Soil Nitrogen, Phosphorus And The Other Chemicals' Release And Transport With Surface Runoff And Mixing Zone Depth On The Loess Slopeland

Field chemicals release and transport with surface runoff under rainfall or irrgation condition have beenplaying important roles in degradation of soil quality, abatement of fertilizer efficiency, expansion of surfacewater pollution, and deterioration of local eco-environment on loess plateau. The soil chemicals involved in lossby surface runoff only exist in certain soil horizons, the depth of which is named as mixing zone depth ofinteraction among rainfall, runoff and soil chemical. It is a useful value to simulate the process of soil chemicalsrelease and transport with surface runoff and sediment, insight into the mechanism of nonpoint source pollution,and evaluate the fertilize efficiency. According to the result of artificial rainfall, the character and factors affectedby soil, rainfall, chemicals, slope length, slope gradient and field mangment practices, for rainfall, runoff,infiltration, soil erosion and soil chemicals, such as NO₃^-, NH₄⁺, PO₄^3-, K⁺ and Br^-, are investigated. Thedeterminate methods, influence factors and practical application of the mixing zone depth are also researched.The results can enrich the dynamics of soil, water and nutrient transport, evaluate the soil water and fertilizerefficiency, and theoretically benefit the research of agricultural NSP on loess hill slope.1. According to the surface transport experiments of soil chemicals under the non-satuation soil and watererosion condiction, for adsorbed PO₄^3- and K⁺, the time of sheet erosion and rill erosion initiation is coincidentthat of chemicals content in surface runoff climbs, and then their abundant export by surface runoff is urged. Therising time of non-adsorbed NO₃^- and Br^- content, however, is lagged behind that. For interval rain events, thesoil PO₄^3- concentration in surface runoff for the 2nd rain is usually more than that of the stable content period forthe 1st. The total soil erosion amount and PO₄^3- surface loss of the former are greater than that of the later, whilethe NO₃^- and Br^- surface loss for the 2nd rain is less. The soil, water and chemicals loss on loess slope land aregreatly enhanced with groundwater- supplying, higher initial soil water content, lower infiltration capacity forclay loam soil, more rainfall intensity and longer rainfall period.2. Rainfall volume increases with the slope length as a line function, while sediment, mean erosionmodulus, and chemicals net loss in surface runoffand sediment rise as a significant exponential correlation withit. The critical slope gradient for soil erosion and chemicals loss is in the range of 15°～20°. There is greaterfertilizer loss for spray-applied land than that for mixed-applied one. More fertilizer applied will absolutely gainthe surface loss and infiltration amount of soil chemicals. Soil erosion can be reduced by grass cover, thechemicals' concentration in surface runoff, however, is urged. The surface transport of soil NO₃^- and Br^- is usually along with overland flow, while the content of NH₄⁺, PO₄^3- and K⁺ in surface runoff is exponentiallyrising with sediment amount. The sequence of infiltration capacity for soil chemicals is Br^-＞NO₃^-＞K⁺＞NH₄⁺＞PO₄^3-. The mineral nitrogen concentration of recession flow is usually more than that of the runoff when therain terminates, especially for a longer slope. There are much more nitrate than ammonium in the recession flow.3. Compared with the exponential function, the power function is more suitable for fitting the changes ofchemicals' concentration in runoff with raindrop splatter erosion or slight soil erosion. The Depths of Uniformand Complete Mixing Zone model, Depth of Effective Runoff Transport model and Kneeing Point Methoddetermining EDI are tested and analyzed for soil chemicals on the loess lope. The EDI got by the Kneeing PointMethod is proved to be the real mixing zone depth. The curve of Br^- concentration in runoff is described as apower function with the fertilizer initially applied depth, which is less the depth, more the fertilizer loss amount.The calculated EDI of Br^- in this experiment is 2.1 cm, while the real mixing zone depth is 6.0cm. The climb ofthe initial soil moisture will aggrandize the mixing zone depth.4. The model for chemicals transport by surface runoff, which can reflect the chemical convection—dispersion and desorption factors is put forward under water erosion condition. This model and the polynomialequation is test to well fit this process. The notion of Effective Erosion Transport Depth (EED), the solute inwhich can only transport with surface runoff, and that of Effective Adsorptive Depth, where the chemicals lostby sediment, are put forward. The models for EED and EAD are founded, in the condition of total solute loss isconsidered. It is shown that the chemical loss amount can be calculated with the correlation equation for EED,EAD and those influential factors.5. Based on the un-uniform and incomplete mixing of rain water, soil water and infiltration water, theformula of soil initial solute concentration, c_o, at the ponding time is exported, which is suitable for theunsaturated soil and the infiltration influence before surface runoff occurs. The quality convection transportcoefficient, k_m, is calculated for different soil layers. Then, the effective quality transport model for describingthe chemical distribution of the mixing zone when runoff occurs is conduced and test by the experimental data.To delete the hazards of fertilizer surface loss and leaching, and according to the theory of mixing zone depthwith the erosive rainstorm and critical slope gradient of returning land for farming, it is suggested that the upperfertilizer-applied layer is more than 6～10 cm below the surface soil layer, more deep in the season with serioussoil erosin and evapouration. The lower limit of fertilizer-applied layer cannot be lower than the range of croproot. Integrated by the chemicals loss amount and influence mechanism, the mixing zone depth can evaluate thenutrient validity and soil contamination export hazard.