Assessment Of Effectiveness Of Fine Roots In Reducing Soil Erosion And Trapping Sediments In Planted Forest Hillslopes By Using¹³⁷Cs Techniques

Implementation of the Grain for Green Program （GFGP） in western China plays an important rolein controlling soil erosion and improving ecological environments. China has the world-largest area ofplanted forests. Soil erosion in the planted forests is still active because of lacking fixation of soils byfine plant roots. So far, few studies have been conducted on the quantitative relationships between soilerosion and fine plant root density at hillslope scale. To determine this relationship is very important forthe restoration of vegetation and its sustainable managments in China.Selecting the representative forest hillslopes in severe soil erosion areas of the Upper YangtzeRiver Basin and hilly and gully areas of the Loess Plateau in Shaanxi Province, relationships betweensoil erosion rates and coverage and root densities of different species of vegetation investigated. Soilerosion rates were derived from traditional¹³7Cstechnique and repeated¹³7Cssampling techniques.Plant root densities with different diameters were determined by undisturbed soil core sampling andvegetation coverage by linear section measurements. Further, the effectiveness of fine plant roots inreducing soil erosion and SOC loss in forested hillslopes were quantified. The main results weresummarized as below:1．In the study hillslope of southwestern China, mean soil erosion rate was25.25t/ha/yr in theentire hillslope, showing severe soil erosion. Soil redistribution rates decreased in the followingvegetation types: grass （0.60t/ha/yr）> shrub （0.53t/ha/yr）> tree （0.57t/ha/yr）> bare land （-36.32t/ha/yr）. SOC was obvious loss induced by soil erosion and with the mean SOC loss rate of0.0202t/ha/yr. SOC redistribution rates of loss or accumulation decreased in the following vegetation types:shrub （0.0686t/ha/yr）> grass （0.0166t/ha/yr）> tree （-0.0169t/ha/yr）> bare land （-0.1489t/ha/yr）.Correlation analysis showed that there existed no correlations between soil and SOC redistribution ratewith coverage of vegetation, but correlations with root density （RD） in planted forest hillslope. Soilerosion and SOC redistribution rate was significant correlated to RD for the <1mm root diameter class（P<0.01）, but uncorrelated to RD for other root diameter classes （1-2mm、2-5mm、5-10mm and>10mm）.The effective characteristics of fine plant roots <1mm diameter in reducing soil and SOC loss indicatethat soil erosion and SOC erosion sharply decreased with the increase when RD was less than60g/m²,Soil erosion and SOC loss erosion were0with the fine RD in the range of60-110g/m²and Soil andSOC were trapped sediments from upland when RD was greater than110g/m².2．In hilly and gully areas of the Chinese Loess Plateau, soil and SOC erosion rates significantlydecreased after15years returning of cultivation to afforestation. Comparing with cultivated hillslope in1996, there was97%decrease with an average of soil erosion rate of1.11t/ha/yr in the entire hillslope.Soil redistribution rates decreased in the following slope positions: summit （2.83t/ha/yr）> middle（-2.24t/ha/yr）> lower （-5.16t/ha/yr）> upper （-6.87t/ha/yr）. Mean SOC loss was0.0172t/ha/yr,decreased89%as compared with that in1996. SOC loss rates increased in the following hillslopepositions: summit （0.05t/ha/yr）> middle （-0.04t/ha/yr）> lower （-0.09t/ha/yr）> upper （-0.11t/ha/yr）.Correlation analysis indicated that soil erosion and SOC redistribution rates were significantly correlated to grass cover and RD <1mm root diameter class （P<0.01）, but uncorrelated to coverage ofshrub and grass and RD for other root diameters classes （1-2mm,2-5mm,5-10mm and>10mm）. Theeffective characteristics of fine plant roots <1mm diameter in reducing soil and SOC loss indicate thatsoil and SOC erosion sharply decreased with the increase in the fine RD in the range of20-30g/m²while soil and SOC obviously accumulated when RD was greater than35g/m².The above results obtained from this study suggests that a key role of shrub and grass cover andfine plant roots in controlling soil erosion and trapping sediments from the upland in forested hillslopesin southwestern China and the Chinese Loess Plateau. The role of fine plant roots is greater thanvegetation cover and coarse roots may accelerate soil erosion in forested hillslopes. Therefore,protection of shrub and grass cover and fine root systems should be considered the importantmanagement measures to control soil erosion and strengthen ecological functions in planted foresthillslopes in western China.