution and rising height of soil water and salt were affected by the composition of soil particles,which were generally manifested as the lower water content,salt ion content and rising height of water and salt in the soil with high content of shell sand.The shell sand particles were irregular,had a layered structure and a large pore size,and had a weak effect on the capillary lift of groundwater.The water and most of the salt ions rise by 30 cm.The mixed soil and fluvo-aquic soil increased by 50 cm.The overall performance was that the appropriate proportion can improve the rising height of capillary water,and the less the content of shell sand,the higher the rising height of capillary water.The mixed soil with the ratio of 2:1 and 1:5 of shell sand and fluvo-aquic soil had a strong lifting effect on water and salt ions,and its water content and salt content were between shell sand and fluvo-aquic soil.Differentiation occurred during the transport of salt ions in the soil.The movement speed of salt ions in different proportion treatment soil was as follows:Cl^->Na⁺>NO₃^->SO₄^2->Mg²⁺>Ca²⁺>HCO^-,there was synergistic effect between Na⁺and Cl^-,increased by 12.69times and 9.56 times respectively.The results of factor and principal component analysis showed that the comprehensive score of mixed soil was 1:1>1:2>1:5>2:1>5:1.From the perspective of salt control and water conservation,the suitable ratio of shell sand and Fluvo-aquic soil was 2:1.The surface enrichment ability of the mixed soil with the ratio of 2:1,1:1,1:2 and 1:5was stronger than that of the shell sand and the Fluvo-aquic soil.The conductivity increased multiple compared with the initial value 29.54,16.03,24.02 and 23.23;salt ions content increased multiple 239.17,132.73,206.49 and 129.97;the conductivity and salt ions content of Flu increased 19.21 and 108.73;Shell sand increased 0.11 and 9.12;5:1 mixed soil increased0.94 and 1.48.The Fluvo-aquic soil was stronger than the shell sand,and the weakest was the mixed soil with the ratio of 5:1.Therefore,The surface enrichment ability of shell sand was weakly.?3?Effect of fresh water and saline leaching on evaporation of different ratio of shell sand to Fluvo-aquic soilFresh water and salt water leaching had no significant effect on the same soil water content,but affected the distribution and migration of soil water and salt ions.In the vertical direction,there was no significant difference in the water content of different soil layers under fresh water leaching,and the overall water content of the soil under salt water leaching increased with the deepening of the soil layer.The water content of soil increased with the decrease of shell sand content.The conductivity and salt ion of shell sand in 10-20cm layer were higher than that in 0-10cm layer under salt water leaching treatment.Cl^-and Na⁺were evenly distributed in soil layer,and Cl^-and Na⁺were positively correlated?P<0.05?,showing synergistic effect during the migration process in soil.Because shell sand was sandy soil with strong infiltration capacity,other ions were easy to enter deep soil.However,most of the salt ions in the mixed soil with high content of fluvo-aquic soil and fluvo-aquic soil were concentrated in the 0-10 cm layer.The leaching loss of Cl^-,Na⁺,K⁺in different proportion of fresh water leaching soil was the largest in the 1:1 proportion of mixed soil,the order of cation leaching loss was as follows:K⁺>Mg²⁺>Na⁺>Ca²⁺.The effect of salinity on the evaporation of soil water was different.Under natural evaporation conditions,the absolute cumulative evaporation of differently treated soils increased with time and was related to the proportion and the salt content of the mixed soil.The cumulative evaporation capacity of soil increased with the decrease of shell sand content.The cumulative evaporation of shell sand under freshwater leaching was 66%lower than that of saline leaching,and the cumulative evaporation of other soils was higher than that of saline leaching.It showed that salt can promote the evaporation of shell sand,and hinder that of Fluvo-aquic soil and mixed soil,which might also be one of the reasons for the low water content of shell sand soil in the coastal zone.?4?Response of soil water content to temperature under different ratio of shell sand to Fluvo-aquic soilThe response of soil moisture to temperature change was different in different ratio of shell sand and Fluvo-aquic soil.Mixed soil?5:1,1:1,1:2,1:5?and Fluvo-aquic soil,the soil water content and residual water content decreased with the increase of temperature and drying time,while the accumulated water loss increased with the increase of time,and the water loss rate increased with the increase of temperature.With the increase of temperature,the accumulated water loss of shell sand decreased,and its unique scale layer structure was conducive to water storage.Whether this part of water could be absorbed and utilized by plants remained to be studied.There was no significant difference between the residual water content and the accumulated water loss of the mixed soil with the ratio of shell sand to Fluvo-aquic soil of 2:1 at 30?and 50??P<0.05?.The rate of soil water loss increased with the decrease of shell sand content,and the accumulated water loss and rate of soil water loss were different in different water loss stages.The average accumulated water loss of seashell sand and tidal soil accounts for 82%?30??and 90%?50??of the total accumulated water loss in the rapid stage of water loss,and about90%of the total water loss of seashell sand in this stage.In the stage of rapid and gentle water loss,high temperature?50??improved the soil water loss rate,while in the stage of stable water loss,low temperature?30??enhanced the impact of water loss rate.From the perspective of effective water use,the transition zone between the beach ridge and the land side of the shell dike was the key area for vegetation restoration and reconstruction.In the process of vegetation restoration,the reasonable allocation of shrubs and herbs could be considered to improve the texture of shell sand soil and enhance the water and soil conservation function of the soil.