Effects Of Microsites On Soil Erosion And Fractal Features In Two Forest Types Of Gaps

In the area of Picea koraiensis-Abies nephrolepis-Pinus koraiensis forest(PA)and Tilia amurensis-Pinus koraiensis forest(TP)in Xiao Xing'an Mountains,the disturbance of wind can cause trees to fall and trees to be uprooted,forming treefall gaps and mound and pit microsites(pit bottom,mound top).Due to biological disturbances and soil migration,uprooting may cause soil erosion,affect soil structure changes and soil formation,change soil physical and chemical properties,and have a serious impact on the spatial variability of surrounding soil.Therefore,effects of microsite types and soil properties on soil fractal features and anti-erosion ability of in different forest types of treefall gaps were analyzed,it was of great significance for preventing soil erosion,stabilizing soil structure and improving the productivity of forest ecosystems.In this study,the two forest types of Picea koraiensis-Abies nephrolepis-Pinus koraiensis forest and Tilia amurensis-Pinus koraiensis forest as the study object of the relatively consistent topography conditions(slope direction,slope,and altitude).In May 2018,each forest type is set with a 170m x 170m rectangular sample plot,select 30 typical mound and pit microsites,and set the surrounding gap and closed stand as controls.To investigate the base diameter and length of the treefall on mound and pit microsites,and to determine the composition of soil micro-aggregates,soil properties and soil fractal characteristics in the pit bottom,mound top,gap and closed stand.Soil anti-scouribility and anti-erodibility were determined by undisturbed soil sample washing experiment method and still water disintegration method.In different fprest types of treefall gaps and microsites,the relationship between soil micro-aggregate composition and fractal characteristics and soil properties was analyzed,and the differences between soil properties and soil anti-scouribility and anti-erodibility were compared.The results are as follows:(1)The content of soil microaggregates 0.05-0.25mm were higher in the two forest types,ranging from 27.00%to 42.77%.The content of soil microaggregates<0.002mm was the lowest,ranging from 4.44%to 8.89%.Soil bulk density was higher in the pit bottom and mound top of the two forest types,and the soil nutrient content in mound top was higher than that in pit bottom.In the pit bottom and mound top of gaps,the soil nutrient content in the TP was higher than that in the PA.Soil particle content at the size of<0.002mm had no correlation with soil properties.Soil particle content at the size of<0.002mm had no correlation with soil properties,but at the size of 0.0002-0.02mm had significantly negative correlation with soil non-capillary porosity,total porosity,aeration porosity,organic matter,total phosphorus,total nitrogen and organic carbon,respectively.(2)On the whole,the soil fractal dimension(D)and the proportion of characteristic soil micro-aggregates(PCM)in the TP were larger than those in the PA.In two forest types,the ratio of soil micro-aggregates diameter(RMD)in mound top and pit bottom was larger than that in the controls.Soil D and PCM had no significant correlation with soil properties.Soil RMD was negatively correlated with capillary porosity and total porosity,and significantly negatively correlated with soil bulk density and aeration porosity.In two forest types,the formation of mound and pit microsites can lead to the decreases in the larger size micro-aggregates in the soil,the stability of soil micro-aggregate,increases in soil D and PCM,and significantly increase in soil RMD.Soil RMD can be used as a quantitative index to describe the physical and chemical properties of soil in pit and mound microsites of forest.(3)In pit and mound microsites of two forest types,the soil sand content is higher and anti-erosion is weaker,but the control group is opposite;There was no significant negative correlation between soil bulk density and soil anti-scouribility and anti-erodibility(P>0.05),there was no significant positive correlation between soil bulk density and them in the PA(P>0.05),while there was significantly positive correlation between soil bulk and them in the TP(P<0.01),between the soil bulk density and them was negatively correlated in the stand;There was no significant positive correlation between soil organic matter and soil anti-scouribility and anti-erodibility,but there was an extremely significant positive correlation in the control group;Soil anti-erosion ability varies with forest and microsite types,soil anti-erosion ability of TP pit and mound microsites was not significantly higher than that of PA.Soil anti-erosion ability of PA control groups is significantly higher than that of TP.It can be concluded that the formation of pit and mound microsites cause a significant decline in soil erosion resistance,resulting in the loss of soil nutrients,the reduction of soil clay content,the process of soil restoration is difficult.