ANALYZING THE EFFECTS OF LAND USE ON SOIL EROSION IN SOUTHWESTERN CHINA USING FALLOUT RADIONUCLIDES

Establishing the effect of deforestation and reforestation on erosion rates is necessary to understand the impact of land use changes on the environment. Widespread deforestation dominated southwestern China from 1950 to 1980. In 1998, the Chinese government enacted the Returning Farmland to Forest Program (RFFP) to prevent flooding and downstream erosion, believed to be caused by deforestation. We quantify the effects of deforestation, reforestation, and agriculture on short-term erosion rates by analyzing the concentration of fallout radionuclides (FRNs) in detrital sediments collected in southwestern China. We hypothesize that deforestation increased surface erosion from the 1950s through 1980s, while reforestation since the late 1990s has slowed erosion rates.

Using 137Cs and 210Pb concentrations in detrital sediment, we are able to estimate the depth and timing of erosion in the upstream watershed. 137Cs, which results from atmospheric testing of nuclear weapons, is a tracer for erosion of surface material exposed during the 1950s-1960s. Samples that contain 137Cs must be the result of both minimal and shallow erosion. 210Pb is a tracer for current surface material, due to its ongoing deposition. The presence of 210Pb indicates slow, surface erosion. Thus, watersheds with high erosion during the deforestation will have little to no detectable 137Cs, while watersheds with slow erosion during reforestation will have detectable 210Pb. The basins studied include Red, Yangtze, Salween, Mekong, Irrawaddy. Prior research measured in situ and meteoric 10Be, which we can compare to the short-lived FRN data.

Preliminary data show that only 3 of 59 detrital samples have detectable 137Cs. The absence of 137Cs indicates erosion rates were high during the time of deforestation. However, 42 of the 59 samples have detectable 210Pb. The moderate levels of 210Pb indicate that erosion has since slowed. Since 137Cs and 210Pb are grain size dependent, future work will include measuring FRN concentration in <63μm fraction and leaching samples to improve detectability for 210Pb. These data suggest that reforestation may have decreased erosion rates in southwestern China.