| The karst area in Southwest China is one of the three major karst concentration areas in the world and the largest distribution area in the world.In the long-term karstification,it has a surface and underground double-layer structure of water,soil,rock and biology.Its unique dual structure makes the area have two forms of surface and underground erosion,and part of the surface water and soil loss passes through karst pores(cracks),holes Pipelines and other channels enter the underground system and are lost,resulting in underground leakage.Sloping farmland is the main site of soil erosion in karst areas.Under the action of rainfall,water and soil loss not only takes away the surface soil,but also the loss of soil nutrients.Water enters the ground through holes(cracks),takes away a large amount of nutrients in the soil,reduces soil fertility,and seriously restricts the local land productivity.Therefore,this paper takes the shallow pore(fissure)soil(calcareous soil)of karst sloping farmland as the research object,through the combination of simulation monitoring test and field test,designs the sloping farmland soil sample plot as the control group,studies the soil nutrient leaching characteristics under different porosity(opening)and different soil depth under natural rainfall conditions,and reveals the water nutrient loss law and influencing factors of karst sloping farmland pore(fissure)soil.The research results can provide a theoretical basis for the prediction and control of soil and water loss / underground leakage in karst areas.The main results are as follows:(1)Through the infiltration test on the simulation test and field sample plots,the infiltration process of each surface soil has an obvious change trend.The soil infiltration rate gradually decreases and tends to be stable with the extension of time.The stable infiltration rate of each test fissure soil has a large decline compared with the initial infiltration rate.The stable infiltration point of the simulated pore(fissure)soil is concentrated in the period of40～60 min.in the field test,the stable infiltration time is relatively faster,The infiltration characteristics of porous(fissured)soil and sloping farmland soil have little difference.The stable infiltration rates of porous(fissured)soil with porosity of 16%,9% and 4% are 13.48mm/min,7.54 mm/min and 6.69 mm/min.The greater the porosity,the greater the stable infiltration rate.The stable infiltration rate and total infiltration amount of the field porous(fissured)soil are less than those of the porous(fissured)soil in the simulation test.Through correlation analysis,the infiltration capacity of fissured soil has obvious positive correlation with soil bulk density and capillary porosity.Combined with the simulation and field test,there is no obvious difference between the shallow pore(crack)gap of the sloping farmland and the surface soil water infiltration law of the sloping farmland,and the crack opening has no significant impact on it.The soil infiltration capacity is the result of the joint action of a variety of external factors.The initial infiltration rate and stable infiltration rate of soil are significantly correlated with the dissolved organic carbon(DOC),total nitrogen(TN),total phosphorus(TP)and total potassium(TK)of surface water,of which the initial infiltration rate is positively correlated with the DOC and TK of surface water,and the stable infiltration rate is significantly positively correlated with the doc,TN,TP and TK of soil flow and underground pore water.The steady infiltration rate of soil has a certain negative correlation with surface water TK and soil flow TN.(2)Under the influence of rainfall characteristics and porosity under natural conditions,the nutrient leaching characteristics of shallow pore(fissure)soil in Karst sloping farmland are quite different.Among the variation characteristics of soil organic carbon,TN,TP and TK leaching loss in the pore(fissure)of the simulated monitoring test,the loss of TP is the smallest,and the rest is larger.The concentrations of TN,TP and TK in the surface water in the pore(fissure)soil of sloping farmland were significantly lower than those in the surface water of sloping farmland due to the existence of pore(fissure);In the surface soil nutrient leaching,the concentration changes of DOC,TN and TP are as follows: porosity 16% > 9% >4%,that is,the greater the porosity,the greater the soil nutrient leaching;The TN concentration of water in different soil depths is underground > soil > surface;The changes of DOC concentration and TP concentration in different soil depths were the same,which was soil > surface > underground;TK leaching loss of different soil depths is surface > soil >underground.The results of field experiments on the fissured soil of sloping farmland show that the concentrations of TN,TP and TK in the soil water of sloping farmland are significantly higher than those of fissured soil.The overall change of TP leaching concentration in the fissured soil of sloping farmland from the surface to the depth of 20～30cm is: 20～30 cm > 10～20 cm > 0～10 cm,and the overall change of DOC,TN and TK is the same,that is,0～10 cm > 10～20 cm > 20～30 cm.After fertilization,the concentrations of TN,TP and TK in soil water at the depth of 0～10 cm increased significantly.The concentrations of TN,TP and TK in soil water at the depth of 0～10 cm in sloping land were higher than those in fissured soil at the same depth.The concentrations of TN,TP and TK in soil water at the depth of 10～20 cm in sloping land were also higher than those in fissured soil,and the time of nutrient increase was faster than that in fissured soil.Due to the influence of pore(fissure)rock soil interface and preferential flow,there was a great difference in the amount of soil nutrient leaching.In the porous and fissured soil,the time of nutrient leaching is faster,which aggravates the leaching of soil nutrients from sloping farmland.(3)Rainfall characteristics have a certain impact on soil nutrient leaching from Karst sloping farmland.Especially under extreme rainfall conditions,rainfall intensity has a significant correlation with soil TN,TP and organic carbon leaching.Through fitting analysis,the correlation between rainfall intensity and soil water TK concentration is poor,while rainfall intensity has a good linear relationship with soil water doc,TN and TP concentrations.TN concentration in soil flow and underground pore water has a certain response relationship with rainfall intensity.Rainfall duration has a significant positive correlation with DOC concentration in soil flow(P < 0.05),and a significant negative correlation with TP in surface water.Rainfall has a good positive correlation with TP and TK of surface water,and it has the greatest impact on nutrient leaching of surface soil;There was a certain correlation between the amount of nutrient leaching and the physical and chemical properties of soil and the size of porosity in Karst sloping farmland,in which the non-capillary porosity was significantly negatively correlated with the concentration of DOC,TK and TP(P < 0.05).There is a certain correlation between soil nutrient leaching and soil properties and rainfall conditions in Karst sloping farmland.The influencing factors of nutrient loss in crevice soil of karst sloping farmland are complex and changeable. |
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