The Heterogeneity Of Soil Water Retention Capacity And Its Controlling Mechanism Across A Slope-gully System

The permanent gully,one of the main types of erosion gullies in the Loess Plateau,has a profound impact on the earth surface processes of hydrology,vegetation,landforms and ecology.Slope is the basic geomorphic unit of soil erosion.The coexistence of permanent gully and slope farmland in the Loess Plateau has experienced great changes since the implementation of the Grain for Green project in 1999.The project converts slope farmlands for agricultural production into forest or grasslands with ecosystem services function.The gully as well as the slope consititute the slope-gully system,which is the place of runoff occurrence and sediment transport in the catchment.Differences in terrain affect the soil hydraulic and physicochemical properties,and then act on vegetation distribution.Due to drought and scarcity of water resources,it is important to improve soil water storage capacity,promote rainfall infiltration and increase soil water use efficience.Characterizing soil water retention capacity as well as its relationships with soil macropores and physicochemical properties are helpful to understand the soil hydrological process in the slope-gully system and reveal the mechanism of soil macropores regulating infiltrated water.Understanding the heterogeneity distribution of soil physicochemical parameters in the slope-gully system can also provide the basis for vegetation restoration and land reclamation in the Loess Plateau.In order to know the influence of gully on the spatial distribution of soil water retention capacity in the slope-gully system,soil water retention curves(WRC)of three topographic treatments,gully bottom,bank and nearby slope,were determined by the centrifuge method.The X-ray CT scanning technique was used to quantify the geometry characteristics of soil macropores,so as to compare the differences of soil macropores in different topographic treatment,slope positions as well as soil depth.The main factors affecting soil hydraulic parameters(saturated hydraulic conductivity and soil water retention capacity)were also determined based on relevant results on macropore parameters and soil physicochemical properties.Following conclusions were derived:(1)From the surface to sub-surface layer,soil water retention capacity was largely consistent in the gully bottom and increased in the nearby slope.Inconsistent results appeared at the three slope positions of the gully bank.In contrast to the surface layer,the soil water retention capacity of the sub-surface layer was consistent,increased and decreased at the upper-(U-),middle-(M-)and lower(L-)-slope positions,respectively.The effect of the gully on soil water supply capacity of surface layer in the gully bank is more prominent.In the gully bank,the surface layer C(?)at less than 100 k Pa was slightly greater than the gully bottom and nearby slope,and C(?)at more than 100 k Pa was the smallest of the three terrains.The sub-surface showed similar changes based on the suction limit of 40 k Pa.During the continuous reduction of soil moisture,the micro-topography along slope may have also contributed to the formation of the spatial soil water retention capacity patterns.For the gully bank,the L-slope of surface layer reached the CRM earlier,and the AWC of the U-slope in sub-surface layer was higher than the L-slope.(2)Soil macropores parameters were significantly effected by three topographic treatments of gully bottom,bank and nearby slope.The mean macropores number of the gully bank was higher than that of the gully bottom and nearby slope in the 0~110 mm soil layer,while it ranked as: gully bottom> gully bank> nearby slope in the 110~220 mm layer.The macropores parameters of various terrains were also affected by the positions.The mean macropore number and macroporosity of the gully bottom and U-slope were higher than those of the M-slope and L-slope.The macroporosity of the M-slope on the gully bank was 6.02%,which was five and four time lower than the L-slope and U-slope,respectively.In the 0~220 mm layer,the gully bank always had the largest mean equivalent diameter.The largest macropore numbers corresponded to the U-slope of gully bottom in the 0~125 mm layer,followed by the L-slope and M-slope of gully.In the 100~220 mm layer,the equivalent diameters in the M-and L-slope of the gully bank were slightly higher than those of the nearby slope.There were significant differences in the three-dimensional macropores parameters between the positions of the same terrain.The M-slope of the gully bottom and nearby slope were smaller than the U-and L-slope positions,while that of the gully bank was higher than the U-and L-slope positions.(3)The topography significantly affected the saturated hydraulic conductivity(Ks)and the bulk density which changed with the depth of the soil layer in the gully bottom,bank and nearby slope.The influence of topography on soil water content was more prominent.There was no significant difference in soil water contents between the gully bank and nearby slope of each soil layer,while they both had significant differences with the gully bottom.The soil water content decreased first along the gully bottom to the bank and then increased to the nearby slope.The soil water content in the gully bottom was high,while the soil water deficit was serious in the 0~30 cm layer of the gully bank.(4)The soil p H of slope-gully system was weakly alkaline.With the increase of soil depth,the soil conductivity(EC)of the gully bank remained unchanged,the gully bottom and nearby slope are decreased,and the nearby slope was significantly larger than that of the gully bottom.Soil organic carbon(SOC)increased from U-slope to L-slope,and it was most obvious in the surface layer.SOC is significantly influenced by soil layer and interaction between soil layer and terrain(p< 0.01).The SOC in shallow layer of gully bank and nearby slope is lower than that in surface layer,and the interlayer of gully bottom is basically unchanged.The content of total phosphorus(TP)on the surface slope was significantly higher than that in the gully bottom.The effect of topography and its interaction with slope position on the(AP)of available phosphorus reached a extremely significant level,and the AP of the corresponding soil increased by 36.09% and 62.17%,respectively,compared with the sub-surface and shallow layers on the nearby slope.The content of total potassium(TK)in shallow layer of nearby slope was significantly lower than that shallow layer of in gully bottom.The available potassium(AK)was affected by the topography,slope position and soil layer,and the maximum AK values of surface,sub-surface and shallow soils on the nearby slope were 4.0,6.4 and 5.5 times higher than those of the gully bottom,respectively,With the increase of soil depth,the soil TN,AP,TP,AK and TK decreased with the increase of soil depth.(5)The main factors affecting the distribution of soil Ks and soil water retention capacity parameters include soil macropore parameters,texture and nutrients.The effect of soil macropore parameters on AWC was greater.With the increase of the number of variables,the prediction effect of field capacity(FC),CRM,permanent wilting point(PWP)and AWC was improved.As far as the Akaike information criterion(AIC)is concerned,the prediction accuracy of AWC on the gully bottom is improved obviously after introducing macropore parameters.Based on RMSE and Nash efficiency coefficient(NSE),and considering macropore parameters,the prediction effect of ln Ks,CRM and AWC on the gully bottom is slightly higher than that of only considering physical and chemical properties.The influence of soil macropore parameters such as branch density,connection point density and so on the gully bottom topography ln Ks is less than that on the gully bank and nearby slope.In addition,pore volume fraction of gully bank topography also has a great contribution to ln Ks,? value is 2.077.Both capillary porosity and total porosity play an important role in predicting ln Ks and soil water retention parameters.However,using the basic physical and chemical properties of soil or introducing macropore parameters at the same time,the contribution of capillary porosity to the change of AWC is obviously.In addition to capillary porosity and total porosity,the variation of FC,CRM and PWP is greatly influenced by clay and sand content in gully bottom.The content of clay and powder in nearby slope contributed greatly to the variation of soil water retention parameters,but the change of ln Ks was mainly influenced by total porosity and capillary porosity.The R2 value of the terrain FC,CRM,PWP and AWC linear regression equation is more than 75%.The prediction of FC,CRM,PWP and AWC on nearby slope based on bulk density,texture,porosity and organic carbon is better than that on gully bottom and bank topography.Overall,the existence of the permanent gully increased the intensity of soil erosion on the gully bank under the action of external forces,and a large amount of sediment is gathered in the gully bottom under the action of Rain Water erosion and transportation.In addition,the spatial distribution of soil pore structure,texture and nutrients changed due to the influence of natural factors such as vegetation and climate,and the spatial heterogeneity of soil water retention capacity in slope-gully system was obviously enhanced.Soil macropore parameters,capillary porosity and soil texture are the main factors that affect the soil water retention performance of slope-gully system.