Effects Of Litter Incorporation In Typical Grasslandon The Soil Detachment Process And Its Seasonal Variation In The Loess Hilly Area In China

Plant litter would be incorporated into the topsoil during the sediment transport,deposition and soil splash.The litter in the topsoil significantly affects the soil properties and then the soil detachment process.We collected the common plant litter and incorporated it into the topsoil.Then we carried out the scoring test under six hydraulics conditions(10o-1L s^-1,10o-2L s^-1,15o-2L s^-1,20o-2L s^-1,25o-2L s^-1,25o-2.5L s^-1)to study the litter incorporation effect on soil detachment process.Specifically,we studied the change of soil physical and chemical properties,soil detachment capacity,rill erodibility,critical shear stress under the litter incorporation effect,quantified the effect of the litter mass density,length,morphological characteristics on soil erosion resistance.We investigated the effects of incorporated litter on soil freezing-thawing process and soil detachment after the freezing-thawing.Furthermore,we also monitored the long-term variation of soil detachment process under the litter incorporation effects,and determined the relationship between soil properties,environmental factors and the long-term change of the soil erosion resistance.The study of soil detachment process under litter incorporation effect can deepen the understanding of soil erosion process in the loess hilly areas,clarify the effect of vegetation restoration on soil and water conservation,effectively improve the accuracy of soil erosion prediction,and provide theoretical support for the soil erosion control in the Loess Plateau.The main conclusions are as follows:?1?The litter incorporation mass density?LMD?greatly affected the soil detachment process.With the increase of LMD,litter incorporation effect showed obvious inhibition on soil detachment.The soil detachment capacity decreased exponentially with the increase of LMD.When the LMD increased from 0.1 to 1.3 kg m^-2,the soil detachment capacity decreased from 2.543 to 0.555 kg m^-2 s^-1,about 10%?80%reduction compared to the control plot?without litter?.Soil erosion resistance improved with the increase of LMD.The rill erodibility decreased exponentially with LMD.Compared with rill erodibility of the control soil(0.233 s m^-1),the rill erodibility decreased 6%,22%,41%,65%,and 72%at the LMD of0.1,0.4,0.7,1.0,and 1.3 kg m^-2,respectively.The rill erodibility varied greatly when the LMD was less than 1.0 kg m^-2.The critical shear stress increased linearly with LMD.It increased 60%,131%,139%,203%,and 212%when the LMD was 0.1,0.4,0.7,1.0,and 1.3kg m^-2.?2?The incorporated litter length had large effect on sol detachment process.Under the condition of the same LMD(0.7 kg m^-2),the soil erosion resistance was weakened with the increase of incorporated litter length.The soil detachment capacity increased with the litter length,when litter length increased from<0.5 cm to 8 cm,the soil detachment capacity increased from 0.780 to 1.907 kg m^-2 s^-1.When the length of litter was more than 3 cm,the soil detachment capacity was no longer increased significantly with the increase of litter length.In addition,the rill erodibility increased as a power function with the length of litter in the soil,and the critical shear stress decreased,but there was no significant quantitative relationship between critical shear stress and litter length.Further,the morphological characteristic parameters of litter in soil?such as litter length density,surface area density and volume ratio?were quantified,and the correlation analysis was carried out with soil detachment capacity and soil erosion resistance parameters.The results showed that the morphological characteristics of litter in soil significantly affected the soil detachment process The morphological parameters of litter in soil vary significantly with litter type and length.In common grassland slopes in loess hilly area,the length density of litter in the soil is between 2.34?118.25 km m^-3,the surface area density is between 27.9?674.2 m² m^-3,and the volume ratio of litter to soil is between 0.002?0.065 m³ m^-3.The morphological characteristics of litter can be used to characterize the influence of litter incorporation effect on the process of soil detachment.Litter surface area density was the best litter morphological characteristic parameter to quantify soil detachment capacity.With the increase of litter surface area density,the soil detachment capacity decreased in a flat-steep-flat curve.When the litter surface area density was in the range of 200?400 m² m^-3,the soil detachment capacity changed significantly.Moreover,in the loess hilly region with litter in the soil,soil detachment capacity by overland flow can be predicted by flow shear stress andThe rill erodibility decreased with the increase of litter surface area density.By quantifying the relation between relative rill erodibility and litter surface area density,it can be used to=0.79,R²=0.79,p<0.01).The critical shear stress increased with the increase of litter surface area density,but no significant quantitative relation was detected,which still needs further study.?3?Litter incorporation can affect the soil freezing-thawing process,and thus affect the soil detachment process.The effect of soil freezing-thawing on soil properties is mainly related to the initial soil moisture content during freezing-thawing.Incorporated litter can inhibit the influence of freezing and thawing process on soil structure.Soil detachment capacity and rill erodibility increased with the increase of soil moisture content during freezing-thawing,while critical shear stress decreased.When the initial soil moisture content was 5%,10%and 20%during freezing-thawing,the soil detachment capacity increased by70%,300%and 870%,rill erodibility increased by 80%,250%and 590%,and the critical shear force decreased by 4%,28%and 62%,compared with the unfreezing-thawing soil.The influence of freezing-thawing on soil detachment capacity and rill erodibility could be reduced by the incorporated litter.Moreover,soil detachment capacity,rill erodibility and critical shear stress can be expressed as function of soil cohesion.The soil detachment capacity decreased linearly with the increase of cohesion,the rill erodibility decreased exponentially with the increase of cohesion,and the critical shear force increased linearly with the increase of cohesion.?4?Litter incorporation has a long-lasting effect on soil detachment process.After the litter incorporation,soil properties fluctuate with time,of which,soil bulk density increased rapidly in the first rainy season,then stabilized,and then decreased;soil organic matter and water stable aggregates decreased in the rainy season,and their variation trends were similar;soil cohesion and crust thickness increased rapidly during the first rainy season and then remained stable;the litter content in soil continued to decrease,and the decomposition rate of Bothriochloa ischaemum?L.?Keng.in the first rainy season was significantly higher than that of Artemisia sacrorum Ledeb.The soil detachment process also showed obvious changes with the change of seasons.The soil detachment capacity and rill erodibility generally showed a trend of stable after rapid decrease with time,and the growth rate of critical shear stress slowed down after rapid increase.The range of soil detachment capacity varied from 0.115 to 6.876 kg m^-2 s^-1,rill erodibility varied from 0.015 to 0.516 s m^-1,and critical shear stress varied from 0.48 to 7.98 Pa.The litter incorporation effect reduced the soil detachment capacity by 14%to 30%,reduced the rill erodibility by 13%to 30%,and increased the critical shear stress by 21%to 24%.Litter incorporation had both short-term and long-term effects on the process of soil detachment.In the short term,soil erosion resistance was mainly enhanced by binding and shading of soil particles,while in the long term,litter improved the physical and chemical properties of soil through decomposition,finally inhibiting the process of soil detachment.The change of soil erosion resistance was closely related to the change of environmental factors and soil properties.The soil consolidation time,wet and dry alternating times,precipitation,rainfall energy,physical crust thickness,and soil cohesion were all significant factors affecting rill erodibility and critical shear stress.Furthermore,using regression analysis,the soil rill erodibility and critical shear stress can be expressed as nonlinear functions of soil crust thickness andincorporation on soil detachment process can also be expressed through the development of soil physical crust structure.The physical crust structure development of soil treatment with litter was better than that of control treatment.Furthermore,in the seasonal variation of soil detachment process,freezing and thawing,plant and animal activities were all potential factors affecting the process of soil detachment.