Soils on Man-Made Islands in Tokyo Bay

Abstract

Man-made islands occupy about 230 ha in Tokyo Bay. Surface soil for green spaces covering the reclaimed land consists of bark compost brought from greenery areas in the Tokyo Metropolis. Limited amounts of degradable organic wastes are also mixed with the surface soil. Surface soil is usually top-dressed to a depth of 30 cm, to ensure root growth of the planting base. The deep soil layer is strongly compacted during the construction process by the use of heavy machines, and contains miscellaneous artificial materials such as asphalt, concrete, bricks, and dredged materials. Even with complicated soil composition, early stages of the soil development process are recognized in soil properties with time and/or depth. On the man-made island established on 1987, artificial soils covered with trees planted within this decade gradually develop as Urbic Spolic Technosols due to high contents of artifacts derived from rubble and refuse of human settlements and industrial waste. The boundary in the soil profile, which changes from abrupt to diffuse with the passage of time from afforestation, suggests soil mixing process by development of plant root systems at the boundary. Land improvement works using heavy machines strongly influences the physical difference between surface and deeper horizons. Soil development processes were verified through analysis of the chemical properties of the surface horizons, which have higher contents of organic carbon and nitrogen, and lower pH values, common properties of soils beneath well-developed vegetation. Development of soil on the man-made islands definitely increases with the growth of vegetation.