| The vegetation restoration has been impelement in Loess plateau for over ten years. Coverting farmland to forest or grass may have varied soil struction, infiltration and water holding capacity, and therefore lead to change in watershed runoff. This change was more significant in climate transition zone. It may affect the local environmental protection scheme which aims to ensure local water security and afforestation. In this study, three land uses(Robina forest, secondary grassland and rape field) were selected in Zhonggou catchment which located in upper reach of Jinghe River, the subhumid-semiarid region of Loess plateau. Precipetation, soil water infiltration, soil struction, soil water-holding capacity and soil moisture contenct was investatged in different season under different land use. The measured date was used to analyze the effect of land use change on soil hydraulic properties and water balance. The results showed that:(1) Soil basic physical and chemical properties have been change after afforestation. Soil under forest and grassland has higher silt content, CaCO3, and root density but lower clay content and bulk density compare to that under cropland, indicating that soil structure and porosity have been improved, which may promote soil water infiltration and reduce soil erosion. N and electrical conductivity was higher in cropland comparing to forest and grassland. However, soil organic carbon content and pH value was similer among different landuses.(2) Soil saturated hydraulic conductivity was highest in grassland(678 cm d-1), followed by forest(320 cm d-1) and cropland(195 cm d-1). This was affected by differentce of fine roots density and the soil management in each landuse. The larger amount of fine roots and more biological activity in grassland and forest resulted in higher saturated and near-saturated hydraulic conductivity. While in cropland, a plow pan was formed by intensivty plowing activity, which reduced hydraulic conductivity.In surface soil of forest and grassland, soil saturated and near-saturated hydraulic conductivity had its hightest value in September and loweset value in March. The seasonal variation was due to the change of plants roots and soil microbes within the year. In cropland, hydraulic conductivity was more likely affect by tillage. Therefore, in September when cropland was plowed for sowing, the surface soil was loose and had plenty of pores which would increase hydraulic conductivity. However, in June(after harvesting) and March(flowering phase), hydraulic conductivity in cropland was relatively low. In all landuses, soil saturated and near-saturated hydraulic conductivity decreased with soil depth. For soil below 30 cm, no significant seasonal variation was found in this study.The hydraulic conductivity in surface soil was high enough but in cropland subsoil, hydraulic conductivity is lower than the maximum rainfall intensity(49.8mm/h), it may lead to surface runoff under saturation condition.(3) Unsaturated hydraulic conductivity was significantly different under forest, grassland and cropland, but as pressure head decreased, hydraulic conductivity gradually became similar. Hydraulic conductivity at pF > 2 was affected by soil texture rather than soil structure. In cropland, soil was high in clay content. Therefore, at lower pressure head, its hydraulic conductivity was larger than forest and grassland. No significant seasonal change was found for unsaturated hydraulic conductivity.(4) Water-conducting macroporosity(pore diameter > 1 mm) showed a lowest value of 0.0143% in cropland and highest value of 0.0701% in grassland. Although the macropores consisted of only 0.08- 0.35% of the total soil volume, they conducted up to 80% of the flow under forest, grassland and cropland. For cropland, the total water flow of topsoil was significantly larger in September as compared to that in June and March, accounting for more than 80% of the total flow. Nevertheless, the contribution of this pore class decreased in subsoil. This was attributed to the effects of plowing.(5) In the soil depth of 0-6 cm and 30-36 cm, forest soil had the largest field capacity, available water capacity and relatively low permanent wilting point. In grassland, the macropore riched soil structure was benifical for water drainage but resulted in low water-holding capacity. Therefore, grassland soil had higher air capacity but lower field capacity and available water capacity. In cropland, topsoil was high in air capacity, but in cropland subsoil, the plow pan had led to a decrease of available water capacity and an increase of permanent wilting point. Soil air capacity, field capacity, available water capacity and permanent wilting point had no significant difference at soil depth lower than 60 cm. Significant seasonal variation of soil water retention curve was observed in this study, but this variation didn’t have a consistent pattern. More repetition and long-time observation may be needed in order to obtain more accurate results.(6) The average value of soil water content in Zhonggou catchment was 24.9%. It was highest in autumn(Sep-Oct) followed by spring(Mar-May) and summer(Jun-Aug). In forest, soil water content was 26.4%, larger than that in grassland(23.6%). The water comsuption within 100 cm depth was 492.6 mm in forest and was 488.4 mm in grassland. During growing season, forest comsumpted more soil water than grass, while during the non-growing season, it was the opposite. Comparing with grass, forest soil has higher water holding ability and could used soil water more efficiently, which is benifical for soil and water conservation and helpful in reducing surface runoff.(7) In topsoil, forest soil has more available water than grassland. For the soil depth lower than 40 cm, the affect of landuse on available water content was insiginificant. The saturation water, readily available water and moderately available water content were higher in forest soil, while the resistantly available water content was higher in grassland. The analysis of soil water deficiency indicated that except for in June, soil water in Zhonggou catchment was sufficient for the growth of plants.In conclusion, forest and grass have had positive effects on soil physical and hydraulic properties after vegetation restoration. Afforestation has decreased soil bulk density, improved soil macropore structure, increased soil water infiltration(including unstaturated infiltration), water content as well as soil available water. Comparing with forest, grassland soil had larger infiltration ability but lower water-holding capacity. Therefore, it has less conservation ability than forest soil. |
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