Electrical Resistivity Characteristics And Water Storage Capacity Of Stony Forest Soils

During the last decades, tree growth declining was found in some areas of the world forthe reason of the climate change. For analyzing the response of tree growth to the globalclimate change, it is necessary to get some information for the moisture status of the soil-rootsystem. Water storage capacity (WSC) which defined as the amount of water stored in soilthat is available for the vegetation, it is an important limiting factor for the tree growth inseasonal drought area. However, most of the foresty soil stands on rocky areas which showsstrong spatial varation, it is diffcult to get an overview of the inner information withtraditional methods. Electrical resistivity tomography (ERT) was applied to stony forest soilsand helped to have an overall grasp of the soil spatial varation. In our research, electricalproperties and the WSC of the stony forest soils were studied through ERT. We analyzed theinfluencing factors of the WSC and their influcing degree was analyzed as well, in addition,the applicability of the ERT for studying the relationship between the electrical resistivity (ER)of soil and the tree growth was evaluated. Some important conclusions were drawn asfollows:(1) Electrical resistivity measurements on fine earth samples with four-electrode methodindicated that the moisture content acted as the main driver in ER variation. The electricalresistivity of samples from the upper layers (0~20cm) is much higher than that in the deeperlayers, which is likely due to the high organic matter content acting as highly resistivecomponent. The water holding capability of fine earth and rock fragments was measured bypressure-plate apparatus, the results indicated that the water stored in the rock fragments ishard to be extracted by the trees and the contribution to the WSC can be neglected comparedto the fine earth.(2) Numerical simulations were performed to validate the applicability of the ERT forRFC evaluation. Stony soils with different volumetric rock fragments content (RFC) werenumerically simulated by regarding the soil as biphasic media with rock fragments embeddedin fine earth continum. With known ER of the two phases, the effective ER of the soil can be related to the RFC, the relationship was well modelled by the mixing law of Bussian's. Themixing law was verified to be efficient and the wettest condition is favored for theapplication.(3) Estimation of RFC using ERT measurements was evaluated against referencesobtained by digging pits. It shows that the ER of fine earth has a strong impact. Considering asingle electrical resistivity for fine earth as160m, we obtained an absolute error for thevolumetric rock fragment content of22%.(4) ERT was also applicated to the evaluation of another important facot affecting theWSC, i.e. soil thickness. Soil thickness was defined as the distance between soil surface andthe bedrock. A threshold value of the electrical resistivity is proposed to be within1200~1600m, it was applied to separate the bedrock and soils media. Pedo-transferfunctions and the elements related were clarified for the WSC evaluation.(5) Estimation of the rock fragments content and the soil thickness were combined withhydraulic properties of the fine earth to map the WSC. The accuracy of the evaluation wasmuch more affected by the RFC estimaton.(6) There exists significantly correlation between the ER of the tree soil stand and thetree growth status on the Transect.3-D ERT survey can be an ideal alternation which canprovide much more accurately and detailedly for the ER of tree soil stand.(7) The research on the ER of the soils stand from3-D ERT survey indicated that the treegrowth characteristics can be correlated to the ER. Big trees are more adaptable to the climatechange on the research site. Under the condition of the soil stand, root-soil interaction insidethe vadose zone causes the increase of the ER of the soil. Different magnitude of thecorrelation was found between the ER of the horizontal slices and the tree growthcharacteristics.Some theoretical analysis and experimental results indicated that the ERT is applicabilityfor object-based stony soils, which can be laborsaving and efficiently. The results can behelpful for the unstanding of the foresty stony soils, can provide an important guide for therelated foresty stony soil problem solving.2-D or3-D ERT survey will be an important toolfor the soil fundamental research, the research on the applicaton of the method will bemeaningful for the soil related research.