Soil Erosion And Soil Carbon Dynamics Assesed By Using Fallout Radionuclides Techniques

Accelerated soil erosion and altered sediment distribution are a growing problem as a result of anthropogenic activities and global climate change. To understand the changes of soil erosion and soil erosion-induced carbon dynamics, it is important to identify the contributions of these factors to sustainable land use and management, but also to take into account the uncertainty regarding the balance of terrestrial carbon. Fallout environmental radioactive nuclides（FRNs）- 137 Cs and 210 Pb and 7Be- were considered to be powerful tracers for quantifying soil and soil organic carbon redistribution under different scale when compared with the traditional methods. However, there are still some shortcomings with these methodologies. We wanted to investigate the role of soil erosion and redistribution in carbon cycling as well as assess the effectiveness of soil conservation measures in China. Thus, we applied the FRN sampling method under variable temporal and spatial scales at study sites in typical water erosion regions（sloped croplands in the Loess Plateau, Upper Yangtze River and black soil region in Northeast China） and wind erosion regions（wind eroded farmland landscape in Fengning, Hebei Province）. These measurements will provide important reference values for establishing the quantitative research methodology for soil erosion and carbon cycling and provide scientific data for the precise evaluation of carbon sequestration effects for regional soil and water conservation and ecological restoration projects. The following conclusions could be drawn from this study:（1） When using FRN to assess soil redistribution, stratified simple random sampling and composites considerably reduces sampling and measurement costs for the allocation of resources across variable scales of variation（fields, catchments, regions etc.）. FRN repeated sampling methods can be used to detect changes over time in 137Cs-derived net soil redistribution in relation to the historical changes in land use and soil management.（2） Vegetation restoration, terrace engineering and conservation tillage measures can effectively reduce the soil erosion in slop fields landscape. Using FRN tracer technology, the study results found that shrubs are more effective than grasses at reducing soil erosion in the southwest mountainous regions. Furthermore, trees without litters can do not effectively control soil erosion. When compared with slope land, terraced fields in Yan‘an had 49% less soil loss whilst forests and grasslands in the region had 80% less soil loss. In Fengning, soil erosion rates decreased by 33%-44% following three years of no-tillage and with stubble measures when compared with conventional tillage practices. In Baiquan in northeast China, soil erosion rates were found to be 14% less in terraced land and 34% less in land that has underwent contour cultivation measures, when compared to conventional tillage practices.（3） Ignoring the change in soil erosion rates will result in an overestimation of the carbon sequestration effect of soil conservation measures. Based on the 7Be tracer technology and soil carbon density measurements, the study results show that a net negative C sequestration（source） in the topsoil（0–20mm） for CG（conservation grassland for 5 years） and CL treatments（minimum tillage plus with stubble for 3 years） were –0.42 and –0.37 t C ha–1 year–1, respectively. The C sequestration rates for the topsoil with the traditional approach（without soil erosion） were overestimated by 126% for CG and 130% for CL treatments. Net C sequestration（0–245 mm） considering soil organic carbon（SOC） redistribution for CG（0.27±0.12 year–1; 5 years） and CL（1.53±0.13 t C ha–1 year–1; 3 years） revealed an overestimate of 196% and 31% by the traditional approach without considering SOC redistribution（CG,0.8±0.03 t C ha–1 year–1; CL, 2.0±0.06 t C ha–1 year–1）, respectively.（4） Sustained high magnitude erosion processes significantly reduce the SOC mineralization and this organic carbon sink should not be neglected in evaluation processes. Using the established model whereby the relationship of SOC redistribution and soil respiration is based on the field monitoring data, the results found that in-situ soil erosion resulted in a CO₂ emission reduction rate of 0.43 ± 0.15 t CO_2C ha- 1yr – 1 in eroded areas of sloped land. The in-situ soil net CO₂ emissions reduction rate was 0.32 ± 0.22 t CO_2C ha- 1yr – 1 across the entire sloped land.