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Evaluating Stream Water Quality through Land Use Analysis in Two Grassland Catchments

Impact of Wetlands on Stream Nitrogen Concentration

^a Laboratory of Soil Science, Graduate School of Agriculture, Hokkaido University, Sapporo, 060-8589 Japan
^b Laboratory of Nutrient Cycling, Field Science Center for Northern Biosphere, Hokkaido University, Sapporo 060-8589, Japan

View larger version (31K): [in a new window]   Fig. 1. Location map of the study sites and distribution of sampling sites in the Akkeshi and Shibetsu catchments. Denotes water sampling sites; denotes sites in riparian forest adjacent to grassland for measuring denitrification potentials; denotes sites of the wetland for measuring denitrification potentials.

View larger version (44K): [in a new window]   Fig. 2. Distribution of mean NO3–N concentrations in the Akkeshi and Shibetsu catchments.

View larger version (26K): [in a new window]   Fig. 3. Relationship between the mean dissolved organic nitrogen (DON) and carbon (DOC) concentrations in the Akkeshi and Shibetsu catchments. Error bars represent standard deviations for data from four seasons.

View larger version (27K): [in a new window]   Fig. 4. Relationship between the proportion of upland in drainage basins and NO3–N concentration in stream water in the Akkeshi and Shibetsu catchments. Error bars represent standard deviations for data from four seasons.

View larger version (13K): [in a new window]   Fig. 5. Results of redundancy analysis. Solid lines denote explanatory variables (proportion of major land use types: uplands, forests, urban areas, wetlands, wastelands); dotted lines denote the response variables (concentrations of total nitrogen [TN], NO3–N, NH4–N, and dissolved organic nitrogen [DON]); denotes sampling sites in the Akkeshi catchment; and • denotes sampling sites in the Shibetsu catchment.

View larger version (69K): [in a new window]   Fig. 6. Denitrification potentials of soils of three land use types. A_ denotes drainage basins in the Akkeshi catchment and S_ denotes drainage basins in the Shibetsu catchment. The values along the x axis show the depths of the soil samples analyzed.

View larger version (17K): [in a new window]   Fig. 7. Relationship between the ratio of water-extractable soil organic carbon (WESOC) to soil organic carbon (SOC) and log10 (denitrification potential) in the Akkeshi and Shibetsu catchments. Error bars represent standard deviations of three replications.

View larger version (19K): [in a new window]   Fig. 8. Precipitation and fluctuation of stream water level and ground water table. Precipitation (a), water table fluctuation in the riparian forest and grassland (b), and water table fluctuation in the wetland (c) near the points where denitrification potentials were measured in the Akkeshi catchment. The diagrams to the right [(b') and (c')] show how the water table sensors were set up and the points at which denitrification potentials were measured. "A" shows the ground level at the river bank and "B" shows the ground level in the wetland 10 m from the edge of the river.