Topographical variations in a plant–soil system along a slope on Mt Ryuoh, Japan

The plant–soil system was studied at different topographic levels (i.e. ridge, backslope and footslope) along a slope in a Cryptomeria plantation. Soil solution chemistry at each representative topographic plot was investigated. Tree height and diameter of Cryptomeria decreased upslope. The understory species composition changed along the slope. The upper part of the slope with Oa horizon soil N transformation was characterized by ammonification, while most of the inorganic N in the lower part of the slope without Oa horizon was nitrified. The inorganic N form in the soil solution corresponded with soil N transformation. Ammonium was the dominant inorganic N at the ridge, while NO3 predominated at the foot of the slope. Soil solution chemistry was similar to throughfall at the ridge. At the foot slope, the chemical composition of the soil solution was different from throughfall due to high NO3− concentrations. This suggests that the inorganic N form regulated not only N concentration but also cation concentrations. The soil N transformation pattern is important in nutrient cycling.