| Vegetation restoration is the main measure for the restoration and reconstruction of the ecosystem on the Loess Plateau.It improves soil fertility and soil structure,and reduces soil erosion by changing unreasonable land use methods and increasing vegetation coverage.The composition and stability of soil aggregates are the main indexes to evaluate soil structure,quality and anti-erodibility.Soil internal forces,including electrostatic repulsive force,van der Waals attractive force and hydration repulsive force,play an important role in controlling the stability of aggregates.Vegetation restoration can improve the stability of soil aggregates by inputting a large amount of organic matter into the soil,however,the problem of why the increase of organic matter(SOM)can improve the stability of soil aggregates is still in the qualitative discussion stage.To explore the internal forces mechanism of improving the stability of soil aggregates in the process of vegetation restoration,the Ziwuling forest region of the Loess Plateau was selected as the research area,and the soils under typical vegetation succession stages(farmland,grassland,shrubland and forest in sequence)were selected as the research objects to study the evolution of surface electrochemical properties of soil particles(including surface charge number SCN,specific surface area S,surface charge density?0,electric field intensity E0,surface potential?0)in different succession stages.The interaction forces among soil particles(including electrostatic repulsive force,van der Waals attractive force and hydration repulsive force)at different succession stages were calculated quantitatively,the variation of aggregate stability at different successional stages was clarified,the quantitative relationship between the interaction between soil particles and aggregate stability at the mesoscopic scale was established,and the internal forces mechanism of soil aggregate stability at the mesoscopic scale was explicated.These results provide a theoretical basis for the cultivation of good soil structures and the establishment of new internal control technology for soil erosion control on the Loess Plateau.The main results are as follows:(1)The effects of vegetation restoration on soil physical and chemical properties were clarified.The results showed that with the progress of vegetation restoration,the bulk density of topsoil(0-20 cm)decreased in the range of 1.04-1.27 g cm-3,with an average of1.17 g cm-3;the soil water content increased in the range of 13.72%-19.85%,with an average of 16.70%;the soil particle composition and clay mineral composition changed little;the soil calcium carbonate content decreased in the range of 80.72-115.66 g cm-3,with an average of 113.10 g cm-3.The content of soil organic carbon(SOC)increased in the range of 7.20-16.93 g kg-1,with an average of 12.04 g kg-1.The content of SOC in grassland,shrubland,early forest and climax forest was 1.27,1.77,1.97 and 2.35 times of farmland soil,respectively;the content of total nitrogen(TN)increased in the range of0.96-1.56 g kg-1,with an average of 1.24 g kg-1;the content of total phosphorus(TP)increased in the range of 0.64-0.75 g kg-1,with an average of 0.69 g kg-1.In the 1 m vertical soil profile,the contents of soil SOC and TN decreased with the increase of soil profile depth in different succession stages.Due to the influence of soil parent material,the variation of soil TP content in different succession stages was small and relatively stable.The variability of soil C/N in different succession stages was small and stable between10-12.Soil C/P and N/P increased with vegetation restoration and decreased with the increase of soil profile depth.(2)The evolution of the surface electrochemical properties of soil particles during vegetation restoration was revealed.The surface charge number(SCN)of soil particles varied from 10.88 to 19.85 cmol kg-1,with an average of 16.18 cmol kg-1,and SCN value increased with vegetation restoration;the specific surface area(S)of soil particles varied from 40.67 to 61.71 m2 g-1,with an average of 54.88 m2 g-1.The surface charge density(?0)of soil particles varies from 0.22 to 0.31 c m-2,with an average value of 0.28 c m-2.?0 value increases with the vegetation restoration,and reaches the maximum at the climax forest stage.The surface electric field intensity(E0)of soil particles reaches the order of 108 V m-1.The increase of soil organic matter(SOM)content is a main factor affecting the electrochemical properties of soil surface.The amount of soil surface charge number,specific surface area,surface charge density and surface electric field intensity are significantly positively correlated with the content of SOM.(3)The effects of soil internal forces on the particle size distribution and stability of aggregates during vegetation restoration were qualitatively evaluated.The particle size distribution and stability changes of dry aggregates in different succession stages were measured by dry sieving method.Deionized water and ethanol fast wet aggregates to simulate the changes in the particle size distribution and stability of the aggregates with and without soil internal forces,respectively.The results showed that the proportion of 5-1 mm large aggregates increased and the proportion of<0.15 mm small aggregates decreased in the process of vegetation restoration.The physical stability and water stability of aggregates increased with the process of vegetation restoration.The particle size distribution of aggregates measured by ethanol fast wetting treatment was similar to that measured by dry sieving method,which showed a trend of first decreasing and then increasing with the decrease of particle size,mainly 5-1 mm and<0.15 mm aggregates,while the particle size distribution of aggregates measured by deionized water fast wetting treatment was mainly<0.15 mm aggregates.The mean weight diameter of soil aggregates in farmland,grassland,shrubland,early forest and climax forest stages measured by ethanol fast wetting treatment(MWDe)was 4.62,1.45,1.31,1.32 and 1.17 times higher than that measured by deionized water fast wetting treatment(MWDw),which indicated that the soil internal force produced in the process of deionized water fast wetting made the aggregates break violently and the stability of aggregates decreased significantly.In addition,during the fast wetting process,soil internal forces mainly destroyed>0.5mm aggregates.The relative mean weight diameter index(RMWD)of soil decreased significantly with the increase of SOM content in the vegetation restoration process,indicating that the increase of SOM content in the soil reduced the fragmentation degree of soil internal forces on aggregates,and the stability of aggregates increased.(4)The magnitude and distribution of internal forces between soil particles in the process of vegetation restoration were quantified,the quantitative relationship between soil internal forces and the stability of aggregates was established,and the mechanism of internal forces in improving the stability of aggregates was revealed.The results showed that the electrostatic repulsive force between soil particles increased with the vegetation restoration and the increase of SOM content,and the electrostatic repulsive force in different succession stages was farmland<grassland<shrubland<forest;the Hamaker constant in different succession stages increased with the increase of organic matter content,and the Hamaker constants of farmland,grassland,shrubland and forest were 4.54×10-20,6.64×10-20,7.75×10-20 and 9.66×10-20 J,respectively,indicating that with the process of vegetation restoration and the increase of organic matter content,the van der Waals attractive force among soil particles increased significantly.The increase of van der Waals attractive force between soil particles is larger than electrostatic repulsive force,which indicates that the stability of soil aggregates will increase with the vegetation restoration and the increase of soil organic matter content.The results of soil aggregate stability test showed that the aggregate stability increased with the vegetation restoration and the increase of SOM content.The variation of soil aggregate stability with electrolyte concentration and organic matter content was consistent with the theoretical prediction,and there was a good exponential relationship between aggregate breaking strength and net forces(P<0.01),which indicated that soil internal force had an important influence on soil aggregate stability in the process of vegetation restoration.In conclusion,the increase of soil organic matter content in the process of vegetation restoration significantly enhanced the van der Waals attractive force between soil particles,thus improving the stability of soil aggregates.(5)The internal forces mechanism of improving soil anti-splash separation ability in the process of vegetation restoration was clarified.The results showed that the proportion of large aggregates increased,the proportion of small aggregates decreased and the stability of aggregates increased with the process of vegetation restoration and the increase of SOM content.At the same time,the increase of SOM content decreased the net forces between soil particles.Therefore,the improvement of aggregate stability in the process of vegetation restoration is caused by the decrease of net forces between soil particles due to the increase of SOM content.The experimental results verify the internal forces mechanism of the increase of aggregate stability.In addition,the soil splash erosion rate decreased with vegetation restoration,and the change of splash erosion rate with electrolyte concentration was consistent with the net forces between soil particles.There was a good exponential relationship between splash erosion rate and net forces(P<0.01),indicating that net forces had an important impact on splash erosion.There was a significant negative correlation between splash erosion rate and aggregate stability(P<0.01),a negative correlation between splash erosion rate and>0.25 mm aggregate content,and a positive correlation between splash erosion rate and<0.25 mm aggregate content.The above results showed that the increase of SOM content in the process of vegetation restoration reduced the net forces between soil particles,reduced the fragmentation degree of aggregates,increased the content of large size aggregates,decreased the content of small size aggregates,increased the stability of aggregates,and improved the anti-splash separation ability of soil.Based on the above results,we draw the following conclusions:with the vegetation restoration,soil organic matter content increased significantly,soil surface charge number,specific surface area,surface charge density and Hamaker constant increased,resulting in the change of soil internal forces,especially the van der Waals attractive force between soil particles increased significantly,thus improving the stability of aggregates and the ability of anti-splash separation.The results quantitatively explained the internal forces mechanism of soil aggregate stability improvement in the process of vegetation restoration from the perspective of soil electrochemistry,and provided a theoretical basis for the cultivation of good soil structure and the establishment of new internal control technology for soil erosion control on the Loess Plateau. |
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