Influence of asphalt pavement construction processes on urban soil formation in Tokyo

Urban soils have been drastically disturbed and modified by human activities. Soil sealing, necessary for transportation and infrastructure development, is one such process, which often results in negative effects on the soil ecosystem functions. However, sealed soils can play a critical role in material cycling through ground surfaces in urban ecosystems. This study investigated the effects of asphalt sealing on the mineral soils beneath the roads in Tokyo. The results confirmed that mineral soils beneath asphalt pavements were characterized by high pH levels, CaO content, inorganic carbon (IC), and electric conductivity (EC) due to liming for road material fixation at the subbase layers. An analysis of the concentrations of IC and Ca, or pH value suggested existence of calcium carbonate (CaCO ₃) mainly due to liming and absorption of atmospheric CO ₂ at subbase layers. Relatively high content of sulfur in the subbase layers is likely attributed to asphalt contaminants originating from petroleum. Removal of the original surface soils, for new road development, decreased oxides and organic matter content. Contents of oxides in top mineral soils were low as compared to those of natural Andosols by mixing of subbase materials. This study suggested that construction of asphalt pavement induced specific soil genesis beneath asphalt pavement such as soil alkalinization, calcium carbonate accumulation, and technogenic material mixing. First, surface soil removal for the construction of asphalt pavement decreases organic matter content in mineral soils. Concurrently technogenic materials mix with top mineral soils. Second, Ca dissolved into water from the subbase layer can react with dissolved inorganic carbon followed by calcium carbonate formation and Ca cation migration into the deeper horizon. As asphalt pavement intercepts litter supply, organic matter accumulation was prevented in mineral soils beneath the asphalt pavement. Our study confirmed that owing to the water infiltration, through pervious asphalt and the cracks developed on the asphalt surface by heavy traffic, material cycling exists even in mineral soils beneath the asphalt pavements.