Effect Of Soil Erosion On Soil Organic Carbon Loss In Sloping Farmlands Of Chinese Mollisol Region

In the black soil?Mollisol?area of Northeast China,soil erosion causes the topsoil with rich organic carbon loss with runoff and sediment,which reduces soil organic layer,causes soil degradation and seriously threatens food security in whole China and regional ecology security.Therefore,it is of great significance to study effects of soil erosion on soil organic carbon loss in sloping farmland of Northeast China.Thus,this study,using combined methods of field investigation,runoff plot observation,indoor simulated rainfall and inflow experiments,and laboratory analysis,quantifies soil erosion impacts on soil organic carbon loss in the sloping farmlands of Chinese Mollisol region.The objectives of this study are 1)to investigate the response of spatial distribution of soil organic carbon to erosion/deposition in typical thinner black soil watershed,2)to analyze effects of rainfall intensity and inflow rate on soil organic carbon loss,3)to quantify impacts of sheet erosion dominated and rill erosion dominated on soil organic carbon loss,4)to clarify soil erosion influences on soil dissolved organic carbon and aggregate organic carbon losses under both near-surface soil hydrological conditions of free drainage and seepage,5)to compare soil dissolved organic carbon and soil organic carbon losses under different tillage systems(no tillage,mini tillage,contour tillage,conventional?traditional?tillage.The research results can provide scientific basis for preventing soil degradation and realizing sustainability of black soil resource utilization in the Chinese Mollisol region.The main results are as follows:?1?Responses of spatial distribution of soil organic carbon to erosion/deposition are revealed based on soil profile investigation in the typical thinner Mollisol watershed?Binzhou River Basin?.Soil organic layer thickness varies from 0 to 80 cm and it concentrates in 0-30 cm thickness,whose area occupies 54.9%of watershed area.Moreover,soil organic carbon content changes from 9.53 to 24.13 g/kg with an average value of 14.21g/kg and soil dissolved organic carbon content shifts from 0.11 to 0.24 g/kg with an average of 0.17 g/kg.At both scales of watershed and hillslope,the spatial distributions of soil organic carbon and soil dissolved organic carbon contents are opposite with spatial distribution of soil erosion rates.Meanwhile,soil organic carbon and dissolved organic carbon contents in the watershed follows sequences of the downstream>the middle stream>the upper stream,and both contents at hillslope positions follows orders of down slope>middle slope>lower slope.Meanwhile,Both soil organic layer thickness and organic contents in the deposition areas are greater than that in the eroded areas.Furthermore,there is positive correlation between soil organic carbon and soil dissolved organic carbon contents with soil organic layer thickness and the correlation coefficient in deposition areas?R²=0.73?is higher than that in the eroded areas?R²=0.65?.?2?Contributions of rainfall and inflow rate to soil organic carbon loss are quantified.Soil organic carbon losses under 50 and 100 mm/h rainfall intensities are significantly greater than those under 50 and 100 mm/h inflow rates,and soil organic carbon losses under simulated rainfall experiments are 76.9 and 190.5 times as much as that under simulated inflow experiments,which indicates that impacts of rainfall intensity on soil organic carbon loss are greater than that of inflow rates.The enrichment ratio of soil organic carbon under simulated rainfall experiments is around 1.14,and it is around 1.24 that under simulated inflow experiments.Moreover,under the combination of rainfall intensity and inflow rate with the same supply water?rainfall intensity+inflow rate?,soil organic carbon losses under combination of rainfall intensity and inflow rate are lower than those under simulated rainfall experiments,while they are larger than that under inflow rates.?3?Effects of sheet erosion dominated and rill erosion dominated on soil organic carbon and soil aggregate organic carbon losses are discussed.Both of soil erosion rates and soil organic carbon losses in rill erosion dominated treatments are significantly higher than those in sheet erosion dominated treatment.When rainfall intensity increases from 50 to 100 mm/h,soil erosion rates and soil organic carbon losses at sheet erosion dominated increases 7.1times and 7.2 times,respectively;and both at rill erosion dominated increases 17.6 and 16.4times,respectively.The different erosion patterns also induce different soil aggregate organic carbon contents in different aggregate sizes.Moreover,at 50 and 100 mm/h rainfall intensities,organic carbon content in<0.25 mm aggregate size increases by 14.9%and14.3%,respectively,and compared with the tested soil,it increases by 33.3%and 22.1%,respectively.The relationship between soil organic carbon loss and soil aggregate loss was revealed by introducing the concept of adhesion coefficient of organic carbon.At the rainfall intensities of 50 and 100 mm/h,the adhesion coefficients of sorganic carbon under rill erosion dominated experiments are 1.08 and 2.42 times higher than those under sheet erosion dominated,respectively.?4?The influences of soil erosion on soil dissolved organic carbon,organic carbon and aggregate organic carbon losses under both near-surface soil hydrological conditions of free drainage and seepage are elucidated.Runoff rate and soil loss under seepage conditions are1.20-1.35 and 3.19-9.01 times higher,respectively,compared with free drainage.For the conditions of 3,4 and 5 m slope lengths and 50 and 100 mm/h rainfall intensities,soil organic carbon loss is dominated,which accounts for 75.3%and 90.0%of the total organic carbon loss.For the conditions of 3 and 4 m slope lengths and 50 mm/h rainfall intensity,soil dissolved organic loss was dominated,which occupies 54.2%-67.4%of the total organic carbon loss.For the condition of 3,4 and 5 m slope lengths and 100 mm/h rainfall intensity,soil organic carbon is dominated,which takes up 54.0%-77.5%of the total organic carbon loss.Meanwhile,the losses of macro-aggregate organic carbon and micro-aggregate organic carbon under both hydrological conditions increase with an increase of rainfall intensity and slope length.?5?Soil dissolved organic carbon and soil organic carbon losses among different tillage systems?no tillage,mini tillage,contour tillage,conventional tillage?are compared based on observation of field runoff.Soil loss from no tillage,mini tillage and conventional tillage is 24,198 and 1308 t/km²a,which is 3,28 and 187 times higher than that from the contour tillage,respectively;while it only is 0.17%,1.43%and 9.47%of soil loss from bare and fallow treatments,respectively.Both of annual soil dissolved organic carbon loss and annual soil organic carbon loss under different tillage systems follow the orders of the bare and fallow treatment>conventional tillage>mini tillage>no tillage>contour tillage.The average annual loss of soil dissolved organic carbon under no tillage,mini tillage and traditional tillage is 137.59?384.78 and 431.43 mg/m²,respectively,which is 1.05,2.93 and3.29 times higher than that under contour tillage,respectively and is only 7.76%,21.70%and 24.32%of average annual loss of soil dissolved organic carbon under the bare and fallow treatment,respectively.The annual loss of soil organic carbon under no tillage,mini tillage and traditional tillage are 0.49,3.67 and 23.80 g/m²,respectively,which is 5.16,38.63 and 250.53 times higher than that under contour tillage,respectively;it is only 0.2%,1.4%and 8.6%of annual loss of soil organic carbon under the bare and fallow treatment.Meanwhile,there is positive correlation between event soil dissolved organic carbon loss with P?rainfall?,I₃₀?maximum 30 min rainfall intensity?,PI?rainfall multiples by average rainfall intensity?,PI₃₀0 and rainfall erosivity?R?under the bare and fallow treatment,no tillage,mini tillage and traditional tillage treatments.Furthermore,there is a significant positive correlation between event soil dissolved organic carbon loss and runoff rate under these four tillage systems and a significant positive correlation between event soil organic carbon losses with soil erosion intensity is also found under these four tillage systems.