Effect And Mechanism Of Different Organic Amendments On Soil Aggregates And Splash Erosion

The degradation of soil structure and function is an important manifestation of ecosystem degradation,soil organic amendments application is an effective way to improve soil structure and function.Aggregate is the basic unit of soil structure,which is critical to the regulation and maintenance of soil function.The frequently extreme rainfall events caused by global climate change have caused serious soil hydraulic erosion.Aggregate breaking is the main reason for soil suffer from hydraulic erosion,and the breaking strength directly determines soil erosion strength.Soil aggregate stability is an index to assesses the ability of soil to maintain its original state when resisting environmental changes.Therefore,it is of great scientific significance to clarify the mechanism of the effects of organic amendments application on soil aggregates stability and anti-erosion ability for quantifying the effect of organic amendments on improving soil function.Soil particle interaction forces have been proved to be the fundamental reason for soil aggregates breakdown in water.The application of organic amendments changes the soil physical and chemical properties,which will inevitably cause the changes of soil particle interaction forces,and further affect the soil aggregate stability and anti-erosion ability.The application effects of organic amendments with different types and application rates exist large differences.However,based on soil particle interaction forces,the difference and mechanism of the effects of different organic amendments application on soil aggregate stability and anti-erosion ability remain unclear.In this study,three organic amendments,biochar,weathered coal,and grass peat,were selected as the experimental material.Lou soil was taken as the research object.First,the three organic amendments were individually applied into soil at 0%,1%,and 3%（w/w）rates and through a field positioning experiment.Then,through indoor simulation and analysis,explored the effects and differences of different organic amendments on soil basic properties and particle surface chemical properties,revealed the differences and internal forces mechanism of the effects of three organic amendments application on soil aggregate stability and anti-erosion ability.The study provides the theoretical basis for the selection and application of different types organic amendments in soil quality improvement practices.The main results of this study are showed as follows.（1）The basic physicochemical properties of different amended soils were significantly different after 4 years of amendment application.Compared with unamended soils,1%\2%\3%weathered coal and grass peat decreased the p H by 0.24\0.46\0.48 and 0.04\0.11\0.12,respectively,the 1%\2%biochar decreased the soil p H by 0.09\0.13 and the 3%biochar had no significant effect on soil p H.The p H of the soil amended by weathered coal was significantly lower than that of the soil amended by grass peat and biochar.Compared with unamended soils,the 1%\2%\3%biochar,weathered coal,and grass peat increased soil total organic carbon content by 61.3%\124.8%\175.2%,45.6%\85.5%\145.6%,and14.7%\25.8%\37.5%,respectively,the differences among the three amended soils were significant under the same application rate（P<0.05）.Whereas,the 1%\2%\3%biochar,weathered coal,and grass peat increased soil C/N by 3.83\7.13\8.83,4.86\9.46\11.80,and1.10\1.19\2.0,respectively.The 1%\2%\3%biochar and weathered coal increased soil mineral bonded state organic carbon content（MOC）by 11.4%\23.2%\38.7%and 15.9%\26.6%\55.9%,respectively,the grass peat had no significant effect on MOC.Both of soil C/N and MOC of weathered coal amended soils significantly higher than that of biochar and grass peat amended soils with the increase of application rates.（2）All three organic amendments application significantly decreased the surface potential（absolute value）,charge density,and electric field intensity,increased the specific surface area and surface charge number,the differences among the application effect of three amendments were mainly reflected in the surface potential,specific surface area and surface charge number.Specifically,under the 1%\2%\3%application rate,soil particle surface potential（absolute value）（m V）followed the order of grass peat treatments（136.5\137.2\137.6）>biochar treatments（136.4\135.9\130.6）>weathered coal treatments（133.2\131.8\129.2）,both specific surface area(m² g^-1)and surface charge number(cmol kg^-1)followed the order of weathered coal treatments（59.7\70.3\78.1 and 26.2\28.9\30.4,respectively）>biochar treatments（56.8\62.3\72.0 and 25.2\25.7\27.5,respectively）>grass peat treatments（56.6\60.0\64.0 and25.3\26.0\26.7,respectively）.The differences among the three treatments were significant under the same application rate（P<0.05）.The charge density and electric field intensity were mainly related to total organic carbon content,while surface potential,specific surface area,and charge number were more related to C/N than total organic carbon content.Further analysis showed that the soil mineral bonded state organic carbon content was the main factor for the variation of particle surface potential,specific surface area,and charge number,the explanation rate and contribution rate were 53.3%and 85.7%,respectively.（3）The effects of soil particle interaction forces on the aggregate stability of bulk soil after adding three organic amendments increased with the increase of the application rates.The soil aggregates stability was obviously reduced by the changes of soil particle interaction forces when a dry aggregate was immersed in water environment,the mean weight diameter of aggregates after dry sieving was 1.47-1.62 mm,the mean weight diameter of aggregates soaked in alcohol（MWD_a）was 1.47-1.62 mm,while the mean weight diameter of aggregates soaked in pure water（MWD_w）is 0.30-0.50 mm,the MWD_a was 3.24～4.90 times of MWD_w.The three amendments application significantly reduced the break degree(R_MWD)of soil aggregate structure caused by soil particle interaction force change.Reducing the R_MWD was beneficial to improve the soil aggregates stability.（4）The differences of net force of soil particles among different amended soil leads to the differences of aggregates breaking strength.The electrostatic repulsion pressure and the net force between soil particles increased sharply first(from 1 to 0.01 mol L^-1)and then remained unchanged(from 0.01 to 0.0001 mol L^-1)with the decrease of the soil electrolyte concentration.Under the same electrolyte concentration,three organic amendments applications could reduce the net force between soil particles by decreasing electrostatic repulsion pressure and increasing van der Waals attractive pressure.With the increase of application rate,the contribution of the van der Waals attractive pressure increase to the net force reduction was higher than 50%at the 2%and 3%rates,and it could reach 90%at the 3%rate.The van der Waals attractive pressure（absolute value）in weathered coal treatment was higher than that in biochar and grass peat treatment.There was an exponential correlation between aggregate breaking strength and net force（R²>0.88,P<0.001）.Three organic amendments applications could significantly reduce the aggregate breaking strength with the increase of application rates.Under the same application rate,the aggregate breaking strength of weathered coal treatments was lower than that of the biochar and grass peat treatments.（5）Soil particle interaction force is the main driving force for soil splash erosion when the soil electrolyte concentration was<0.1 mol L^-1.Reducing particle interaction force is beneficial to improving soil anti-erosion ability.Under the same rainfall kinetic energy,with the decrease of soil electrolyte concentration,the soil splash erosion rate increased gradually,meanwhile,the contribution rate of raindrop force decreased and soil particle interaction force increased to the soil splash erosion.When the electrolyte concentration was<0.1 mol L^-1,the contribution rate of soil particle interaction force was higher than 50%,and the particle interaction forces became the main driving force when occurs splash erosion.There was an exponential correlation between net force and soil total splash rate（R²>0.78,P<0.001）.Three organic amendments applications could effectively reduce the soil splash erosion rate and the contribution rate of internal force,and the effect increased with the application rates.Under the same application rate,the effects of weathered coal application on reducing soil splash erosion rate and the contribution rate of internal force were larger than that of biochar application,and the effects of the both were obviously larger than that of grass peat application.In this study,the three amendments improved the surface electrochemical properties of soil particles mainly by increasing soil organic carbon content and C/N,and then reduced the electrostatic repulsion pressure（positive）,increased the van der Waals attractive pressure（negative）,and decreased the net force（positive）among soil particles.The decrease of net force mainly depends on the increase of van der Waals attractive pressure.The reduction of net force was beneficial to the improvement of soil aggregates stability and anti-erosion ability.The difference in surface electrochemical properties of soil particles among different amended soils was mainly reflected in the surface potential,specific surface area and surface charge quantity,and the difference were mainly affected by the soil mineral bound organic carbon content.Under the same application rate,the mineral bound organic carbon content shows the order of weathered coal treatment>biochar treatment>grass peat treatment.Thus,weathered coal was more beneficial than biochar and grass peat to increase van der Waals attractive pressure,reduce net force,and finally improve the soil aggregates stability and anti-erosion ability.