The Characteristic Of Red Soil Surface Crust Andits Effect On The Hydraulic Conductivity

The surface crust of red soil formatted after rain has great impacts on runoff and sediment. It can affect the infiltration, evaporation and moisture of soil. Meanwhile the cycle of soil water may be changed by the surface crust. However, the relationship between surface crust and infiltration of soil is still a dispute. The report of surface crust and soil unsaturated infiltration is rare. Based on the investigation of the characteristics of formation for surface crust under different land use and farmland tillage practices, we studied the status of soil infiltration and the impact of surface crust on soil unsaturated infiltration capability. The experiment was carried out in quaternary red soil at He sheng qiao town in Xian an distract, Hubei province. Three kinds of land use, including woodland, plantation and farmland were selected. Besides, we setted five different tillage practices at an8°slope farmland. The tillage practices were check(CK), summer corn (C), corn intercropping Bahia grass with (C+B), the corn surface straw coverage (C+S) and corn intercropping bahia grass strip at the same time surface straw cover (C+B+S).In rainy season and dry season, we had measured the characteristics of the surface crust and rate of soil unsaturated infiltration using disc tension infiltration instrument in-situ measurement. Meanwhile the index model was used to estimate the unsaturated hydraulic conductivity curve. The main results are as follows:In different land uses for different seasons, the surface coverage could inhibit the generation of surface crust, the surface coverage was the determinants of red soil surface crust formation. In this study, woodland and tea having coverage in spring and winter are not observed crust formation. The crust coverage for poor coverage farmland was respectively10.7%and7.1%in spring and winter. In the rainy period and dry period for different farmland measures, compared with the CK treatment,the coverage of the surface crust in the cultivation of corn reduced from16.4%to11.6%, a decline of38.2percent, which had a effect to inhibit the formation of the crust. But land covered straw (C+S, and C+B+S)wasn’t the formation of the crust, completely inhibited the development of the crust. This showed that the surface coverage was the determinants for crust formation on the red soil surface.Crust occurred arable land for relatively little coverage.In the topsoil crust of farmland, the biggest of single crust area was about17cm2. The crust numbers of a single crust less than1cm2area accounted for21%-37%of the total crust numbers. The crust numbers of a single crust less than5cm2reached more than90%of the total crust numbers. The crusts coverage accounted for9.7%-17.9%on ground. Analysis of the Open crack degrees variation in crust showed it varied from0.02-0.35cm/cm2. Crusts thickness were1.2-1.7mm.Soil strength for crusts in wet condition were146-166KPa.In different periods, the surface covering of soil saturated hydraulic conductivity are significant differences. Saturated hydraulic conductivity of the tea plantations B and forest B for surface coverage during the summer were0.16cm·hr-1and0.54cm-hr-1, greater than the saturated hydraulic conductivity rate in the winter,and they have the significant difference. Saturated hydraulic conductivity of the farmland C+S and C+B+S for surface straw coverage in the dry period (September) were1.07cm·hr-1and1.50cm·hr-1, greater than the saturated hydraulic conductivity rate in the rainy period (August) and they have the significant difference. In the study area, saturated hydraulic conductivity for cultivated red soil (stable infiltration performance) close to the middle level of penetration (0.527-0.547cm·hr-1), and more than tea garden (0.108-0.337cm·hr-1) and part of the woodland (0.173-0.917cm·hr-1). Although the cultivated soil surface had the formation of crust, the steady infiltration rate changes with the seasonal changes smaller than the tea gardens and woodland.Arable soil covered by straw inhibited the surface crust, while improving the performance of its topsoil infiltration. As the infiltration Miriam water supply tension near saturated infiltration, less than0.6KPa, infiltration rate of straw mulching (C+C+B+S) is1.2-1.5times than that of the coverage (C and C+B). The unsaturated curve of hydraulic conductivity shows that straw mulching of soil unsaturated hydraulic conductivity decreased, as the rapid decline of soil matric potential. The negative correlation between nearly saturated soil infiltration capability and soil coverage, the thickness of the crusts was not significant. However, the opening of the crust analysis degree has a positive correlation with nearly saturated soil infiltration capability, but the p of significance is larger than0.05. These results above indicate that the status of land coverage is a decisive factor for the development of red soil crust. Surface soil crust reduces the capability of infiltration as the red soil nearly saturated, increasing the performance of the unsaturated hydraulic conductivity. Whether crust could promote the evaporation of topsoil have to be studied in future.