Volcanic ash soils contain appreciable quantities of anion adsorption capacity (AEC) due to the variable charge, and they adsorb anions by non-specific adsorption. To study the effect of this characteristic on the NO 3 dynamics in forest soil, the NO 3 adsorptiondesorption process by the A-horizon and the B-horizon soils that are under the influence of volcanic ash were determined by the batch and the column methods. The batch experiment indicated that the adsorbed NO 3 by the B-horizon soils increased with the additive KNO 3 concentration. This result revealed the characteristics of the anion adsorption due to the variable charge soils. The A-horizon soils adsorbed less amount of NO 3 than B-horizon soils. The NO 3 adsorption amounts were negatively correlated with the total C concentration in soils, suggesting the amount of positive charge in the soil was controlled by the organic matter. Meanwhile, the NO 3 adsorption amounts were negatively correlated with pH in the A-horizon soils. Therefore, the A-horizon soils have variable charge due to the presence of humic substances, which was different from the B-horizon in composition of the positive charge. From the column experiment, the NO 3 concentrations in soil water of the field before and after cutting, and the thickness of the B-horizon soil, the increment of NO 3 adsorption amounts in the subsoil after cutting was calculated to be approximately 60 kgN ha 1 . This value indicated that the subsoil temporarily retained large amounts of N decomposed and released from forest floor, which inhibited the rapid increase in the NO 3 concentration of stream water. When the soil columns filled with adsorbed NO 3 were combined with deionized water, the entire additive NO 3 was finally leached out from the columns. Therefore, it is expected that, while the increase in NO 3 leaching from the cutting watershed would be reduced, the period of the increase would be long. From the column experiment and field observation, SO 4 2 leaching is also expected to increase followed by NO 3 leaching, in the future.

Effects of Anion Adsorption Characteristics of a Volcanic Ash Soil on Long-term NO 3 Leaching from a Partial Clear-cut Cedar and Cypress Watershed. J. Jpn. For. Soc. 91: 184 191. Long-term changes in the water quality of soil water and streamwater were monitored before and after cutting at the lower slope position in a small forested watershed composed of artificial stands of old aged Japanese cedar and cypress in the northern Kanto region in Japan. The NO 3 concentration in soil water directly reflected the N dynamics, i.e., the reduced nutrient uptake and accelerated nitrogen mineralization in the surface soil layer immediately after the cutting, and the increased nutrient uptake induced by regenerated trees several years after the practice. In contrast, the NO 3 concentration in streamwater maintained higher level than before cutting in the sixth year after the practice. The NO 3 concentrations in high and low discharge, which is dominated by direct flow and base flow, respectively, were calculated from the relationships between discharge rates and the NO 3 concentration in streamwater. The results suggested that the NO 3 distribution in soil affects the NO 3 concentration in streamwater. Changes in the NO 3 concentration in the subsoil were calculated using the HYDRUS-1D model, and these changes were consistent with previously measured seasonal and long-term changes in the NO 3 concentration in streamwater. This finding implies that NO 3 adsorption characteristics of the volcanic ash subsoil affects NO 3 leaching into streamwater. In order to clarify the effects of the aging of artificial forests, the method of cutting, and environmental changes on N dynamics in soil and streamwater, it is necessary to consider the NO 3 adsorption characteristics of volcanic ash soil.