5:15 PM - 6:30 PM
[HTT21-P13] Effects of additional nitrogen input and different river slope on the outflow from upstream nitrogen-saturated forests
Keywords:nitrogen saturation, stream water quality, nitrogen isotope, land use, Topographic Index
The objectives of this study were to clarify (1) whether the nitrogen dynamics (the concentration of NO3-, δ15NNO3 and δ18ONO3) in the downstream were affected by the nitrogen-saturated forests in the upstream, (2) the influence of nitrogen runoff from agriculture and urban area in the downstream, and (3) the influence of the river gradient on denitrification.
NO3- concentration and the stable isotopes, δ15NNO3 and δ18ONO3 were measured in the Tatara River Basin, located in northern Kyushu, western Japan. Water samples were mainly collected from branch rivers; Ino River, Sue River and Umi River. Deciduous mature forest dominates the ridges in the upstream, with paddy, farmland and urban dominating in the middle and downstream. Topographic Index (TI) was calculated by using Digital Elevation Model (DEM) with ArcGIS software.
Our results showed that the NO3- concentration in the upstream of three rivers (33.9-82.8 µmol/L) were higher than the other non-saturated forests. Lower δ18ONO3, higher NO3- and δ15NNO3 in the downstream were be found in Sue River and Umi River. On the other hand, Ino River showed different trend that lower NO3- concentration, higher δ15NNO3 and δ18ONO3 concentration in the downstream. The higher NO3- concentration in the downstream in Sue River and Umi River was affected by the increasing agriculture and urban areas in the downstream. However, the NO3- concentration in the downstream of Ino River decreased with the increasing agriculture and urban areas.
The average TI value showed no difference between rivers. However, the distribution of TI values in Ino River was different from other rivers. Ino River has smaller changes in gradient from upstream to downstream and fewer agriculture and urban areas in the downstream than the other two rivers. These differences could result in the higher probability of denitrification occurring and the lower NO3- loading in the downstream in Ino river.
The nitrogen-saturated upstream forests in this watershed has large impact on the nitrogen sources to the downstream. In the areas with steeper gradient, the high concentration of nitrogen runoff was supplied to the agriculture and urban areas. On the other hand, in those areas with gentle gradient, the nitrogen loading in the downstream also decreased due to the gentle gradient of the watershed and it was suggested that the rate of denitrification and decrease NO3- concentration in the downstream was also high.