| Soil erosion is a critical environmental problem throughout the world’s terrestrial ecosystems. Soil erosion inflicts multiple, serious ecological damages in various ecosystems. In particular, soil erosion exacerbates the soil degradation, pollutes surface water. Recently, the impacts of human-induced land use change on soil erosion have received increasing attention worldwide. At the small watershed scales, improper land utilization accelerates soil erosion and affects many processes in lacustrine systems. The radionuclide 137Cs as a tracer has been used successfully in soil erosion and sedimentation studies. It can characterize the interactions among land use, erosion and soil nutrients. At the same time, the sediment analysis associated by 137Cs method could be applied to track the relation between human activities and environmental changes and may provide insight into past changes in catchment soil erosion and its spatiotemporal distribution. As a result, at the small watershed scales, the information about spatial distribution characteristics of soil erosion and relationship between soil nutrients and land use can provide some important basis for ecological restoration and reconstruction of degraded ecosystem.In this study, the southwest and northwest catchments of Dianchi watershed were investigated to determine the effects of land use on soil erosion and nutrients based on 137Cs and GIS. All catchments were intensely influenced by human activities. The soil quality status was evaluated by soil nutrients and element stoichiometry. The catchment erosion and trophic status changes were studied by an interpretation of the variations of multi-proxies, including grain size, total organic carbon, total phosphorus, total nitrogen and organic carbon isotope. Sediment accumulation rates and ages were determined using a model of the 210Pbex. Meanwhile, soil organic carbon and total nitrogen storage as affected by land use in every small watershed were estimated. The main results obtained are as follow:1. Land use effects on soil erosion in small catchment.At the small watershed scales, the distribution of 137Cs with depth in the soil profile and its content differed among the land use types. The majority of 137Cs contained in 0-20cm soil depth.137Cs concentration in tillage was uniform at top layer. For forestland and grassland,137Cs concentration decreased exponentially with soil depth. In Dongda catchment, the average contents of 137Cs in tillages, abandoned-farmlands, grasslands and forestlands were 391.00,173.17,908.41,601.93 Bq/m2, respectively. The sites under tillage and abandoned-farmlands ranged up to the highest and the grassland site had on average the lowest erosion rate. In Baoxiang catchment, the average contents of 137Cs in tillages, grasslands and forestlands were 847.41,746.27,660.29 Bq/m2 respectively. There was no significant difference in 137Cs concentration and erosion rate between cultivated farmlands and abandoned-farmlands at Dounan area, however, the cultivated or abandoned fixed number of year had significant effect on it. For all land use types, the erosion rates estimated by 137Cs under tillage were higher than non-cultivated sites, indicating that plant covers were effective in reducing soil erosion. In addition to land uses, topography and vegetation had also typical coupling influence on soil erosion, under the GIS support, the use of the whole soil erosion status simulation of space was implemented by established multiple linear regression model. The annual soil erosion rate is 2575.87 tkm-2y-1 for Dongda catchment and 2206.15 tkm-2y-1 for Baoxiang catchment. The average soil erosion rate is 2104.89 tkm-2y-1 in the Dounan region of Maliao catchment. The soil erosion in Baoxiang and Maliao catchment basically belongs to the light erosion in intensity, which in Dongda catchment belongs to the medium erosion.2. Effects of different land uses on soil nutrient variability and stoichiometry characteristics in small catchments.The soil nutrient concentrations in different land use types decreased with depth at all study areas.The differences of TOC, TN and TP contents among various land uses types were of great significance (P<0.001). In Dongda catchment, the comparison of TOC contents is shown as:grassland>forestland>tillage>abandoned farmland. The TN contents follows:grassland>tillage>forestland>abandoned farmland. The TP contents follows:tillage>grassland>forestland>abandoned farmland. In Baoxiang catchment, the comparison of TOC and TN contents is shown as: grassland>tillage>forestland. The TP contents follows:forestland>tillage>grassland. In Dounan area, the soil TOC, TN, TP contents of cultivated farmland were significantly higher than that of abandoned farmland influenced by fertilization. The average values of δ13C in the soil samples from all sites differed among land uses. In the Dongda catchment,δ13C value in soil ranged from -26.63%o to -20.01‰. It was then deduced that the organic matter could mainly derived from terrigenous C3 plant. In Baoxiang catchment, the average values of δ13C in the soil samples from tillage and forestland were -20.35%o±0.15%o and -17.98%o±0.75%o, respectively. Generally, reflecting that organic matter in the study areas maybe mainly derived from terrigenous C4 plant. The variation ranges of δ13C values in different soil profiles reflected the degree of soil organic matter decomposition. Combined with the feature of soil TOC profile, the variations of δ13C show that the distribution of soil organic matter has obvious regional characteristics and the organic matter transformation is more complex. Different nitrogen forms in the soil under various land uses are different, but organic nitrogen is the dominant form of transferable nitrogen. In addition to the Dounan region, the relative concentrations of different nitrogen form ranked according to the following sequence of SOEF-N>SAEF-N>IEF-N>WAEF-N in the other two catchment. Overall, grassland and forestland are of great significance to maintain and improve the regional soil quality because of the higher TOC, TN and various forms of nitrogen content. Although there are some differences in soil C/N, C/P, N/P ratio under different land use, the soil N and P efficiency is better in the study area.3. Effects of different land uses on soil nutrient storage and loss in small catchments.Results related to C,N,P density and storage differed for different land use types. The tillage exhibited the highest C,N,P density at studied areas. The density of C, N and P in grassland was higher than forestland. In Dongda catchment, the average amounts of C, N and P storage was 124.96×107kg,141.11×106kg and 52.61×106kg, respectively. In Baoxiang catchment, the average amounts of C, N and P storage was 114.07×107kg,131.53×106kg and 35.07×106kg, respectively. The storage of C, N and P in cultivated farmland at Dounan area was up to 87.26×106,11.62×106,8.53×106 kg, respectively. Land use significantly affects the surface nutrient loss. The surface nutrient loss in the tillage was significantly higher than that in other land uses and the grassland exhibited the lowest nutrient loss at the surface soil in Dongda and Baoxiang catchment. At Dounan region, the surface nutrient loss in the cultivated farmland was higher than that in abandoned farmland. The decline in soil quality associated with soil erosion and decreased soil nutrients. The soil nutrient loss of cultivated land was the largest under different land use in three research areas, which has the greatest potential to non-point source pollution, especially, the problem of P loss should be highlighted. The grassland and forestland would be beneficial for regional environmental protection, resulting from their reduction of the nutrients loss. At the small catchment scales, the relationship between soil nutrients and soil erosion can be modeled using the mathematical function.4. Sedimentary record of catchment erosion and trophic status changes.Bulk geochemical parameters were measured in sediment cores at two sites in Dianchi watershed to track the impacts of anthropogenic activities on the coastal environment over the last 100 years. The catchment erosion and trophic status changes were studied by an interpretation of the variations of multi-proxies. The average sediment rate was 0.51 cm/y in the Dongda catchment and 0.58 cm/y in the Baoxiang catchment. The interaction of the large-scale land reclamation and rainfall were related to high sediment accumulation rates (SARs) and the variation of SARs could reflect the catchment soil erosion well. The TOC, TN and TP showed an upcore increasing tendency in two sediments. Organic matter within the sediments was deduced mainly from C3 land plants and algaes in the Dongda catchment, whereas the changes of δ13C are closely linked with the succession of aquatic and terrestrial plants. Nitrogen forms in reservoir sediments play an important role in its biogeochemical cycling. IEF-N, WAEF-N, SAEF-N and SOEF-N were the potential bioavailable nitrogen form. SOEF-N was the highest content of bioavailable nitrogen in the sediments. Moreover, NH4+-N was the main component of IEF-N, WAEF-N and SAEF-N and play a key role in the eutrophication degree of overlying water. The results of sedimentary records indicated enhanced agricultural activities such as land use changes, population increase, extensive use of fertilizers, fish aquaculture and the discharge of domestic waste water resulted in dramatic fluctuations in the lake environment and contributed to the eutrophic process. |
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