Soil erosion rates on forested mountain hillslopes estimated using137 Cs and210 Pbex
Wakiyama, Y.; Onda, Y.; Mizugaki, S.; Asai, H.; Hiramatsu, S.
ISSN/ISBN: 0016-7061 DOI: 10.1016/j.geoderma.2010.06.012
Although the excess of lead-210 ((210) Pb(ex) ) has been described as an alternative or complementary nuclide to cesium-137 ((137) Cs) for soil erosion assessment, the potential use of(210) Pb(ex) in forested environments has not been examined sufficiently. In this study, to investigate the potential use of(210) Pb(ex) for the assessment of soil erosion on various forested hillslopes, we compared soil erosion rates derived from(210) Pb(ex) to those from(137) Cs obtained in forest stands. The study was conducted in four types of forest: a Japanese cypress stand with a bare forest floor, a Japanese cypress stand with a fern understory, a Japanese cedar stand with sufficient litter cover, and a natural broadleaf forested stand in the Tsuzura River basin of southeastern Japan.(137) Cs and(210) Pb(ex) inventories for seven potential reference sites on flat ridge tops and along transects in each stand were measured using gamma-ray spectrometry to calculate soil erosion rates. To describe the behavior of organic matter in each stand, the amounts of eroded sediment and organic matter were observed for approximately 2 years in erosion plots (1.7-2.0 m long, 0.5 m wide) in the Japanese cypress stand (bare), Japanese cedar stand, and broadleaf forest stand. From the preliminary assessment of potential reference sites, spatial variability in(210) Pb(ex) was comparable to that of(137) Cs. Mean erosion rates estimated from both Cs and Pb were highest in the Japanese cypress stand (bare), followed by the Japanese cypress stand (fern), the broadleaf forest stand, and the Japanese cedar stand. This magnitude relationship was consistent with that of observation results of eroded sediment in erosion plots. In the Japanese cypress stand (bare), a significant correlation was found between Cs- and Pb-based soil erosion rates in the Japanese cypress stand (bare) while no significance was found in the other three stands. These results provide evidence of the feasibility of(210) Pb(ex) for soil erosion assessment in forest stands with bare forest floors. In the other three stands, the preferential loss of(210) Pb(ex) associated with organic matter may explain the poor correlation between the two erosion rates.