| Ecological construction project is one of the most important measures to control soil erosion and restore soil fertility on the Loess Plateau.Under the different measures,such as returning farmland to forest,grass,terraces and check dams,the research of soil erosion and migration-deposition of organic carbon is of great significance to planning of soil and water conservation,the implementation of measures,and the evaluation of sediment reduction benefit.In this paper,through field investigation,sampling and laboratory experiments,the migration,deposition and fixed process of eroded soil organic carbon in the Nianyegou watershed with ecological construction was studied.This could illustrate the coupling relationship between organic carbon migration and sedimentation process and sediment yield typical with ecological construction.The main research results are as follows:(1)The growth trend of 137Cs loss and soil erosion rate on the slope showed similar fluctuation change.The upper part of the slope showed erosion and deposition,the erosion rate of the middle slope has trend down,and the lower slope erosion increases gradually.With the increase of slope gradient,the 137CS loss zncreased by years and the soil erosion rate with exponential growth(y = 3582.5 e0.037x),The order of the '37Cs loss was the south slope>north slope>east slope,the soil erosion rate was the south slope>north slope>east slope.According to the erosion modulus,it showed as slope land>grassland>forest-shrub land>terrace.Grassland.The slope cropland was extremely eroded,and grassland,forest-shrub land,terrace was lightly eroded.(2)The correlation equation of deposited sediment and the corresponding rainfall erosion was y = 1.6434 x0 4249.The silt and clay content in the sediment of main section has significant negative correlation.The silt content decreased in sedimentary cycle from front to back of the dam and vice versa for sand.From 1984 to 1990,the total deposition thickness was 3.59m,and the accumulated sediment volume was 264.74t which accounted for 66.26%of the sludge deposited during the running period of the check dam.In Nianyegou watershed,69%of the sediment was from farmland,the forest-shrub land,grassland and terrace was 3%,13%and 14%,respectively.Compared to the ungoverned period,the sediment from farming and barren slope accounted for more than 85%and 8%from steep cliff about.Through decades of control,the sediment source had considerable changes.(3)Different ecological restoration measures can significantly increase the organic carbon content of 0-20cm soil layer.The organic carbon content of forest-shrub land,grassland and terraced was enriched in the soil surface.But it had not show obvious enrichment in slope cropland.The enrichment of organic carbon mainly appeared on the upper and middle slope.For the grassland it appeared in the middle slope position,terraces in upper slope.With ecological construction,the organic carbon,light fraction organic carbon(LFOC),heavy fraction organic carbon(HFOC)in the 0-20cm layer significantly increased.While the easily oxidized organic carbon(EOC)was reduced in the 0-20cm soil layer.The organic carbon reserves of the lm soil profile were shown as the forest-shrub land(1.32kg/m)>grassland(1.21kg/m)>terrace(1.17kg/m)>sloping farmland(1.13kg/m).Ecosystem restoration measures,landform and the interaction had significant effect on the spatial distribution of organic carbon.Returning farmland to forest-shrub land,grassland and terrace can significantly increase the organic carbon content.For the study region,it can increase sequestration ability of the soil organic carbon and regional carbon storage correspondingly.(4)The organic carbon profiles of dam 1 and 2 displayed irregular distribution patterns.For dam 3,the organic carbon deposited as sediment deposition which can be divided into three stages:the stable period(1971-1982).the fluctuation period(1982-1987),the growth period(1987-1987).The average content of organic carbon in dam 1,2 and 3 was 2.39g/kg,2.25 g/kg and 2.16 g/kg,respectively.In the check dam,part of organic carbon was released due to increase of EOC,the rest of the carbon was locked as HFOC.Therefore,EOC played a vital role in the content of soil organic carbon.The organic carbon of deposited sediment in the dam was positively correlated with clay and silt(p<0.05),which was negatively correlated with sand(p<0.05).The total accumulation of deposited organic carbon of dam 1 was 175.48t,for dam 2 and 3 was 122.78t and 207.87t,respectively.The total amount of organic carbon in the dam area is 506.13t.The organic carbon content of sloping farmland is the highest,which reach 65%of the total deposition,followed by terraces and grassland which is 16%and 14%,respectively.The forest land is the lowest which accounted for 5%.Based on the results of this study,the organic carbon reserves in the deposited sediments of the Jiuyuangou are estimated to be about 1540t.(5)The spatial distribution of organic carbon concentration in the farmland,forest land,grassland and terraces is relatively consistent that of soil 137Cs,and significant positive correlation existed(p<0.05).This showed that soil erosion processes with differ landscape will significantly affect the migration process of soil organic carbon.In addition to the farmland,on the slope of forest-shrub land,grassland and terrace,erosion-deposition of organic carbon occurred with erosion.According to the Occurrence Site,the“source”and"sink" formed from slope to check dam.The distribution of active organic carbon was different with that of 137Cs and the erosion intensity determined the content of EOC.The organic carbon migration of the slope was 40.03t,and that of the dam was 207.86t,which could be estimated that the organic carbon migration of the ditch sides and the valley was 164.83t in 19 years.It accounted for 20%and 80%,respectively.From the slope to the dam,a"source" and "sink"of organic carbon are formed in the watershed. |
PDF Full Text Request