Hydro-geomorphological study for the formation of soil water chemistry in hilly grassland within small watershed

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To reduce the nutrient outflow from mountain pasture, it needs to know the hydrological processes and chemical material transport within a watershed. The aim of this research is to clarify the variation of soil water and the formation of water chemistry on the landforms of hilly grassland within a small watershed. Experimental watershed was selected in a pasture of research station of NILGS, Nagano, Japan. Soils are Andisols. We traced the soil water condition and the nitrate-nitrogen content on convex ridge slope and concave valley bottom in the watershed. Soil water condition was measured by using tension meters as the matric potential. Water samples were collected by porous cup method.Rainwater on the ridge slope mostly infiltrated into soil. The gradient of matric potential is large between the layers of 20cm depth and 50cm depth. This shows the soil water moved laterally at the 20-40cm deep layer on the ridge slope. Matric potential of soil water on the valley bottom increases into deeper soil layers. Due to the high hydraulic conductivity and the abundant macro pore of valley bottom soil the soil water from surrounding areas accumulates and the water percolates into deeper soil-horizons.Nitrate-nitrogen concentration was below 1 mgN/L on the ridge slope. The concentration on the valley bottom was high over several mgN/L. The concentration increases almost linearly along the valley bottom in the fertilizer-applied area and decreases exponentially in the un-fertilized section. Along the valley bottom the increase of soil water pH also occurs. Thus the nitrogen decrease suggests that the denitrification processes within the valley-bottom reduce the concentration of nitrate-nitrogen.These results indicate that the nitrate-nitrogen accumulates into the valley bottom accumulated through the downward movement process from the 20-40cm deep zone of ridge slope in the fertilized area.