Effect of deforestation on the transport of particulate organic matter inferred from the geochemical properties of reservoir sediments in the Noto Peninsula, Japan

Abstract

The effects of artificial deforestation in the 1970s and the subsequent recovery process on the transport of particulate organic matter were investigated using reservoir sediments obtained from the Noto Peninsula in Japan. In 1975, the sedimentation rate increased to about 2.7 times its previous value because of deforestation and plantation activities in the catchment. The sedimentation rate remained high until 1991, suggesting that the erosion rate of soil and organic matter increased during the 15 years after the deforestation. This intensive erosion was induced by exposure of bare soil, forest management activities to remove understory vegetation, and heavy rainfall. Conversely, the d13C and d15N of the sediment organic matter has continued to decrease from the mid-1980s to the present time, despite the fact that the sedimentation rate has recovered to pre-deforestation levels. The decrease of the δ13C and δ15N of the sediment organic matter reflects a decrease in the contribution of soil organic matter, although post-depositional diagenesis and variability of the isotopic composition of the aquatic organic matter also affect the δ13C and δ15N. The decrease in the soil organic matter contribution suggests gradual recovery of vegetation, accumulation of forest floor organic matter, and reduction of soil erodibility. These results indicate that the change in the transport of organic matter continued for at least 35 years after the deforestation and plantation.