The Spatiotemporal Variability Of Soil Hydrological Properties Under Artificial Caragana Korshinskii Plantations On The Loess Plateau

The Loess Plateau is located in a semi-humid,semi-arid and arid climate zone,with water shortage,serious soil erosion and fragile ecological environment.Vegetation construction is the most direct and effective method to control soil erosion and improve ecological environment.Because of the adaptability of Caragana korshinskii to drought stress,it has become the main shrub type of vegetation construction on the Loess Plateau.By 2017,the planting area of Caragana korshinskii on the Loess Plateau has reached 1.33×10~6 hm~2.Previous studies paid more attention to caragana korshinskii in reducing soil erosion,increasing soil nutrients and soil aggregates,ignored the influence of caragana korshinskii on soil hydrological properties.In this study,the typical plantation vegetation,Caragana korshinskii,was selected to investigate the spatiotemporal variability of soil hydrological properties,and to clarify its dominating factors at the plot,hillslope and region scales.This study was conducted to provide theory support for the research of water cycle mechanism,to provide scientific basis for rational utilization of water resources,vegetation reconstruction and the ecological environment sustainable development.The main conclusions of this study were as follows:(1)At the plot scale,the saturated hydraulic conductivity(Ks)under shrubs was significantly higher than that between shrubs.Ks at the margin of shrub canopy was typically highest,and had the greatest spatial variability.Soil water content(SWC)varied little with the increasing distance from shrub stems.At the plot,the areas with the highest Ks tended to be the areas around the shrubs.Soil organic matter and soil texture were the dominating factors influencing Ks and SWC variations at the plot scale.Soil bulk density and soil texture can be used to well estimate Ks and SWC by state-space approach.(2)At the hillslope scale,SWC exhibited a decreasing trend from upper to lower slope position.No regular changes,however,were observed in soil bulk density(Bd),Ks,silt and clay contents along the slope.SWC,Bd,clay and silt contents for the whole profile had strong spatial autocorrelation;Ks and surface soil texture had no obvious spatial structure.Continuous drought increased soil desiccation under the plantation of Caragana korshinskii;extreme precipitation events could supply SWC for the 0-200 cm soil layer.The planting density of caragana korshinskii was the dominanting factor affecting soil water spatial variation at the hillslope scale.(3)At the region scale,SWC of the 0-100 cm soil layer decreased with the reduced rainfall from the semi-humid to the semi-arid region,and there was a significant difference between SWCs at different sampling sites(p<0.05).In the area of semi-arid area,with the decrease of rainfall,no significant difference existed between SWCs of the 100-300 cm soil layer(p>0.05).The quantity and profile distribution of fine roots of Caragana korshinskii greatly changed with the decrease of rainfall.The fine root biomass in the semi-humid zone was significantly higher than that in the semi-arid area(p<0.05).The fine roots at the site located at the north end of the rainfall gradient zone were confined to the soil layer of 100-160 cm in the soil profile of 300 cm depth.This was the adaptation of root system of Caragana korshinskii to the extreme soil desiccation.Rainfall was the dominanting factor affecting soil hydrological properties at the region scale.(4)Considering the temporal variability of SWC,the time stability of SWC gradually increased as depths of soil layers enlarged.By the time stability analysis of SWCs at different depths of sampling sites,it was found that the most time stable depth(MTSD)representative of mean SWC in the profile was typically below 160 cm depth.The prediction error for the hillslope mean SWC with MTSD was significantly higher than that with most time stable locations(MTSL).But the results still reached the prediction accuracy.Soil texture is the dominating factor affecting the time stability of SWC in different soil layers.(5)There were significantly correlations between the spatial patterns of SWC in the shallow soil profiles(0–10,0–20,…,0–100 cm)and that in the 0–300 cm deep soil profile(p<0.01),and the mean SWC in the deep profile on the hillslope could be well estimated using the SWC measurements only at several time stable locations(TSLs)identified from the SWC measurements in the shallow soil profile(within 0–100 cm depth).Moreover,the optimal number of TSLs and the optimal depth of the shallow profile varied with soil moisture conditions.In the semiarid region,the optimal depth of the shallow profile was 0–40 cm and the optimal number of TSLs varied from 2 to 3;in the semi-humid region,the optimal depth of the shallow profile was 0–30 cm and the optimal number of TSLs was 3.The proposed approach could greatly reduce the expenditures of time and cost in monitoring the mean SWC for the deep profile at various spatial scales and could be applied well to the regions with similar hydrological and climatic conditions.