Effects Of Straw Returning On Soil Microaggregates Characteristics And Zeta Potential

Straw returning as the main way to cultivate soil fertility in cultivated land can not only improve soil fertility,but also improve soil structure.In this paper,the in-situ simulation experiment of straw returning was carried out by using nylon mesh bag method to qualify the composition and stability of soil microaggregates,the organic carbon content and humic substance composition of different size microaggregates with straw returning for two years at different depths.In the experiment,four levels of R0（0%）,R1（0.44%）,R2（0.88%）,R3（1.32%）and three different returning depths of0-15cm,15-30cm,30-45cm were carried out.The change of the zeta potential of the colloid affects the cohesion and dispersion between colloidal particles,thus affecting the formation of the good soil structure.In order to determine the stability of soil organic-mineral complex,the change of the variable charge on the soil surface with pH was studied.The clay fraction extracted from soil with the particle size of<2μm will be the research object,through adding electrolyte solutions with different ion species(Na⁺,K⁺,NH₄⁺,Ca²⁺,Fe³⁺,Al³⁺)and different ion concentrations(10^-3,10^-4,10^-5M),the zeta potential change of soil colloidal particles with pH was also qualified.When the organic matter in the colloid is removed or the amount of straw added is1.32%,the difference in organic matter content of the colloid changes its zeta potential.（1）The microaggregates of 0.25-0.02 mm are the dominant size fraction,and the straw returning to the field increases the content of 0.25-0.02 mm and>0.25 mm aggregates of each soil layer while the content of 0.02-0.002 and<0.002 mm microaggregates decrease,and the largest change in the three-particle content is in the S1 layer.The average weight diameter（MWD）and geometric mean diameter（GWD）of microaggregates of different size fractions in different soil layers were increased compared with no straw returning,but the significant difference analysis showed that the increase was not significant.At the same time,with different amounts of straw incorporation the organic carbon content of microaggregates of different size increased in different soil layers,and the contents of humic acid,fulvic acid and humin show the similar trend.（2）In different soil layers,the amount of variable charge soil raises with pH,which explains the reason why the absolute value of zeta potential in this paper increase with pH.Compared with monovalent ions（Na⁺,K⁺,NH₄⁺）,divalent(Ca²⁺)or trivalent ions(Fe³⁺,Al³⁺)are more likely to enter the electric double layer,leading the thickness of the electric double layer reduced and the absolute value of zeta potential of colloidal particles decreases,promoting cohesion between the colloidal particles and formation of good structure.Excessive application of chemical fertilizer in agricultural land would make the divalent or trivalent ions be replaced by these monovalent ions,which colloidal particles tend to disperse and soil structure is destroyed.Besides,for different concentrations of NaCl solution,when the electrolyte concentration is higher,the absolute value of the zeta potential shows an increases trend and the colloidal particles tend to aggregate.Since the organic matter carries a large amount of variable negative charge,the removal of organic matter makes the absolute value of the electrokinetic potential of the colloid significantly lower.Compared with no straw incorporation,the absolute value of the zeta potential decreased after 1.32%straw was returned to soil,that is,the thickness of the diffusion layer of colloidal particles is reduced,leading to cohesion between colloidal particles,which are conducive to the formation of soil structure.