Distribution Of Gravel And Its Effect On Water Properties In Stony Soil Of Northwest Guangxi

The karst region is very fragile eco-environment with slow soil formation process,shallow and discontinuous soil,small water holding capacity,and low vegetation coverage.Due to the long-term intense karstification function,the uncoordinated pattern of water and soil resources(the soil is above,the water is below) has been formed in karst area.Consequently the agricultural development is heavily restricted,and the impoverished situation is serious.One of the efficient approaches is to carry out vegetation restoration and to consolidate its function for soil and water conservation.To investigate the stony soil physical properties,to understand the distribution of gravel on hillslope,and to analyze the effect of gravel on water properties are important for the reasonable utilization of water and soil resources,and ecological environment construction.Based on field experiment and corresponding simulation in the Huanjiang Experimental Station of Agro-ecosystem,Chinese Academy of Sciences,the distribution of gravel and its influencing factors along hillslope are analyzed from September in 2006 to May in 2007,and water infiltration and evaporation in soil with different gravel sizes(5-20mm,20-35mm) and contents(gravimetric gravel content is 0,10%,30%,50%,respectively) are investigated from May to September in 2007. The main results are shown as follows:(1) Stony soil is calcareous soil leaned to alkalinity.Upward along the slope,soil pH value,organic matter content,total porosity and capillary porosity increase,and soil bulk density decreases.Soil clay contents reduce but the sand contents increase gradually.With the increase in soil depth,soil pH value increases,but soil organic matter content reduces.The decrease of soil bulk density upward along hillslope may primarily have a close relationship with soil organic matter content,clay content,soil structure,compaction and porosity Total porosity is between 55.24%-61.34%,but capillary porosity is between 51.57%-57.81%.(2) The gravimetric gravel contents increase from bottom to top on hillslope. With the increase in soil depth,they decrease in the soil profile at the bottom but increase at the top of the slope.The order of the gravel sizes for gravimetric contents in the topsoil is 20-5mm＞75-20mm＞5-2mm on the uncultivated slope and in shrub-grass land in the depression area.However,there is significant difference for the gravimetric contents of various gravel sizes on another hillslope above the plant garden.The 20-5 mm gravel contents are higher at the bottom but 75-20 mm higher at the top of the hillslope.Obviously,gravel content and size have a significant relationship with slope gradient. (3) Soil water infiltration capability increases with the increase of gravel contents gradually.It increases with the increase in the gravimetric gravel content from 10% through 30%to 50%when gravel size is the same.Soil water infiltration ability increases with the increase of gravel size with the same initial moisture content,and decreases with the increase of the initial moisture content with the same gravel content.The accumulated infiltration quantity is higher under air-dried condition than that under near saturated condition.It increases with the increase of gravel size when the initial moisture content is the same.For the highest gravel content and the biggest initial moisture content,the stable infiltration time and the penetration initiation time are the shortest.When the gravimetric gravel content is 30%and gravel size is 20-35 mm,and the initial moisture content is near saturated,the average and stable infiltration rates are the smallest.(4) During the initial stage of water evaporation,the effect of gravel content is small.However,gravel cover reduces the time to get the stable evaporation stage. During the stage of water evaporation intensity influenced by initial soil moisture content,the change of water evaporation loss decreases with the increase of gravel content.During the water vapor diffusion stage,the range of water evaporation loss increases with the increase of gravel content.During the preliminary evaporation stage,the evaporation speed increases with the increase of gravel content from 10% through 30%to 50%.However it is the inverse order during the end of evaporation stage.During the process of water evaporation,the accumulated water evaporation amount decreases with the increase of gravel content from 0 to 50%,and it is the highest in soil with gravel cover.