Reserach On The Stability Of Soil Aggregates And Carbon Pool Characteristics Of Ecologically Restored Slopes

At present,the development of domestic slope ecological restoration technology has shown a blossoming situation,but the slope ecological restoration work is mostly focused on the reconstruction of slope surface vegetation,while not enough attention is paid to the soil habitat conditions such as structural stability,fertility and carbon pool characteristics in the later stage of slope ecological restoration,which is not conducive to the effectiveness and longevity of slope ecological restoration work and the transformation of slope ecological restoration concept to carbon sink function under the background of double carbon.In view of this,this study selects typical ecological restoration techniques for slope restoration,mainly including vegetated concrete ecological protection technology（CBS）,vegetated cement soil habitat construction technology（VCS）,imitation vegetated concrete vegetation restoration technology（CCBS）,high sub-agglomerate vegetation restoration technology（CS）,and guest soil spraying technology（TBS）,and different restoration years（1a、3a、5a、18a）of CBS.The response of soil aggregate composition and stability,organic carbon content and oxidizable organic carbon fraction,carbon pool management index and carbon sequestration potential to the ecological restoration of slopes was investigated.While evaluating the ecological restoration effect of slopes,the linkage between the stability of aggregates and organic carbon pools was analyzed to provide basic data for the in-depth study of soil quality and carbon pool changes after ecological restoration of slopes,and to provide theoretical support for the transition of the concept of ecological restoration of slopes to carbon sink function in China.The main findings obtained are as follows.（1）CBS,VCS-CBS,CCBS,CS,TBS,NR ecological restoration slope air-dry soil agglomerates are mainly>5mm particle size,with the least content of 0.5～0.25 mm particle size,and overall show a“V”shaped distribution of high left and low right with decreasing particle size.Compared with the bare slope,the content of large aggregate R0.25 increased from19.79%to 30.59%,indicating that ecological restoration could significantly promote the formation of large aggregate in slope soil.Under the three crushing mechanisms of LB wet sieving,the distribution of soil aggregates in typical ecological restoration slopes varied greatly.The range of R0.25_FW,R0.25_WS,and R0.25_SW were 28.24%～65.37%,56.11%～83.44%and75.58%～94.7%respectively.The overall performance was that R0.25_SW>R0.25_WS>R0.25_FW.Further analysis of the cluster stability indexes showed that the cluster destruction rate PAD₅and PAD_0.25 under the three fragmentation mechanisms were FW>WS>SW,and the average mass diameter MWD values showed D>FW>WS>SW,and the relative dissipation index RSI>relative mechanical fragmentation index RMI.The main mechanism of fragmentation of soil agglomerates on each ecological restoration slope is the dissipative effect of rainstorm conditions,next is the mechanical destructive force generated by raindrop strikes or runoff shear,and the least destructive force on agglomerates on ecological restoration slopes is the non-uniform swelling of clay particles under slow wetting.And the soil aggregates on each ecological restoration slopes were more sensitive to the dissipation effect.In addition,the water stability of CBS and VCS-CBS ecological restoration slope aggregates was better than other ecological restoration slopes.With the increase of restoration years,the trend of change in agglomerate content of wind-dried soil with different grain sizes on CBS slopes was different,but there was no significant difference in R0.25 content between restoration years（p<0.05）.LB under the three fragmentation mechanism treatments PAD₅ and PAD_0.25 both showed FW>WS>SW,MWD all showed D>SW>WS>FW,and the main mechanism of agglomerate fragmentation did not change with age.Meanwhile,RSI gradually increased several times more than RMI with the increase of restoration years,indicating that the sensitivity of soil agglomerates on vegetated concrete ecological restoration slopes to dissipation increased year by year.（2）The variation of whole-soil organic carbon of slopes restored by different ecological restoration techniques ranged from 7.07 g/kg to 24.4 g/kg.The soil organic carbon contents of CBS,VCS-CBS,CCBS,CS,and TBS sample sites were 3.25,2.66,1.78,5.75,and 3.05 times higher than that of CK.According to the nutrient content grading standard of the national soil survey,the soil organic carbon content of each vegetation restoration model sample site was not very abundant,only CS and NF sample sites were abundant,CBS,VCS-CBS and TBS sample sites were moderate,CCBS and NR sample sites were deficient,and CK sample site was extremely deficient.The ROC content of easily oxidized organic carbon of each typical ecologically restored slope soil ranged from 3.37 g/kg to 20.29 g/kg.Compared with bare slopes,the ROC content of CBS,VCS-CBS,CCBS,CS,TBS,and NR slopes increased by 195%,148%,70%,469%,183%,and 43%respectively.In terms of easily oxidized organic carbon fractions,high oxidized reactive organic carbon（F1）,medium oxidized reactive organic carbon（F2）,low oxidized reactive organic carbon（F3）,and inert organic carbon fractions（F4）differed,and both F1 and F2 fractions were higher than F3 and F4 fractions,while F1 and F2 fractions accounted for a significantly higher percentage of TOC than F3 and F4.（3）The variation of whole soil organic carbon of CBS restored slopes in different restoration years ranged from 10.82 g/kg to 18.47 g/kg,showing a trend of decreasing and then increasing with the increase of restoration years.In terms of the contribution of organic carbon of agglomerates,the contribution of organic carbon of large-size agglomerates was always higher than that of small-size agglomerates.The effect of the change in the number of agglomerates on the long-term effect of slope ecological restoration is higher than that of the increase in organic carbon content.The change of easily oxidizable organic carbon content of CBS restored slopes in different restoration years ranged from 7.22 g/kg to 17.70 g/kg,and the ROC content gradually decreased with the increase of restoration years.F1 and F2 components gradually decreased with the increase of restoration years,F3 components gradually increased with the increase of restoration years,and F4 components did not change significantly in the early stage of vegetation restoration,and increased significantly after vegetation restoration.This means that the carbon sequestration capacity of the disturbed slope restoration reaches a certain number of years,while the time-efficient fertility gradually decreases.（4）The carbon pool management index CMI of each typical ecological restoration slope varied from 308 to 1403.74,ranked from largest to smallest as CS>TBS>CBS>VCS-CBS>CCBS>NF>NR,and the artificial ecological restoration measures had a significant contribution to the improvement of organic carbon pool quality of slopes.Compared with the bare slopes,the carbon sequestration index RI was significantly improved,but it was slightly lower than the natural restoration mode,and the difference between the natural Horsetail pine forest and the natural one was larger.The carbon pool management index CMI of CBS slopes in different restoration years varied from 362.48 to 1042.01,ranked from large to small as CBS1>CBS3>CBS5>CBS18,showing a decreasing trend with increasing restoration years.Overall,both the carbon sequestration indices RI1 and RI2 showed an increase with the increase of restoration years,which implies that the carbon sequestration capacity of slope soils restored by vegetated concrete ecological protection technology is gradually increasing.