Comprehensive Evaluation Of Soil Quality And Its Soil Organic Carbon Storage In Coastal Wetlands Of Jiaozhou Bay

Coastal wetland is an important part of the global ecosystem.As the most basic carrier of wetland ecosystem,wetland soil plays several important roles in providing nutrients to plants and animals and participating in the geochemical cycle of various substances.Soil quality can not only directly restrict the sustainable development of regional soil resources,but also affect the development and stability of ecosystem.However,the evaluation of wetland soil quality in current studies is mainly based on the analysis of single or multiple soil physical and chemical properties.This cannot reflect the ecological function of soil at a macro-level.Therefore,it is important to develop the theory and method of soil quality evaluation from the soil ecological functions perspective,in which an integrated evaluation method system,combining macro and micro-technologies of data quantitation,is also constructed.In addition,the carbon storage function is not only one of key ecological functions of coastal wetland,but also a key index affecting soil quality.Thus,to further clarify the content,component structure,and spatial distribution characteristics of soil organic carbon（SOC）can examine the soil quality level of coastal wetlands more comprehensively.However,in previous studies,the evolution trend and internal mechanisms of SOC characteristics were generally investigated by the association analysis(e.g.,correlation analysis with influencing factors.The quantitative analysis of relative importance of different influencing factors to the changes of SOC characteristics is lacking,and the underlying mechanisms related to key influencing factors is still needed to be ascertained.Based on the current research gaps mentioned above,the main research contents and conclusions of this study are as follows:（1）This paper constructs a comprehensive system of soil quality evaluation based on soil ecological functions.According to the characteristics of coastal wetland soil,three functional ecological function indexes,including nutrient maintenance,carbon storage,and biodiversity maintenance,were defined in this evaluation system.Moreover,an integrated data quantification method using remote sensing,In VEST empirical model,and measured data was established.As a whole,the constructed evaluation method has multi-advantages,including clear function,high evaluation data availability,easy operation,and wide applicability.（2）Using the constructed evaluation method,the soil quality of Jiaozhou Bay coastal wetland was evaluated to address two aims:The first one is to verify the effectiveness of the evaluation method,and the second one is to systematically study the temporal and spatial evolution of soil quality in this region.The results demonstrated that the annual decreases in habitat quality value,carbon storage,and soil nutrient status in Jiaozhou Bay coastal wetland were contributed by the degradation of beach wetland and expansion of urban area.The different human disturbances to the environment resulted in different degrees of damage to the soils.The overall situation of soil quality was ordered as follows:cultivated land>woodland>tidal flats>unused land>grassland>aquaculture pond>construction land.Based on the results obtained,the ecological protection strategy in Jiaozhou Bay coastal wetland should be to carry out ecological restoration for the degradation of tidal flats,and reasonably reduce the development degree of aquaculture pond and construction land as much as possible while paying attention to economic development.（3）Taking the Yanghekou wetland in Jiaozhou Bay as the research object,the changes of distribution characteristics of SOC under different vegetation cover types and soil depths were studied.In the horizontal direction,the type of sample plot（Suaeda glauca,Spartina alterniflora and bare wetland）indicated a significant effect on the SOC content（P<0.05）;The SOC content in different sample plots was ordered as Spartina alterniflora>bare wetland>Suaeda glauca.In the vertical direction,the SOC content in the three sample plots showed a relatively consistent law among different soil depths:the SOC content in the middle layer（15～30 cm）was higher than those in the surface layer（0～15c m）and deep layer（30～45 cm）.（4）Based on the data obtained,including SOC content and basic physical and chemical properties of soil,the relative importance of influencing factors and relevant action mechanisms were explored using the correlation analysis,random forest（RF）model,and structural equation model.In terms of prediction performance,the RF model showed a better model fitting degree and prediction accuracy than other two machine learning models（artificial neural networks and support vector machine）.The relative importance of influencing factors revealed by RF model was ordered as dissolved organic carbon DOC/SOC>bulk density>microbial biomass carbon MBC/SOC>DOC>salt content>p H>TP>Mg²⁺>MBC>β-Glucosidase>Ca²⁺.The results of correlation analysis,RF model,and structural equation model consistently indicated that SOC content was significantly positively correlated with DOC,total phosphorus,salt content,Ca²⁺and Mg²⁺（P<0.01）,and significantly negatively correlated with bulk density,MBC/SOC and DOC/SOC（P<0.01）.（5）Based on the results of（3）and（4）,the main factors affecting SOC content in Jiaozhou Bay coastal wetland were the DOC/SOC,bulk density,MBC/SOC,DOC,salt content and p H.Meanwhile,soil salt content can indirectly affect SOC by affecting DOC content and p H value,soil bulk density can indirectly affect SOC by affecting salt content,total phosphorus and DOC content.