Soil ripening following dam removal

Abstract

The onset of pedogenesis in dewatered reservoir sediments following a dam removal was evaluated using a ripening index that was initially developed to describe the condition of drained, marine sediments behind Dutch dikes. The drained reservoir exposed lahar terraces, mantled with reservoir sediment, upstream of the former dam at a similar geomorphic position to a downstream lahar terrace. Three years following dam removal, the exposed sediment has physically ripened (n-values are less than 0.7) over 1 m to the depth of a buried soil. However, the sediment has only chemically ripened to a depth of approximately 16 cm, the depth at which pH values become higher than those of the downstream terrace soils. Even at the surface, the sediment has not biologically ripened, as indicated by the low carbon-to-nitrogen ratios (13:1) relative to those of the downstream soil surface (28:1). These results indicate that chemical and biological ripening happen more slowly than physical ripening. As such, dewatered reservoir sediments likely reach field capacity before other crucial edaphic conditions have developed, such as the accumulation of plant-available iron and nitrate. This difference will greatly affect vegetation successional pathways in these newly created upland environments.