Effect Mechanism Of Wheat Straw Incorporation On Soil Erosion Resistance

Straw incorporation is of great importance to straw resource utilization,which has ecological,economic and social benefits,e.g.,improving soil structure and fertility,promoting nutrient cycling,increasing crop yield and avoiding environmental pollution risk caused by straw incineration.Straw incorporation into topsoil and its decomposition and humification could change soil erosion resistance（soil erodibility K_d and soil critical shear stressτ_c）,and then soil erosion processes and modelling.However,the effects of wheat straw incorporation regimes,i.e.,straw length and straw returning amount,incorporated with pure nitrogen application and soil amendment on soil erosion resistance are not clear.Taking a silt loam soil from the earth-rocky mountain areas of northern China as an object,this study used the orthogonal table of L₉（3⁴）to arrange four influencing factors at three gradients(i.e.,straw length of 2,5 and 8 cm,straw returning amount of 2 000,4 000 and 8 000 kg hm^-2,pure nitrogen application amount of 100,140 and 180 kg hm^-2,and pure polyphenol application amount of 0,40 and 80 kg hm^-2).A total of 10 treatments including blank control（CK）with wheat straw incorporation combined with nitrogen and polyphenol.In addition to nitrogen fertilizers and soil amendment,factors such as wheat straw quality and external environment also affect straw decomposition and soil properties.Therefore,it is necessary to study the response law of soil physicochemical properties and soil erosion resistance after wheat straw incorporation without nitrogen fertilizer and polyphenols.Wheat straw incorporation experiment with CK was conducted in 5 treatments,including different straw length（2,5 cm）and straw returning amount(4000,8000 kg hm^-2).Soil erodibility and critical shear stress were measured by the submerged jet equipment,and the soil physicochemical properties were determined.The effects of wheat straw incorporation on soil physicochemical properties,soil erosion resistance and its temporal variation were studied,and the quantitative relationship between soil physicochemical properties and soil erosion resistance was established.The main conclusions are as follows:（1）With the time prolonged after straw incorporation combined with nitrogen and polyphenol,the soil bulk density increased by 2.5%,soil total porosity decreased by 2.4%,respectively,and the number of soil macroaggregates（>0.25 mm）decreased but water stability increased.Compared with 60^th day after straw incorporation,straw decomposition rate increased by 13.7%after 1 yr.and 60^th day,but soil organic matter decreased from 6.57 g kg^-1 to 5.73 g kg^-1.Nitrogen fertilizer and polyphenols amount had significant effects on soil organic matter（P<0.05）.With the increase of nitrogen fertilizer and polyphenol,soil organic matter decreased first and then increased.Humic acid and humin increased by 14.0%and 24.3%,respectively,and fulvic acid decreased by 51.4%.Straw length,straw returning amount,nitrogen fertilizer amount and polyphenol amount had significant effects on fulvic acid,and straw returning amount had the highest contribution rate of 39.5%.（2）Soil erodibility（K_d）and soil critical shear stress（τ_c）showed significant temporal variability after straw incorporation.With the extension of straw incorporation time,K_d decreased from 123.60 cm³ N^-1 s^-1 to 53.06 cm³ N^-1 s^-1,τ_cdecreased from 2.23 Pa to 1.67 Pa,K_d andτ_c decreased by 57.1%and 25.1%,respectively.After 60^th day of straw incorporation,K_d andτ_c of each returning treatment decreased compared with CK.After 1 yr.and 60^th day,straw length,straw returning amount and nitrogen fertilizer amount had significant effects on K_d.Straw returning amount and polyphenol amount had significant effects onτ_c.Straw returning amount is the key factor affecting soil erosion resistance,and its contribution rate to K_d andτ_c is the highest with the value of 42.1%and 50.9%,respectively.With the increase of straw returning amount,K_d first increased and then decreased,andτ_cincreased.Under the condition of straw incorporation combined with nitrogen and polyphenol application,K_d had a good linear relationship with soil capillary porosity and soil humic acid（R²=0.81）,and theτ_c could be simulated by the aggregate greater than 0.25 mm and soil capillary water content（R²=0.90）.（3）After wheat straw incorporation,the soil aeration porosity,water-stable aggregate greater than 0.25 mm and hydrolytic nitrogen of each treatment increased compared with CK,and soil total phosphorus decreased.After straw incorporation,the change of soil properties led to the change of soil erosion resistance.Compared with CK,the mean of soil erodibility（K_d）and soil critical shear stress（τ_c）decreased by 16.9%and 38.2%,respectively.When the flow shear stress was greater than 5.62Pa,straw incorporation increased soil erosion resistance compared with CK.After wheat straw incorporation,water-stable aggregate greater than 0.25mm and soil total porosity could better simulate the dynamic change of K_d with the R² of 0.98.