Characteristics Of Organic Carbon Fraction Decomposition In Sediment In Check Dams

Soil organic carbon is an important part of the terrestrial carbon sink,and the storage of organic carbon deposited in the dam sediment is as high as 21.6Tg,constituting an important "carbon sink" in the terrestrial system.At present,the researches on organic carbon deposited in dam land mainly focus on carbon storage,vertical distribution characteristics,sources,mineralization and stability of organic carbon deposited in dam land,but so far,there are few researches on the mineralization and stability of deeper carbon deposited in dam land sediment.In view of this,this study takes the typical silting dam in a typical small basin of the Loess Plateau as the object,conducts the mineralization culture experiment based on the simulated sediment environment（hypoxia condition）under the original deposition condition through the analysis of laboratory samples,and compares the decomposition rate of organic carbon under the assumed dam break condition when the sediment is exposed（aerobic condition）.This paper analyzes the characteristics of organic carbon and its component content and decomposition rate varying with depth in sediment of silting dam,analyzes the influence of the physicochemical properties and microbial characteristics of significantly sediment in dam site on organic carbon and its component,and reveals the main influencing factors of the variation of organic carbon and its component in sediment at different depths of silting dam in Loess Plateau.The main research conclusions are as follows:（1）The content of organic carbon and its components in the sediment profile of the dam fluctuates with increasing depth.The content of particulate organic carbon（POC）in sediment organic carbon（SOC）is relatively small,with a range of 20.30%-64.91%and an average of 34.47%.The content of mineral-bound organic carbon（MOC）in SOC is relatively large,with a range of 35.09%to 79.70%and an average of 65.53%.MOC was 1.9 times the average amount of POC.SOC ranged from 0.97-3.99 g·kg^-1,with an average of 2.18±0.89 g·kg^-1,and POC ranged from 0.42-0.96 g·kg^-1,with an average of 0.70±0.17 g·kg^-1.The MOC ranged from 0.40 g·kg^-1 to 3.06 g·kg^-1,with an average of 1.49±0.78 g·kg^-1.（2）During indoor culture,SOC decomposition rate of 56.66 mg C·kg^-1 soil·d^-1 under aerobic condition was 7.6 times that under anoxic condition(7.44 mg C·kg^-1 soil·d^-1).Under anoxic condition,the decomposition rate of particulate organic carbon is 1.3 times of the total organic carbon decomposition rate,and that of mineral bonded organic carbon is 49.1%of the total organic carbon decomposition rate.Under aerobic conditions,the decomposition rate of particulate organic carbon is 3.9 times of that of total organic carbon,and that of mineral bonded organic carbon is 3.9%of total organic carbon..（3）Structural equation model was used to analyze the effects of oxygen conditions,deposition depth,sediment chemical,physical and microorganisms related to organic carbon decomposition on the decomposition rate of organic carbon and its components.Oxygen condition is the most important factor affecting the decomposition rate of organic carbon and its components,followed by microorganisms related to organic carbon decomposition and sediment chemical properties,while sediment physical properties and deposition depth have little effect.The effects of sediment depth,sediment chemical,physical and microbial properties on the decomposition rate of organic carbon and its components under different oxygen conditions were further analyzed based on structural equation model.Under anoxic condition,the chemical properties of sediment had the greatest effect on the decomposition rate of total organic carbon,followed by the microbial properties of sediment,while the physical properties of sediment had little effect,with the standard path coefficients of 1.83,1.65 and 0.08,respectively.Microorganisms related to organic carbon decomposition had the greatest effect on the decomposition rate of particulate organic carbon,followed by chemical properties and physical properties,with the standard path coefficients of 1.89,1.35 and 0.20,respectively.Sediment chemical properties have the greatest influence on the decomposition rate of mineral bonded organic carbon,followed by microorganisms related to organic carbon decomposition and physical properties,with the standard path coefficients of 1.79,1.68 and 0.41,respectively.Under aerobic conditions,related microorganisms had the greatest effect on the decomposition rate of total organic carbon,followed by chemical properties and physical properties,with standard path coefficients of 2.08,1.64 and 0.71,respectively.Related microorganisms had the greatest effect on the decomposition rate of particulate organic carbon under aerobic conditions,followed by chemical properties and physical properties,with standard path coefficients of 1.61,1.49 and 0.52,respectively.Related microorganisms have the greatest influence on the decomposition rate of organic carbon in the mineral binding state under aerobic conditions,followed by chemical properties and physical properties,with the standard path coefficients of 1.82,1.15 and 0.63,respectively.In conclusion,oxygen conditions have the greatest influence on the decomposition rate of organic carbon and its components,and sediment chemical properties and microorganisms related to the decomposition of organic carbon are also obvious,while sediment physical properties and depth have little influence on the decomposition rate.