| The Dianchi watershed is located in Yunnan Province of China. Since the 1980s with the constant increasing of population and the rapid booming of economy, local land resources have been unreasonably abused to such an extent that the ecological environment has been devastated, which then led to serious soil erosion and nutrient losses along Dianchi watershed. A direct result of soil erosion is the decrease of soil fertility. What's worse, the nutrient elements that the soil lost flowed down into the Lake, which may bring about potential detriments such as silted courses and water eutrophication. The balance of local ecological environment is thus broken.Accordingly,a quantitative analysis of soil erosion and sandy-soil source is beneficial to the positive development of local ecological environment. Methodologically,nuclide tracing technology,which is increasingly popular in researching soil erosion,has been proven to be an effective method in accurately reflecting LUCC's impact on soil erosion, revealing the reciprocal mechanism between soil erosion amount and nutrient loss amount, measuring concentrations of PAHs in sediment core from small catchment in north of the Dianchi watershed, and quantizing the relationship between human activities and environment changes.The present study, which selects two small catchment Songhuaba and Shangyou as the target areas that are situated on the north of Dian Lake, aims at analyzing the distribution features of the nutrient elements in 3 types of land utilization, estimating the soil erosion degree of the two target areas with 210Pbex nuclide tracing technology,discussing the correlation between soil erosion amount and nutrient loss amount,investigating how human activities exert their influence on soil erosion and thus providing scientific instructions for the effective utilization of land, prevention of soil erosion, maintenance of soil fertility and remediation of eutrophic water along the watershed.The main findings of the present study are as follows:(1)In the two small catchments, soil TN contents are all high in 3 different types of land utilization, while the contents of TOC, TP and TS are all below the average.Among them, the nutrient contents in forestland and grassland are overall higher than those in abandoned land. In Songhuaba and Shangyou, soil TOC and TN contents in forestland and grassland are higher; the nutrient contents in 0-20 cm soil layer are significantly higher than those in 20-40 cm;nutrients contents in the topsoil of grassland are much higher than those in abandoned land. In Songhuaba, a positive correlation is observed between any two among soil TOC, TN, TP and TS,which indicates that there is potential balance mechanism among different soil nutrient contents. However, no significant correlation is detected among soil TOC, TN, TP and TS in Shangyou.(2) In the two small watersheds Songhuaba and Shangyou, types of land utilization don't exert significant influence on the contents of soil trace elements. Cr content in soil is lower than the natural background value,while Cu,Pb and Zn contents are much higher than the natural background values; Cd content far exceeds the value in the 3rd level of Environmental Quality Standard for Soils, which manifests there is potential risk of heavy metal pollution in Songhuaba and Shangyou,and particular emphasis should be placed on the prevention and control of soil Cd pollution. In addition, soil Fe and Al contents don't make significant differences in Songhuaba and Shangyou, while soil Ca content in Songhuaba is far below that in Shangyou, which indicates different human activities have exerted different effects on soil quality.(3) Land utilization exert significant influence on the contents of soil erosion and nutrient losses. 210Pbex distribution in soil profile varies greatly according to the types of land utilization: 210Pbe, contents in both forestland and grassland are significantly higher than those in abandoned land. Basically, 210Pbex contents in the sampling sites of forestland and grassland decrease, contrary to the case in abandoned land, exponentially with the increase of soil depth, which demonstrates the topsoil in forestland and grassland is more resistant to soil erosion in virtue of vegetation cover and root distribution. Abandoned land soil of the two small catchments suffers from moderate and extreme intense erosion respectively, while soil in forestland and grassland only suffers from slight erodibility, or even deposited. In 3 types of land utilization, topsoil nutrient loss amounts are all above average amounts.The nutrient losses amounts of abandoned land in Songhuaba are significantly higher than those of forestland and grassland. In Shangyou, the loss amounts of TOC, TN, TP and TS follow such a sequence as abandoned land > grassland > forestland. In the two small catchments, highly significant correlation between soil nutrient losses amount and soil erosion amount is verified, manifesting that soil erosion is the direct cause of topsoil nutrient losses.(4) Multiple environmental proxies reveal the influence by human activities of small catchment in north of the Dianchi watershed environment.There is high potential water eutrophication in forestland, grassland and abandoned land of two small catchments, indicating it is critical to prevent nitrogen from flowing down into the lakes. OI and TP contamination degrees in abandoned land are below those in forestland and grassland, while OI, TN, TP implying high potential ecological risk in the two small catchments. The PAHs in sediment core were mainly composed of 2-3 rings, and Naphthalene and Phenanthrene were the dominant compounds throughout the core. The profiles of FLA/(FLA+PYR), BaA/(BaA+CHR), IcdP/(IcdP+BghiP)shows that the main source of PAHs was originated from the combustion of fossil fuels,coal and biomass. According to the results of risk assessment,the PAHs may not induce adverse effects on the aquatic ecosystem in small catchment in north of the Dianchi watershed. |
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