The Nitrogen Loss Feature Of The Throughflow On Weathering Granite Soil Slope

The soil which was developed from weathering granite is widely distributed in southeastern China,where locates in the tropical or subtropical areas,and is rich in heat and precipitation..This kind of soil contains so much sand that its texture is very loose.In addition,there are many hills in the region,and the rainfall mainly concentrated in summer.In those ways,water and soil loss is easily to happen,thus triggering the soil nutrient loss and water eutrophication pollution,which is harmful to the human production and livelihood seriously.On the basis of summarizing relevant research materials at home and abroad,this study chose the soil developed from weathering granite which was collected from Anji county,Huzhou city,Zhejiang province as the research object,and the indoor artificial rainfall simulation test method as the main mean.Therefore,we conducted 20 effective indoor simulating rainfall on the bare slope of weathering granite soil,to explore,under the condition of rainfall,the rule and internal mechanism of throughflow（surface runoff,subsurface flow）and sediment nitrogen loss under different slope gradient and rainfall intensity.The slope gradient（25°,15°,8°,5°）and rainfall intensity（30 mm/h,60 mm/h,90 mm/h,120 mm/h,150 mm/h）were selected as the variable factors.The rainfall duration of each experiment was 90 min after the surface runoff appeared,and the size of each runoff slot was 2 m×1 m×0.6 m in length,breadth and height.The main conclusions are as follows:（1）The curve of TN,NO₃^-N,NH₄⁺-N loss concentration in surface runoff were all descending,and the decline of the loss concentration with the increasing of rainfall intensity would be more obvious when the gradient falling down.The loss concentration curve of TN,NO₃^--N,NH₄⁺-N in subsurface flow were like peaks,and the steeper the slope or the heaver the rain was,the higher the peak height would be.The average loss concentration of TN and NO₃^--N in subsurface flow was significantly higher than that in surface runoff,while NH₄⁺-N was the opposite.The average loss concentration of TN and NO₃^-N in subsurface flow increased with the increase of rainfall intensity,and reaching the maximum value when the slope gradient was 8°。There were obvious correlations between the loss concentration of TN and NO₃^--N in surface runoff or subsurface flow.（2）The curve of TN,NO₃^--N,NH₄⁺-N loss quantity in surface runoff decreased in the early stage,but would rise up in the later under heavy rain.The loss quantity curve enhanced with the increase of rainfall intensity on the whole.The curve of TN,NO₃^--N,NH₄⁺-N loss quantity in subsurface flow were like peaks too.The total loss of TN,NO₃^--N,NH₄⁺-N in surface runoff were 2.08 mg-183.48 mg,1.80 mg-109.43 mg,0.01 mg～44.98 mg;while that in subsurface flow were 370.54 mg～2288.99 mg,325.33 mg～2219.58 mg,7.56 mg～75.05 mg.They all increased with the increment of slope gradient or rainfall intensity.Subsurface flow was the main approach of nitrogen loss on slope,but it would gradually decrease with the increase of rainfall intensity or slope gradient.The heaver the rainfall or the steeper the slope,the erosion yield and sediment TN loss would obviously increase,and the impact of the rainfall intensity on them was greater than that of the slope gradient.NO₃^--N and NH₄⁺-N were the major loss forms of nitrogen in surface runoff or subsurface flow,among which NO₃^--N was the main form,and the proportion of NO₃^--N in subsurface flow was higher than that in surface runoff.（3）There were significant positive correlations between rainfall intensity and the total nitrogen loss in surface runoff,subsurface flow or sediment.However,the slope gradient had no obvious significance correlation with them,indicating that rainfall intensity was the key factor of nitrogen loss on slope.The influence of flow volume on slope nitrogen loss were all very significant,indicating that the rainfall intensity affected nitrogen loss by influencing the surface runoff volume and subsurface flow volume.The regression equation could be used to describe the influence of slope gradient,rainfall intensity,surface runoff volume（or subsurface flow volume,or sediment field）to the total nitrogen loss of surface runoff,subsurface flow or sediment.The degree of fitting were followed as:sediment>surface runoff>subsurface flow,TN>NO₃^--N>NH₄⁺-N.（4）The impact on TN,NO₃^--N,NH₄⁺-N total loss in throughflow were subsurface flow volume>surface runoff volume>rainfall intensity>slope gradient,and the influence of the former three were all significant positive correlation,but there were no significant relationship between slope gradient and all of that.The influence of slope gradient,rainfall intensity,surface runoff volume,subsurface flow volume on total loss of TN,NO₃^--N or NH₄⁺-N in throughflow could be described as regression equations,and the regression coefficients further illustrated that the impact of rainfall intensity on nitrogen loss in throughflow was greater than that of slope gradient,and the flow volume played a decisive role on throughflow nitrogen loss.