Variation Characteristics Of Soil Organic Carbon,nitrogen And Estimation Of Soil Erosion In Water-fluctuation Belt Of The Pubugou Reservoir Area

With rich ecological benefits,the reservoir water-fluctuation belt is an important buffer area to reduce pollutants,enhance biodiversity,protect water quality and maintain the diversity of water ecosystem.However,due to the repeated submergence of reservoir water,the water-fluctuation belt is the sensitive and fragile zone of ecological environment.Under the action of periodic fluctuation of water level and rainfall erosion,soil loss,organic carbon and nitrogen loss in the water-fluctuation belt are very serious,which not only directly relates to the ecological environment of the reservoir,but also affects the operation and life of the reservoir,while organic carbon and nitrogen loss affects the water environment and the greenhouse effect of the atmosphere.Located in the upper reaches of Dadu river in Sichuan province,the Pubugou reservoir has a falling area of about 29 km² and a falling water level difference of up to 60m.Pubugou reservoir belongs to the dry-hot valleys climate,dry and wet climate obviously,rain focused entirely on a hot summer,these will have a great impact on soil structure,soil organic carbon and nitrogen loss,so it is very important to estimate the soil erosion,spatial and temporal variation characteristics of soil organic carbon and nitrogen in the water-fluctuation belt.In this paper,the soil in the water-fluctuation belt of the Pubugou reservoir is taken as the research object,the land use and land cover（LULC）,submersion duration（altitude factor）and terrain factor（LS value）is taken into account in space,and the influence factors of dry and hot valley climate are taken into account in time.The spatial-temporal variation characteristics of soil organic carbon and nitrogen content in the water-fluctuation belt are comprehensively analyzed;The nutrient reserve estimation GIS method based on land use and land cover（LULC）was improved,the effects of submergence duration and topographic factors were added to estimate the organic carbon reserves in the water-fluctuation belt;The universal soil loss equation（USLE）is improved,and the soil erosion equation is established for the reservoir water-fluctuation belt.The following main research results are obtained in this paper.（1）The spatial heterogeneity of soil nutrients was strong in the study area.The total nitrogen（TN）,alkali-hydrolyzed nitrogen（AN）contents of soil organic carbon（SOC）in the study area have obvious distribution rules under the influence of different LULC,terrain value and submergence duration.On the whole,the average values of TN,SOC and AN in the soils was decreased with the increase of soil layer in the study area.In 0-10 cm soil,the TN was 1.21g·kg^-1,the AN was 136.80 mg·kg^-1,and the SOCt was 14.75 g·kg^-1.In 10-20 cm soil,the TN was 0.97g·kg^-1,the AN was117.20 mg·kg^-1,and the SOCt was 12.19 g·kg^-1.In 20-40 cm soil,the TN was0.79g·kg^-1,the AN was 93.88 mg·kg^-1,and the SOCt was 9.70 g·kg^-1;The average values of SOC and TN contents under different LULC showed as follows:sludge>grassland>dry land>bare land.The AN content has no obvious size order;The concent of TN,AN and SOC was inversely proportional to LS values,there was no significant difference between soil TN,SOC with different LS values.There was a significant difference of the content of AN in 0-20 cm soil when the LS value was0.03-1.16 and the LS value was 5.04-13.78（P<0.05）.There was a significant difference of the content of AN in 20-40 cm soil when the LS value was 0.03-1.16and the LS value was 2.83-13.78（P<0.05）.Under the influence of topography,the content of AN in soil is more easily changed than that of TN and SOC;The contents of SOC,TN and AN at 790-820 m elevations in the study area were higher than 820-850 m at different soil depths;The river course in the water-fluctuation belt gradually widens from west to east,the terrain becomes relatively flat,and the population density is relatively increased.The distribution of SOC,TN and AN content is affected by human interference factors,which shows an increasing trend.（2）The climate of the hot and dry valleys will aggravate the loss of soil nutrients in the study area.In three different time periods in the study area（spring,summer,spring）of soil samples analysis results,showed that second times to collect soil,high temperature and rainy summer would lead to TN,AN and SOC loss,but in the soil samples of the first and third time to collect soil samples,TN,AN,SOC were not significant difference（P<0.05）,and in the dry-hot valleys climate,keeping soil structure has a worse trend.In different LULCs,The bare land with 0-20 cm in the water-fluctuation belt,the TN,AN,SOC in the first sampling was significantly different from that in the second sampling（P<0.05）.In the dry land of 0-20 cm,the AN in the first sampling was significantly different from that in the second sampling（P<0.05）,and the variation reached 27.40%;Different LS values in the study area,there was no significant difference in TN,AN,SOC contents in three times samples;At the altitude of 820-850 m,the contents of TN,AN and SOC in the soil from 0 to20 cm showed significant differences in the first and second soil samples（P<0.05）.Under the dry-hot valleys climate,the soil bulk density and sand content increased in the water-fluctuation belt,while the porosity,clay content and silt content decreased.The physical structure of soil under vegetation cover changed little.（3）The organic carbon reserves in the study area were calculated by an improved GIS estimation method.The nutrient reserve estimation GIS method based on land use and land cover（LULC）was improved,the effects of submergence duration and topographic factors were added to estimate the organic carbon reserves in the water-fluctuation belt;By calculation,the total storage of SOC in 0-40 cm of the study area is 184134.12t,with an average density of 6.29 kg/m²,and an average density of 6.80 kg/m² after deducting the area of 2.21 km² of dam and bare rock.Among them,the reserve of SOC in bare land is 94639.13 t,the reserve of SOC in grassland is 57,435.46 t,the reserve of SOC in sludge is 32051.94 t,and the reserve of organic carbon in dry land is 7.59 t.（4）An improved USLE model was used to estimate the soil erosion.Due to the water-fluctuation belt having different conditions from terrestrial areas,it is periodically submerged by water,it takes less time for stormwater runoff to erode than land,and people use the land without managing it,so the USLE cannot be used to directly calculate soil erosion.Therefore,in the present study,a modified USLE method to account for the vacancy of the soil erosion algorithm in the water-fluctuation belt.This modified method can provide reference for the study of soil erosion in other reservoir water-fluctuation belts.According to the calculation,the total annual soil loss in the water-fluctuation belt is 509978.09t,with an average loss of 175.89t/hm².The soil loss in dry land is 9.90t and the erosion modulus is 75.73t/hm².The soil loss in bare land was 471403.53t and the erosion modulus was 310.19t/hm².The soil loss in grassland soil was 38565.48 t and the erosion modulus was50.85 t/hm².At the elevations of 790-810 m,the soil loss was 65926.35 t and the erosion modulus was 71.34 t/hm².At the elevations of 810-830 m,soil loss was281880.72 t and erosion modulus was 20892 t/hm².At the elevations of 830-850 m,soil loss was 162171.02 t and erosion modulus was 259.07 t/hm².To sum up,this paper takes the water-fluctuation belt of the Pubugou reservoir,which is in dry-hot valley climate as the research object,analyzes the spatial and temporal distribution characteristics of SOC,TN and AN in the water-fluctuation belt;An improved GIS estimation method was used to calculate the organic carbon reserves in the study area;The improved USLE model was used to estimate the soil erosion and the soil erosion distribution,providing scientific data support for the ecological and environmental protection work in the water-fluctuation belt of Pubugou reservoir.