| The southwest,with its shallow soil,is one of the world’s largest contiguous areas of karst and is experiencing serious soil erosion.Karst depressions are a unique geomorphic type in southwest China,which can trap erosive sediments like reservoirs or dams.This provides an important opportunity to estimate sediment deposition rates in small basins of depressions,study sediment sources in depressions,and study the effects of erosion on nutrient deposition.Taking typical karst basins in southwest China as the research object,this research reconstructed the variation characteristics of sediment yield in karst basins in recent 60 years by using nuclide dating model based on the sediments in karst depressions.194 soil samples were collected from three sediment profiles in the depression of Guzhou small watershed,and 58 source samples were collected from farmland,crack soil and forest land,using a composite fingerprint identification method based on 45 geochemical properties and an IsoSource multivariate mixed model.The relative contributions of sediment sources and their changing trends in different periods in the past 60 years were estimated and identified.The relationship between soil erosion and nutrient deposition was evaluated based on the sediment characteristics of soil nutrients(SOC,TN and TP),grain size(clay,silt and sand)and nuclide(137Cs and 210Pbex)profile in the depression in recent 60 years.The main conclusions are as follows:(1)Three commonly used 210Pbex dating models—constant sedimentation(CFCS),constant initial concentration(CIC)and constant supply rate(CRS)—are not suitable for peaking cluster depression,while the modified CRS model based on 137Cs age markers improves the determination of sediment chronology.(2)From 1949 to 2015,sediment yield firstly increased and then sharply decreased,which may represent the general trend of soil erosion dynamics in karst watershed of southwest China.The annual sediment thickness and soil erosion modulus reached the maximum values of 6.33 cm yr-1 and 8636.36 t yr-1 during 1949-1956.After the implementation of the policy of "returning farmland to forest" in 1999,the average soil erosion modulus was only 80.21 t km-2 yr-1,which was also larger than the allowable soil loss in karst areas.(3)The best combination of fingerprint identification factors is determined,and the sediment source of the depression is quantitatively analyzed.Only 39 of the 45 metallic elements were detected.Kruskal-wallis H test(P<0.05)and stepwise discriminant analysis(DFA)were performed.Finally,five recognition factors(Cu/Pb,Cu/Cd,Cr/Ta,Cu/Bi and W/Ca)were selected as the optimal combination of recognition factors,and the cumulative discriminant rate was 86.2%.The results showed that cropland was the main sediment source in recent 60 years(89.2%),followed by fissured soil(9.6%)and woodland(1.2%).Compared with the period from 1949 to 1999,the sediment contribution of fractured soil reached the maximum value from 1999 to 2015,which was about 20%.This shows that the policy of "Grain for Green" implemented in 1999 has significantly reduced soil erosion of cultivated land.Therefore,this policy may have a great impact on sediment deposition in karst depression.(4)The effects of soil erosion on nutrient deposition were evaluated.There was a significant negative correlation between nutrient concentration and clay content(P<0.001).Overall,compared with 137Cs,210Pbex has a higher correlation with SOC and TN.In undisturbed sediment profiles,Pb/Cs is a better indicator of nutrient dynamics than a single nuclide.Nutrient deposition rates increased before 1953,reached a maximum in 1954-1956,and then declined rapidly from 1957-2015.Sediment deposition rate was negatively correlated with nutrient concentration(P<0.01),but positively correlated with nutrient deposition rate(P<0.01).It is shown that the temporal variation of nutrient deposition rates over the past 60 years is determined by soil erosion rather than nutrient concentration.The modified CRS model(C-CRS model)can accurately date the sediments in karst depressions,determine the deposition ages at different depths of the profile,and then calculate the sediment deposition rates in different periods.The composite fingerprint identification technology based on the element ratio as the identification factor can identify the sediment source of karst depression,in which the cropland is the most important source,followed by the fissured soil,and the woodland has the least contribution.Nutrient deposition rate is mainly affected by sediment deposition rate rather than nutrient concentration.The policy of " Grain for Green " implemented in 1998 not only significantly reduced the sediment deposition rate,but also affected the sediment source of the depression.The results of this study will be helpful to understand the present situation of soil erosion in small watershed in karst depression,and provide scientific basis for controlling and preventing soil erosion in small watershed in karst area and making reasonable soil and water conservation measures. |
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