Effects Of Moisture And Salinity On Basic Physical And Chemical Properties Of Soil And Its Ecological Risks In Coastal Wetland

In this study,a simulated soil incubation experiment is designed,in which the used surface soil(0~20 cm)is taken from the coastal wetlands of Jiaozhou Bay.The effects of different moisture and salinity on the basic physical and chemical properties of soil and on its ecological risks.UV-Vis spectrophotometry and three-dimensional fluorescence spectroscopy are used to study DOM content and structural.Inductively coupled plasma(ICP)is used to investigate the capacity of DOM to absorb heavy metal ions(Cr(?)Cd(?)and Pb(?))in soil under different moisture and salinity.Soil structure stability,potential release risk of TN and TP,as well as the risk of heavy metal pollution under different soil moisture and salinity are evaluated.The aim of this study was: 1)reveal the relationship between soil DOM and the surrounding ecological environment;2)provide data support and theoretical basis for coastal wetland ecological environmental protection.The following main results are obtained through research:(1)The distinct trends are detected for the change of tested soil physical and chemical properties along with the change of moisture and salinity.Among them,the content of micro-aggregate(< 0.25 mm)is the highest,which ranges from 47.55% to 70.81%.Its content first decreases and then increases with the rise of soil moisture,and increases with the rise of salinity.The content of aggregate between 0.5 mm and 1 mm has the second-high content,which accounts for 15.33%~33.85 % in the soil.The content of other grades is low and the difference is not obvious.The soil DOM first raises and then decreases with the rise of moisture,and reaches the maximum at 30% moisture.DOM rises with increasing salinity.The structure of DOM has five kinds of fluorescence peaks under different moisture and salinity,and it is mainly composed of protein-like substances.The humification degree of DOM is weak.In the low salinity areas,more than two types of humus fluorescent groups appear in DOM and the development degree of humus substances is high.(2)Soil p H slowly rises as moisture is raised and decreases as salinity is raised.This is attributed to the addition of strongly acidic ions(such as SO42-)and the replacement of soluble salt ions.The bulk density(BD),total organic carbon(TOC),total nitrogen(TN),and total phosphorus(TP)exhibit the trend of “inverted-U” along with the increase of moisture.Too high or too low soil moisture inhibits the activity of microorganisms,enzymes,and biochemical reactions.With the increase of salinity,BD and TN show a downward trend;TOC and TP show an upward trend.Increase of salinity inhibits the activity of microorganisms and enzymes,reduces the rate of mineralization and decomposition,and increases the content of TOC and TP.The decrease of TN content is related to the influence of soluble salts addition on the process of denitrification.(3)The soil structure stability and potential release risk of N and P in the coastal wetlands are different under different moisture and salinity.Firstly,the soil structure has the best stability under the 30% moisture and 0.9% salinity.Aggregates greater than 0.25 mm(R0.25),fractal dimension(GMD)and mean weight diameter(MWD)rise first and then decrease with the increase of soil moisture,and decrease with the increase of salinity,while D has the opposite trend.Secondly,the potential release risk of N and P is the lowest in the treatment of high moisture and low salinity,respectively.It is concluded from the correlation analysis of soil indicators that the effect of soil salinity is greater than moisture.p H,TOC,TP,C/N and N/P,as well as the adsorption amount of three heavy metal ions,are all related to the DOM content in the soil to varying degrees,and all of them are closely related to some structures of DOM.Thirdly,different moisture and salinity of soil have obvious effects on the the ecological risk of heavy metals.On one hand,DOM has effect on the adsorption of heavy metal ions in the soil.The increase of DOM promotes the adsorption of Cr(VI)and inhibits the adsorption of Cd(II).However,when the concentration of Pb(II)is high(150 mg/L),the increase of DOM inhibits the adsorption of Pb(II).This is because the increase of DOM promotes the increase of the number of microorganisms,which is conducive to the adsorption of Cr(VI)by soil.Cd(II)and Pb(II)adsorption of soil is inhibited due to the effects of complexation and competitive adsorption.The background concentration of heavy metal ions also affects the amount of adsorption.The adsorption of Cd(II)and Pb(II)by soil increase significantly with the increase of the solution background concentration,but has a little effect on the adsorption of Cr(VI).On the other hand,in the comprehensive ecological risk assessment of heavy metal ions,the value of ecological risk is small in low salinity area.This shows that seawater intrusion has increased the moisture and salinity of the soil,damaged soil structure,reduced soil stability,and increased the risk of the potential release of eutrophication and heavy metals.(4)The correlation analysis results of each soil indicator reveal that the effect of soil salinity on each soil indicator is greater than soil moisture.Soil p H,TOC,TP,C/N and N/P,as well as the adsorption amounts of the three heavy metal ions,are all related to the DOM content within to varying degrees,and all of them are closely related to some structures of DOM.Preventing the soil from seawater intrusion can reduce soil moisture and salinity,and change soil DOM content.As a result,stability of soil structure is improved.At the same time,potential release risk of P and the ecological risk of heavy metals are reduced,which are beneficial to the protection of the coastal wetland ecological environment.