Vertical Distribution Characteristics Of Soil Organic Carbon In Conversion Of Cropland To Forest Land In Semiarid Loess Region

The Loess Plateau is characterized by diverse ecosystems,abundant vegetation species,complex topography and landforms,and fragile ecological environment,up to now,the characteristics of soil organic carbon and its influencing factors of different vegetation types in this area have not been fully understood,the change of soil organic carbon in converting farmland to forestland is one of the important contents in evaluating the measures of converting farmland to Forestland and its ecological benefits.In this paper,soil organic carbon of sixteen typical vegetation communities of arbor,shrub and grass in four different towns of Wuqi County in Semiarid Loess Region were studied,using combining field investigation with indoor analysis,the vertical distribution of soil organic carbon in different vegetation types were analyzed comprehensively,in order to provides basic data and scientific basis for vegetation restoration and reconstruction,forest structure transformation and optimization of forest soil quality evaluation in the future.The main conclusions are as follows:1.Caragana Korshinskii Kom.had the largest soil unit weight,the smallest is grassland and significant difference among the four vegetation types of grassland,Amygdalus davidiana,Populus simonii Carr.and Caragana Korshinskii Kom;soil moisture grassland was the largest,Caragana Korshinskii Kom.was the smallest and the difference was extremely significant;soil porosity was different,and there was no significant difference among them;soil conductivity was Populus simonii Carr.> grassland > Caragana Korshinskii Kom.> Amygdalus davidiana.Soil pH was the highest in Amygdalus davidiana and the lowest in grassland,and the largest soil organic carbon was Populus simonii Carr.,followed by grassland,Caragana Korshinskii Kom.,and there was significant difference.Grey correlation analysis showed that soil organic carbon of grassland was closely related to water content,but not to conductivity;soil organic carbon of Amygdalus davidiana was closely related to conductivity,but not to capillary porosity;soil organic carbon of Populus simonii Carr.was closely related to non-capillary porosity,but not to bulk density;soil organic carbon of Caragana Korshinskii Kom.was closely related to non-capillary porosity,but not to water content.2.Among four slopes of bottom platform,mid-slop,downslope and mid-slope level,the soil unit weight of the mid-slope level was the largest,while the bottom platform was the smallest and the difference was not significant.The soil moisture distribution in 0-100 cm soil layer was roughly "S" type,the bottom platform was the largest,the middle and mid-slop were the smallest,and the soil moisture content and capillary holding capacity were very significant differences.There was no significant difference between soil porosity and water content;soil conductivity was bottom platform > mid-slope > mid-slope level > downslope,and the difference was significant;soil organic carbon was the highest at the mid-slop and the lowest at the mid-slop level,and the difference was extremely significant.Grey correlation analysis showed that soil organic carbon in bottom platform was closely related to non-capillary porosity,but not to soil unit weight;soil organic carbon in mid-slop was closely related to conductivity,but not to non-capillary porosity;soil organic carbon in downslope was closely related to non-capillary porosity,but not to capillary water holding capacity;soil organic carbon in mid-slope level was closely related to total porosity,but not to conductivity.3.Among the four vegetations planted on the level bench,Armeniaca sibirica + Platycladus orientalis had the largest soil unit weight,and there were no significant difference.The vertical variation of soil moisture in the pure plantation was not significant,and the mixed plantation showed a fluctuating trend,but the soil moisture in the pure arbor forests and mixed Arbor-shrub forests on the lecel bench was not significantly different from that in the pure Arbor-shrub forests.The total porosity and capillary porosity of Armeniaca sibirica + Platycladus orientalis were the largest,Pinus tabulacformis and Pinus tabulacformis + Armeniaca sibirica were the smallest,while the non-capillary porosity were slightly different,but there was no significant difference between them.The soil conductivity was Robinia pseudoacacia > Pinus tabulacformis > Pinus tabulacformis + Armeniaca sibirica > Armeniaca sibirica + Platycladus orientalis,and there was no significant difference between them.The soil pH Robinia pseudoacacia was the largest,Pinus tabulacformis was the smallest.Grey correlation analysis showed that soil organic carbon of Robinia pseudoacacia was closely related to conductivity,but not to water content;soil organic carbon of Pinus tabulacformis was closely related to non-capillary porosity,but not to conductivity;soil organic carbon of Pinus tabulacformis + Armeniaca sibirica was closely related to non-capillary porosity,but not to soil unit weight;soil organic carbon of Armeniaca sibirica + Platycladus orientalis was closely related to soil unit weight,but not to pH.4.Among four different slopes of sunny slope,semi-sunny slope,shady slope and semi-shady slope,the soil unit weight of sunny slope was the largest,while semi-shady slope was the smallest,and there were very significant differences.Soil moisture was particularly prominent between the sunny slope and the shady slope,the shady slope was the largest,the sunny slope was the smallest,and there were very significant differences;soil porosity was different and there was no significant difference between them.The soil conductivity was semi-shady slope > semi-sunny slope > shady slope > sunny slope,and there was very significant difference between them.The soil organic carbon content of semi-sunny slope was the highest,while that on sunny slope was the lowest,and the difference was extremely significant.Grey correlation analysis showed that soil organic carbon in semi-shady slope was closely related to conductivity,but not to pH;soil organic carbon in shady slope was closely related to non-capillary porosity,but not to soil unit weight;soil organic carbon in sunny slope was closely related to non-capillary porosity,but not to water content;soil organic carbon in semi-sunny slope was closely related to non-capillary porosity,but not to soil unit weight.