Abstract
Acid rain and nitrogen-based fertilizers are the main contributors to soil environmental pollution in many parts of the world. According to the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), approximately 44.3 to 44.6 tgN/yr of nitrogen oxides are added to the troposphere by fossil fuel emissions. These additions can lead to acid rain. Another environmental problem is the contamination of the soil by the addition of excess nitrogenbased fertilizer. The ultimate result of acid rain and excess N-fertilizer use is acidification of soil water leading to serious problems of eluviation of aluminum and clay or dissolution of harmful heavy metals. The volcanic ash soils in Japan are known as being resistant to acidification. However it is not clear how volcanic ash soils are affected by acid rain and fertilizer.
This study examines the response of Kanto loam to pollution of subsurface layers by nitrate. Chemical properties of Kanto loam soil samples were assessed by exchangeable acidity and soil column tests. The pH and pK of samples were determined by exchangeable acidity tests using distilled water and potassium chloride solution. The saturation column test was conducted to obtain a break-through curve and to calculate the dispersion coefficients by the one dimensional convective-dispersion equation. The CDE helps our understanding of the underlying mechanisms of subsurface solute movement. The extent of nitrate transfer was confirmed by percolation of 0.15mmol nitric acid through columns packed with 25g of the soil sample and 2, 800g of silicate sand.
The shift in exchange capacity was due to a change in the allophane and halloysite contents of the clay mineral fraction. The pH change from 5 to 7 over the full duration of the experiment. The study revealed the following: 1) It is considered that the buffering capacity is derived from the clay minerals in the loam. 2) However, some parts of the shallow layer have a tendency to decrease in buffering capacity. 3) The Kanto loam at the investigated site was assumed to either be susceptible to a collapse of clay or to consist of a less stable form of allophane.
The environmental pollution of the subsurface layer of this Kanto loam was thus attributed to excess nitrate in the soil water. If an acidic pollutant was introduced into this Kanto loam, the soil would be seriously damaged.
