Study On Regulation Impact Of Earthworm Casts On Soil Erosion And Nutrient Loss With Runoff On Slope Farmland In Loess Region Of China & Establishment Of Analytical Model For Prediction Of Nutrient Loss

In this paper,earthworm casts were used to improve soil aggregate structure and fertility,which combined with sediment and nutrient loss with runoff in the loess area.Earthworm casts that take into account the effects of‘water conservation and fertilizer control'was explored application rate and method,through indoor soil analysis and field rainfall experiments.At the same time,based on the characteristics of nutrient loss with runoff in the loess area,an approximate analysis prediction model suitable for runoff nutrient loss were established.The parameters of model were estimated and verified based on field experiments under five different rainfall intensities.The main research results are as follows:?1?The effects of earthworm casts on soil aggregate composition,water-erosion stability and water transport characteristics during infiltration were studied by indoor soil aggregate composition analysis and one-dimensional soil vertical infiltration test.The results showed that:i.The earthworm casts could effectively reduce the soil bulk density and increase the number of large aggregates of 0.25² mm.The fractal dimension D decreased from 2.84 to 2.65 by 6.69%,improved the water-resistant stability of soil aggregates significantly.ii.When the application ratio was 1/20,the migration distance of the wet front could be significantly increased,and then the wet front distance was reduced to some extent as the application ratio increases,but both were larger than the treatment without earthworm casts.And the relationship between the wetting front migration distance and the infiltration time under each treatment could be described by a power function.iii.The increase of the proportion of earthworm casts could significantly increase the cumulative infiltration and infiltration rate,when the ratio was 1/3 at the same time,the cumulative infiltration amount increased by 48.74%compared with the non-applied.The Kostiakov-Lewis model had the highest simulation accuracy,which was closer to the measured value.?2?The application of earthworm casts has significant effects on soil infiltration and agglomerate composition.This study was carried out in the test area of the idle farmland of the loess area applied and which were applied in two application methods:?A?earthworm casts mixed with the soil of the?0-5 cm?surface layer and?B?mulched 5 cm of soil layer after broadcast application of earthworm casts.Different levels of earthworm casts were designed to study on sediment yield and nitrate nitrogen loss with runoff under rainfall conditions.The results showed that:i.Both application methods effectively delayed the time of runoff occurring.The time of runoff occurring under layer application of 800 g/m² was delayed by 3.88 min compared with CK,and the delay effect was most obvious.ii.The layer application of 800 g/m² could significantly reduce the rate of runoff increase in the early stage and delay the time of entering the stable stage.The variation of cumulative runoff with time could be well described by a power function.iii.Under the mixed application and layer application at 800 g/m²,the sediment yield in the stable phase was Significantly reduced by 79.61%and 86.74%than CK.the cumulative sediment amount could be well described by the power function with the time,and the initial sediment yield was significantly reduced with the increase of the amount of earthworm casts.iv.Mixed application could significantly reduce the nitrate concentration in runoff.When the amount of application was large and the application method was layer application,it could play a good role to prevent the sediment and chemical solute loss with runoff.?3?The layer application method effectively reduced the nitrate concentration in runoff,and the equivalent mixing depth model can well simulate the nitrate loss process in runoff.In the process of nitrate-N loss with runoff,the layer application can significantly reduce the nitrate concentration in runoff,and the greater the application amount is,the more obvious the effect is.Under the two application methods,the soil profile nitrate nitrogen distribution was in the form of single peak,and the peak position appeared in the range of 5—10 cm.The equivalent mixing model can well simulate the loss process of nitrate,and the‘depth of mixing layer'in the models of complete mixing,incomplete mixing and equivalent mixing decreased with the increase of the applied amount.?4?Under natural rainfall,the surface runoff erosion of slope farmland tends to remove large quantities of soil particles;such erosion constitutes nonpoint source pollution.The existing sediment and nutrient loss models focus on estimating the total amount of pollutants in the long term.There are few studies on the process of nutrient loss in individual rainfall events,and there are still some shortcomings in the existing mathematical models that describe the nutrient loss process on slopes have not taken into account the effect of infiltration on nutrient concentrations in the exchange layer before the time of runoff starts.In this study,according to the characteristics of nutrient migration in different time periods of a rainfall event,the entire process of a rainfall event is divided into three phases:i.from the beginning of rainfall?t₀?to the time when the exchange layer is completely saturated(t_sa),ii.from the time when the exchange layer is completely saturated(t_sa)to the time when runoff occurs?t_p?,and iii.from first runoff?t_p?to the end of the rainfall.Based on the approximate analytical solution of the motion wave model,an approximate semi-analytical model of sediment yield and nutrient loss based on surface runoff processes was established,and the artificial simulated rainfall experiments were performed to calibrate model parameters and verify model reliability.This model had incorporated raindrop driven process,diffusion and water infiltration effect on nutrient transfer in the exchange layer.In this model,nutrients in the exchange layer were driven to migrate to the deeper soil layer by infiltration and hydrodynamic dispersion before the runoff occurs.Regarding the loss of nutrient in surface runoff,raindrop splash played a leading role in nutrient transfer into runoff from the exchange layer.The results indicated that the simulated calculation values of runoff,sediment and nutrient could be accurately matched with the measured values?R²>0.8,Nash–Sutcliffe efficiency>0.347?.In this nutrient loss model,the raindrop-induced water transfer rate e_r was valued in the range of 0.006⁰.023 cm·min^-1,and the exchange layer depth d_e in the range of0.68—1.32 cm.The raindrop-induced water transfer rate e_r in the nutrient loss simulation could dramatically affect the peaks of nutrient loss rates,whereas the depth of the exchanged layer could conspicuously affect the range of the overall changes of nitrate nutrient loss rates.Some field management measures,such as revegetation,deep fertilization,etc.should be adopted to reduce raindrop kinetic energy and lower the content of nutrients in the exchange layer,so as to realize the purpose of reducing nutrient loss.In general,this approximate semi-analytical model has fully considered the influence of water infiltration in unsaturated soil on nutrient transport in the exchange layer,so it can be used to predict nutrient loss processes on bare slopes in arid and semi-arid regions.However,it should be emphasized that accurate calculation of runoff process is the basis of the nutrient loss simulation,so it is advisable to select an appropriate infiltration formula and solute adsorption coefficients in the light of soil texture,nutrient type,and nutrient concentration in rainwater.