Dynamic Change Of Eroded Organic Carbon And Its Influence Mechanism At Watershed Of The Hilly Region In Loess Plateau

The dynamic change of soil organic carbon(SOC)induced by soil erosion has become an important factor affecting the global carbon cycle.Understanding how soil erosion affects the redistribution and delivery of SOC is essential for clarifying the source,budgets,and composition of SOC.Additionally,in the past few decades,large-scale soil and water conservation(SWC)have significantly changed the state of soil erosion,which further affected the transportation and redistribution of erosion-induced SOC.However,informative reports about the effects of soil erosion and SWC on the source,composition,and redistribution of SOC remain unavailable.Therefore,this thesis takes Shayangou(SYG)small watershed(0.69 km²)and Chabagou(CBG)watershed(187 km²)on the Loess Plateau as the study areas.We firstly estimate the soil erosion rate and sediment deposition rate of SYG and CBG respectively based on the high-density electrical resistivity tomography(ERT)method and Unmanned Aerial Vehicle(UAV)photogrammetry method.On this basis,we carry out field sampling and laboratory analysis,combined with geochemical properties fingerprinting method to clarify the source,spatial variation,and stability of SOC on the small watershed scale.Then,we analyze the different erosion processes and the SOC budget under the influence of SWC measures on the watershed scale through long-term hydrological observation data and the sediment/SOC budget model.Finally,we quantify the composition of SOC in the process of erosion,deposition,and output based on the endmember mixing model based on the radiocarbon isotopes.The main conclusions are as follows:(1)We determine the appropriate method for estimating sediment deposition and soil erosion rate on small watershed scale and watershed scale.For the watershed with small area and obvious lithologic boundary between sediment and channel bedrock,the ERT method can accurately identify the sediment bottom.Combined with the on-site investigation to obtain dam cross-sectional shape and other topographic parameters to construct 3-D topography,the corresponding soil erosion rate of small watershed can be estimated.For the watershed with large area,numerous check dams,and similar topography,the UAV photogrammetry method can be used to simulate the sediment deposit process of check dams and fit the relationship between dam silted land area and sediment volumes,so as to estimate the total sediment yields and soil erosion rate of medium-sized watersheds.(2)We analysed the SOC source and budget of the two stages before and after the Grain for Green Project(GGP)in the dam-controlled small watershed of the Loess Plateau.From 1969 to 2015,the SOC mobilized by soil erosion in the source area was1085.8±170.5 t.The primary source of SOC was the agricultural activity area(AGR),accounting for 68.5%,followed by the vegetation restoration area(RES)and the gully,which contributed 23.6%and 7.9%,respectively.The total amount(rate)of SOC buried in the check dam was 532.9 t(0.17 t·ha^-1·yr^-1),of which 409.1 t(0.20 t·ha^-1·yr^-1)and123.8 t(0.11·t·ha^-1·yr^-1)were buried before the GGP and after the GGP,respectively.Furthermore,we estimated the SOC lost during the transport process was 552.9±170.1 t,accounting for approximately 50%of the SOC mobilized by soil erosion.The total amount(rate)of SOC lost before GGP and after GGP were 389.2±127.8 t(0.19±0.06t·ha^-1·yr^-1)and 162.3±49.1 t(0.14±0.04 t·ha^-1·yr^-1),respectively.(3)We clarified the radiocarbon age and composition of SOC involved in the mobilization-transport-deposition process in dam-controlled small watershed.In SYG small watershed,the SOC in AGR and RES is mainly composed of younger biospheric organic carbon(OC_bio),while the gully is mainly composed of ancient petrogenic organic carbon(OC_petro).The lost carbon in the process of sediment transport was dominated by OC_bio,including modern OC_bio and pre-aged OC_bio.Almost all OC_petro mobilized by soil erosion was buried behind the check dam.In the depositional profile,the OC_petro accounts for approximately 64%of the trapped SOC,and the mean radiocarbon age of the trapped SOC was 9349±2026 yr.(4)We analyzed the effects of SWC measures on the dynamic changes of SOC on watershed scale.In CBG watershed,under the influence of vegetation restoration and check dam construction,the soil erosion rate of Stage-2 and Stage-3 decreased by 31.5%and 75.4%compared with Stage-1,respectively.Vegetation restoration mainly produces significant carbon sequestration synergistic benefits through the following three functions:reducing lateral mobilization of OC;reducing the decomposition of OC;increasing the annual net primary production(NPP)and SOC stock.In Stage-2 and Stage-3,the eroded SOC was buried behind the check dam at the rate of 2125±478 Mg C yr^-1and 1420±282 Mg C yr^-1,respectively.This buried carbon can be effectively preserved on decades time scale.(5)We determine the influence mechanism of vegetation restoration and check dam construction on different components of SOC.Under the synergistic effect of check dam and vegetation restoration,the output of SOC from CBG watershed outlet decreased significantly from Stage-1 to Stage-3.Additionally,the SOC export from the watershed outlet is mainly composed of pre-aged OC_bio.About 52%of the SOC in the deposition areas is OC_petro,and the rest is mainly modern OC_bio.The burial of OC_petro has no net effect on the atmospheric carbon pool,whereas the effective burial of modern OC_bio in the deposition areas constitutes a carbon sink.Check dam construction and vegetation restoration not only significantly reduce the output flux of SOC but also reduce the proportion of OC_petro and modern OC_bio.These effective soil conservations significantly reduce the decomposition of OC_petro and modern OC_bio in long-distance transport,which may be long-term and short-term sources of atmospheric CO₂,respectivelyThis paper reveals the effects of soil erosion and SWC on the dynamic changes of SOC and provides the theoretical basis for further understanding the environmental effects of soil erosion.Our study is also of great significance for decision-makers to implement further SWC planning on the Loess Plateau.