| The severe soil erosion leads to the decline of land productivity,aggravate the development of drought and desertification,and cause the comprehensive degradation of agricultural,forestry and graziery industry chain,and destroy the local ecological environment.For example,the increasing sediments in rivers leads to the turbidity of the water,then leads to water-permeability of the water layer becomes worse,the growth of phytoplankton is blocked and the primary productivity of water is reduced.The influx of fine-grained silt blocks the river bed gravel gap,it would compress the aquatic habitat and spawning space,finally,and the aquatic environment would be destroyed,which would directly threat to the incubation and growth of aquatic organisms.As an important carrier,the fine-grained sediment redistribution significantly controls the transport and fate of nutrients,heavy metals,organic and inorganic contaminants,it would directly threat to water quality safety.A series of water environmental effects caused by sediment has become a hot topic and a focus for a long period of time.The Loess Plateau is the most serious erosion area in the world,especially in the “coarse sandy hilly area” of the Loess Plateau,where contributed the most sediments in the Yellow River.Constructing check dams in gullies has been the most widespread and effective strategy to reduce soil and water loss in the Loess Plateau region.Currently,more than 110,000 check dams exist,storing a total of 21 billion m3 of sediments and 0.0952 Gt of organic carbon on the Loess Plateau.The main functions of these dams are to stop and store sediments,repair gully beds,raise the eroded base surface,and prevent gully bed undercutting and gully bank expansion.At the same time,the sediments trapped by check dams can serve as natural archives for reconstructing the environmental history of soil erosion at a given location.For each couplet,the top layer was fine,while the middle and bottom layers were coarse because coarse particles fall faster than do fine ones in water.The boundary between the couplets that were associated with individual floods was easily defined because the bottom layer in a couplet was coarse,while the top was fine.In the case of absencing data from monitoring catchments,the flood couplets information of check dams is the only source of information that can be used to record the sediment source and the information on environmental changes.At present,most studies have focused on the effects of reforestion or natural fallow on soil moisture in the Loess Plateau.Few studies have been conducted to evaluate the effect of soil and water conservation between reforestion and fallow from sediment sources.Based on the analysis of molecular markers and geochemical elements of different land-use types and flood couplets samples in dam-controlled catchment,using a fingerprint model to identify sediment sources.This research is aimed to accurately identify sediment sources in the gully and hilly zones of the Loess Plateau at the beginning of Grain for Green.The main conclusions are as follows:(1)This study used a sediment source-tracing technique,incorporating both statistically verified multi-component signatures and a multivariate mixing model,which provided valuable information of the response of the main sediment sources in an agricultural catchment with artificial or non-native vegetation.The results demonstrated that gully is the main sediment source in this catchment,constituting 34.7%,while cropland contributed 27.9% of the sediment;forest and grassland contributed 21.7% and 12.7%,respectively;and fallow land contributed the least sediment,with only 3.0% of the total samples.Changes in the hydro-ecological environment lead to the leaf litter and understory being poorly developed and the soil being bare in the forest,making it more vulnerable to erosion.In this area,forest and fallow land contributed less sediment than did cropland,indicating that both of these catchment management strategies could reduce soil erosion.Afforestation is a more suitable choice in areas where precipitation is suitable,but in the vulnerable arid and semi-arid agricultural regions of the present study,it would take considerable research to identify suitable species.A natural fallow may be a better-designed soil erosion control strategy.From a management perspective,this study suggests that reforestation may not be appropriate for landscape restoration in such semi-arid loess hilly areas,while fallow may be highly recommended;it is very meaningful for the rational allocation of catchment management strategies and resources.(2)The majority of existing fingerprinting studies have focused solely on inorganic sediment provenance,and the apportionment of organic matter in sediments remains largely undeveloped.This study used the composite fingerprinting method and new fingerprint properties(individual n-alkanes)to identify sediment sources in a small catchment.The new fingerprint featured properties that depended on the land use/cover and resisted diagenetic modifications and degradation.However,because of the activities of microbes,only the middle and long chain n-alkanes could be used as effective fingerprints in check dam.The optimal composite fingerprint consisted of a total of 6 individual properties(CPI,OEPM,C28,C21,C24,and C26)and correctly distinguished 86.7% of the samples that were used to characterize each of the four source types.Meanwhile,the higher the concentration of biomarkers was,the higher the source classification accuracy became.The results demonstrated that young forest is the main sediment source in this catchment,i.e.,50.5%,and cropland,grassland,and gully contributed 25.6%,14.4%,and 9.5% of the sediment,respectively.Variations in hydrodynamic conditions can result in different sediment fraction distributions in different parts of the check dam.The forest and grassland contribution gradually increased from upstream to downstream,and the sediment contributions of cropland gradually decreased in the direction of the runoff pathway.Cropland also displayed high erosion rates because of tillage.In a comparison of biomarker and geochemical fingerprinting data,the latter may overestimated forest inputs to catchment sediment yields because of a mixed land use history(i.e.,forest and grassland).Although shortcomings in the geochemical fingerprint approach limit its ability to fully discriminate sources based on land management regimes,n-alkanes have the potential to discriminate between a greater number and different types of sediment sources(e.g.,between different crop types or tree species)and provide greater detail regarding sediment sources.(3)The soil and geological conditions were generally homogeneous in this small agricultural catchment;therefore,the geochemical properties could not be used to distinguish the sediments derived from gullies and steep slopes.However,as fingerprint factors,n-alkanes could be used to trace long-term sediment sources.In the sediment sequence studied,the short and middle components exhibited obvious even-over-odd predominance in all the sediment samples.Bacterial and algal activities increased shortchain n-alkanes(C15-C20),while highly saline carbonate environments increased evenchain n-alkanes because of the conversion of n-fatty acids.Therefore,only long,oddchain n-alkanes could be used as fingerprint factors to track sediment sources on the short-term timescale.A composite fingerprinting method based on long-chain n-alkanes(> C27)in 27 source samples from gully,cropland,and steep slope areas and one sediment core(48 flood couplets)was used in this study.The fingerprinting factors selected through a two-step statistical analysis involving the Kruskal-Wallis H test and a discriminant function analysis identified three biomarker properties(C29,C27,and C31)that correctly distinguished > 95% of the three sediment sources.The biomarker fingerprinting results using genetic algorithm optimization demonstrated that the primary sediment source was gully areas,which contributed to 45% of the total,followed by cropland(38.2%)and steep slope areas(16.8%).Meanwhile,this biomarker fingerprint has significant advantages in identifying sediments derived from different landform units(e.g.,gullies and steep slopes).Sediment cores record important information regarding the environmental processes related to soil erosion and deposition.Thus,reconstructing the environmental history using biomarkers and composite fingerprinting method constitutes a new application of this method and is meaningful for designing sediment management and soil erosion-control strategies. |
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