| Soil erosion is one of the serious environmental problems in our country,it could cause serious soil erosion,result in soil nutrients loss,decline land productivity,and severely damage land resources.In addition,soil erosion caused soil organic carbon?SOC?loss,which had an important impact on the regional carbon cycle and global climate change.At present,whether soil erosion is the“carbon source”or the“carbon sink”of the atmosphere has long been controversial.In recent years,more and more researchers have shown that erosion constitutes a C sink,but the magnitude of the C sink remains uncertain.A large source of error in the aforementioned estimates is lack of data on the flux and fate of eroded C?including partitioning into dissolved versus particulate C transported?at different scales.It is the key to determine the source of eroded SOC for exploring the uncertainty of fate of SOC induced by erosion.Therefore,this thesis selected the hilly mountain area of the Loess Plateau as the research object.Based on the isotope technology,biomarker and spectrum fingerprinting methods combined with isotopic mixing model through field investigation,sampling and laboratory analysis,the response characteristics of soil organic carbon and chemical composition in eroding sites to different landscape slope,land use types,and intensity of erosion of small watershed were studied,and its influencing factor was also analyzed.The sources of organic carbon in sediments under different sedimentary environment and its main influencing factors were identified.The role of water-extractable organic carbon?WEOC?in carbon cycle driven by erosion was clarified,and the response characteristics of organic carbon sources in sediments to hydrological process and management measures during rainfall process were also discussed.Finally,the redistribution process and budget balance of SOC caused by erosion and deposition processes in dam-controlled watershed were clarified.The main conclusions of this thesis were as follows:?1?From the landscape slope scale,the vertical and horizontal distribution patterns of SOC and its storage under different land use types after vegetation restoration in the Loess Plateau were studied,indicating that artificial grassland is the best scheme for soil carbon sequestration and soil and water conservation in the Loess Plateau.The results showed that SOC presented an exponential declining trend with increasing soil depth on different slopes.It was mainly distributed in the surface soil?0-10 cm?.The variation of SOC in the surface soil was larger compared with the deeper soil?10-100 cm?.Compared with other land use types,artificial grassland showed greater carbon sequestration potential.After vegetation restoration,the soils under different land use types suffered different degrees of erosion,showing the order of artificial forest>artificial grassland>natural grassland.The losses of SOC at the slope landscape were mainly concentrated in the middle slope.?2?Based on biomarker and isotopic techniques,the sources and redistribution processes of organic carbon in sediments in dam-controlled small watershed were studied,revealing that the n-alkane indexes were not applicable in the Loess Plateau,that the cultivated land was the main sources of sedimentary organic carbon,and that erosion and deposition processes might act as“carbon source”in the carbon cycle.The analysis of One-way ANOVA showed that no significant difference?P>0.05?in the content and derivative indexes of n-alkane in soils under different land use types was found,while significant differences in the carbon and nitrogen isotopes and element composition of the soils were found in the eroding sites?P<0.05?.The variable values of sediment were within the range of variable of soils.Cropland was the main contributor of eroded SOC in sediments in dam-controlled small watershed?>50%?,while gully and forests contributed 29.28%and 17.18%,respectively.The contribution proportion of forests decreased gradually with the direction of flow path,while the cropland and gully were the highest in the middle part and front part of the dam,respectively.The sources of organic carbon also showed obviously different in sediment profiles.Soil erosion and deposition resulted in loss of 1569.8 Mg SOC?Mg=106 g,2004-2016?,and the average annual SOC erosion rate was 0.051 Mg C ha-1,respectively.Erosion and deposition processes leaded to the loss of 52.8%C in total SOC erosion,which might become an important"C source"in the carbon cycle.?3?The spectral characteristics and fluorescence components of WEOC in eroded soils and sediments in dam-controlled small watershed were studied based on spectroscopy combined with parallel factor analysis and principal component analysis,demonstrating that biological production of new organic compounds?autochthonous sources?are likely the major source of sediment WEOC in depositional settings.Results showed that the WEOC concentration was the highest(30.23±2.57 mg L-1)in forest soils and the lowest(6.90±1.42mg L-1)in gully.The WEOC concentrations in other land use types were not significantly different?P>0.05?.The structural and chemical characteristics of WEOC among different land use types?cropland,grassland,fallow?and gully showed no obvious difference,while the WEOC of forest soils had higher aromatic and hydrophobic components and molecular weight compared with other land use types.The concentration,aromatic structure and molecular weight of WEOC in sediments were lower and the degree of humification was higher than those of land use type and gully soils.In addition to humic components,unique protein components?tyrosine and tryptophan,accounting for more than 42.77%?in the WEOC of sediments in depositional sites were found.The WEOC in sediments mainly came from autobiogenic microbial residues and secretions.?4?The sources and influencing factor of SOC in sediments during rainfalls in small watersheds under different control measures were studied.Results showed that gully erosion was the influencing mechanism of SOC loss in sediments during rainfalls in the region of Loess Plateau.The SOC content in the small watershed with management practice was significantly higher than that in the small watershed without management practice and the variation was also larger during rainfall process.Flow rate and sediment yield,rather than rainfall characteristics,negatively correlated with organic carbon content in sediments?P<0.05?,which were the main influencing factors for the dynamic variations of organic carbon in sediments.Gully soil was the main contributor of eroded SOC in sediments during rainfall?watershed without management practice:38.50%and watershed with management practice:35.63%?,while the total contribution proportion of C3 and C4 plant residues was less than25%.?5?The chemical composition and sources of eroded SOC in sediments in small watersheds with different erosion intensities were studied,implying that erosion intensity was one of the main factor influencing the sources of sedimentary organic carbon.The results showed that the effect of erosion intensity on soil quality could be neglected.Biological production of soil matrix also played a key role in the dynamic variation of WEOC induced by erosion.In the watershed with different erosion intensities,gully bank was the main source of SOC?contribution proportions for moderate,high and extremely high erosion intensity watershed accounted for 100%,36.18%and 99.25%respectively?.In watershed with high erosion intensity,orchard and grassland were also the main sources of SOC in sediments,accounting for 29.93%and 33.89%,respectively.In a larger watershed,cropland was the main source of SOC in gully sediments,accounting for 26.5%,followed by“gully bank”?25.4%?,“grassland”?25.1%?and“forests”?23.0%?.No obvious variation of contribution proportion from different land uses to eroded SOC in sediments was found from the upstream to the downstream in space. |
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