環境

The substrate of sediment bar in the mid-stream of a river is usually coarse due to ‘bed armoring’ caused during flooding events. Therefore, the nitrogen content of substrate is very low, and the growth of herbaceous plants is often restricted. Symbiotic N-fixation by legume plants is one of the most important processes of N-enrichment of these bars. Colonization of legume plant in infertile substrate could change the trophic level especially that of N. Succession of such plants also brings about favorable changes in soil particle size, nutrient and moisture content of substrate which may usher drastic change in the floristic composition.

In this study, we have compared clay and sandy sediments of mid-stream Tama River as tracers of factors associated in ecosystem changes. A total of 18 factors viz. moisture content, grain size distribution of soil (D₅₀), total nitrogen (TN), total carbon (TC), total phosphorus (TP), sulphur (S), stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N), inorganic nitrogen (NO₂^–2, NO₃^– and NH₄⁺), calcium (Ca), potassium (K), magnesium (Mg), copper (Cu), manganese (Mn), zinc (Zn) were analyzed. There were significant differences (Student's t-test, P<0.05) in concentrations of TN, TC, δ¹⁵N, Mg and Zn between clay and sandy soils. Therefore, these factors can be used as indicators of ecological changes in river system induced by fine sediment deposition.

In recent years, many N-fixing legume plants e.g. Pueraria lobata have been growing with native plants such as Miscanthus sacchariflorus in the river floodplain. Upstream floodplain areas also have been encroached by these kind of plants and as a result the soil became nutrient rich.

Tetsuo Uchida