Study On The Relationship Between Water Repellency And Soil Physicochemical Factors Of Different Cover Soils In The Loess Plateua

Soil water repellency（SWR）can reduce soil infiltration capacity and increase surface runoff,resulting in the risk of soil erosion.In the Loess Plateau,the soil organic matter（SOM）accumulation and the soil desiccation induced by vegetation restoration and seasonal drought,respectively,can stimulate SWR,thereby interfering with precipitation infiltration and increasing the risk of soil erosion.However,the SWR distribution under different vegetation cover types in the Loess Plateau,as well as the relationship between SWR and soil properties（SOM,soil water content,and soil aggregates）,remain unclear.In this study,soil sample plots were randomly selected from four vegetation cover types（Pinus tabulaeformis Carr.,Robinia pseudoacacia L.,Hippophae rhamnoides L.and Grassland）to determine the on-site SWR by water droplet penetration time method（WDPT）in soil on-site 0-3 cm and 3-10 cm layers.Soil samples were collected according to the on-site SWR performance classes,transported to a laboratory,and air-dried to determine the off-site SWR by WDPT and CA（contact angle）method.Then the sub-samples air-dried were used for soil physical and chemical properties determination.In order to explore the generative mechanism of SWR under different vegetation cover types in the Loess Plateau,we analyzed:（1）the on-site SWR distribution,and the difference between on-site and off-site SWR;（2）the relationship between SWR and SOM;（3）the relationship between SWR and soil water content;and（4）the relationship between soil aggregates and SWR.The main results are as follows:（1）The on-site SWR in the Loess Plateau mainly occurred in the 0-3 cm soil layer,and its exhibited punctate distribution spatially and varied in different vegetation cover types（except grassland）.The on-site soil from Pinus cover type showed the most extensive and the highest SWR（five SWR classes ranged from hydrophily to extreme water repellency）,followed by Robinia（slight and strong water repellency）,and Hippophae cover type（slight water repellency）,while the on-site soil from grassland exhibited hydrophilic.The SWR of different cover soils in descending order was tree（Pinus>Robinia）>shrub>grassland.However,the water repellency of different soil covers from outdoor to indoor was greatly reduced,which indicated that the SWR of loess is unstable.（2）The SOM content and composition significantly affected the SWR.WDPT increased significantly with the increase of soil organic carbon（SOC）content（r=0.85,P<0.01）,both of which had a strong fitting relationship（R²=0.76）.The contents of humic acid,fulvic acid and humin were also significantly positively correlated and linearly fitted with SWR（P<0.01,R²=0.81,0.83,0.85）.Compared with hydrophilic functional groups（amide C=O）,hydrophobic functional groups（aliphatic C-H）exhibited more pronounced and abundant peaks in highly hydrophobic soils and WDPT was significantly positively correlated and linearly fitted with C-H（P<0.01）.At the same time,C-H/C=O（SH）,one of the functional group ratios,was linearly fitted and significantly positively correlated with WDPT（R²=0.96,P<0.01）.It can be seen that the SOM component has a greater impact on SWR than its content,and the hydrophobic functional group C-H and the functional group ratio can reflect the variation of soil water repellency than the hydrophilic functional group C=O and the relative content of functional group,respectively.（3）The water repellency of loess showed dynamic changes with the change of soil moisture content.With the increase of soil water content,the SWR first increased,reached a maximum,and then disappeared successively.The peak value of soil water content differed among different vegetation cover types,but all soil WDPT peak values were lower than that of on-site SWR,indicating that soil water content was not the unique factor affecting SWR.In addition,the critical water content of water repellency disappearance differed among different vegetation cover types also varies,but under the same vegetation cover type,the critical moisture content of the soil increases with the increase of SWR level.（4）The SWR among different vegetation cover types increased with the decrease of aggregates size.The SOC content was the highest in 0.25-0.5 mm aggregates among all vegetation cover types,followed by<0.25 mm aggregates.The SOC was positively correlated with the contact angle（P<0.01）in 0.25-0.5 mm and<0.25 mm aggregates,respectively.In addition,SWR was positively correlated with the content of water-stable aggregates content(M_{（>0.25 mm）})and Geometric mean diameter（GMD）,while negatively correlated with the percentage of aggregates destruction（PAD）（P<0.01）.SWR has a promoting effect on the stability of aggregates.This study explored the SWR and its relationships with soil properties（SOM,soil water content and soil aggregates properties）under different vegetation cover types in Wangdonggou watershed.Our findings could provide the necessary guideline for the rational optimization of vegetation restoration and the evaluation of soil and water conservation in the Loess Plateau.Since the government-implemented project of returning farmland to forest and grass,the SWR has become a common phenomenon in the Loess Plateau.In the future,attention should be paid to optimizing the local vegetation restoration treatments and formulating rational soil management measures to reduce the risk of SWR.It is important for increasing on-site infiltration of precipitation and preventing soil erosion in eroded regions.