Abstract
We compared nitrogen uptake by riparian plants at various distances from the river channel along the Chikuma River, using a stable isotope analysis. Nitrogen stable isotope ratios δ15N of plants in the lower floodplains were relatively high, and increased from upper to lower reach, suggesting use of nitrogen from the river water, which was affected by human activities in the watersheds. On the other hand, δ15N of plants in the higher floodplains were relatively low, and not statistically different among sites, indicating no more use of nitrogen derived from the river water. A nitrogen fixing plant, black locust (Robinia pseudoacacia) forms riparian forests in the higher floodplains, where we observed a 42-fold increase in nitrate concentration of soil compared with that in the lower floodplains. Black locust forests may bring reactive nitrogen into the river ecosystem. The higher floodplains were different from the lower ones not only in physical conditions such as deposition and hydrology but also in nitrogen uptake by plants, and they had little relationships with river water in nutrient uptake.
