The Effect Of Salt And Water Intrusion On Coastal Wetland Soil Quality

In this study,a simulated soil incubation experiment was designed,in which the used soil was taken from the coastal wetlands of Jiaozhou Bay,to explore the effects of water and salt content on the key soil property indicators（physico-chemical,nutrient and biological indicators）.Furthermore,three soil quality evaluation indices（the Additive Soil Quality Index（SQI_a）,the Weighted Additive Soil Quality Index（SQI_w）and the Nemoro Soil Quality Index（SQI_n））under two data set evaluation scenarios（Minimum Data Set（MDS）and Total Data Set（TDS））were calculated to evaluate the soil quality for all the soil cases with different water or/and salt gradients.Meanwhile,the impacts of water or/and salt content on soil quality were also analyzed.Additionally,the optimal data set method and soil quality index were determined by comparing the evaluated results between different data set scenarios or/and indices.The aims of this study are to:i)supply a comprehensive analytical method and assessment system for the study of soil property evolution under different water content or/and salinity scenarios;ii)reveal the change rules of key soil property indicators under different water content or/and salinity scenarios;and iii)supply the theoretical foundation for the construction of efficient remediation technologies for coastal wetlands soil and vegetation.The main findings are as follows:（1）The distinct trends were detected for the change of tested soil property indicators along with the change of water/salt gradient.Soil pH decreased significantly with the increase of salt gradient,which was contributed by the substitution of soluble salt ions and the increase of strong acidic ions.However,the water gradient showed no significant impact on soil pH.Soil BD exhibited the trend of‘inverted-V’along with the increase of water gradient,and it can be attributed to the different water pressure on soil particles.The soil property indicators of TOM,NH₄⁺-N,AP,AK,SA,UA,APA illustrated the‘inverted-V’trends along with the increase of water gradient,and the highest values were acquired at 30%water gradient,which indicated that the soil enzyme activity and biochemical reaction process may be inhibited at too high（e.g,60%）or too low（e.g.,15%）water content level.The indicators of TOM,AP,AK,SA,UA and APA decreased continuously with the increasingly salt gradient.The high salinity of soil may accelerate the decomposition of TOM,and inhibit the synthetic rate of available nutrients and activity of enzyme.It should be noted that the NH₄⁺-N content showed the opposite trend that increased with the increase of salinity gradient,and the possible mechanism mainly because the salt affected the nitrification process.Our correlation analysis results indicated that the soil biological indicators（SA,UA and APA）were positive correlated with the soil physico-chemical or nutrient indicators.The significant positive relations between SA or APA and TOM,AP or AK were recorded,respectively,and a significant positive relation between APA and BD was also detected.（2）The soil quality may be reduced when the soil water content at a too high or too low level.Increasing soil salinity may also result in the reduction of soil quality.As our soil quality assessment results indicated that all the three soil quality indices,SQI_a,SQI_w and SQI_n,illustrated the‘inverted-V’trends along with the increase of water gradient,and the highest soil quality indices appeared at 30%water content level.The lowest soil quality under 60%water content may be attributed to the low soil nutrients availability and enzyme activity,and the highest soil quality under 30%water content was consistent with the relative higher values of soil nutrient and biological indicators.The increase of soil salinity may induce the inhibition of soil enzyme activity and the reduction of AP and AK availability,and thus decrease the value of SQI_s.In the case of MDS approach,three principal components can explain 72.23%variability of the original data,and the minimum data set including SA,BD,UA,and NH₄⁺-N.The contribution of each soil index to SQI_a and SQI_w illustrated the same trend with the increase of water and salt gradient,and this trend can be explained by the index average value and the linear scoring function.（3）As the results of soil quality rating showed that all the soil samples under different water and salt gradient were mostly located in medium（Grade III）or below,and only a few samples can reach up to‘Grade I’.Comparing the difference between water factor and salt factor,the water content level exhibited higher influence on the soil quality than the salt factor.（4）Comparing the results derived from different soil quality indices（SQI_a,SQI_w and SQI_n）or data set（TDS and MDS）by direct comparison and kappa analysis,the SQI_w soil quality index and TDS data set were recommended for the evaluation of the costal wetlands soil quality...