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TECHNICAL REPORTS

Surface Water Quality

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

^* Corresponding author (a-hykw{at}chem.agr.hokudai.ac.jp)

Received for publication September 11, 2005. We evaluated the impacts of natural wetlands and various land uses on stream nitrogen concentration in two grassland-dominated catchments in eastern Hokkaido, Japan. Analyzing land use types in drainage basins, measuring denitrification potential of its soil, and water sampling in all seasons of 2003 were performed. Results showed a highly significant positive correlation between the concentration of stream NO₃–N and the proportion of upland area in drainage basins in both catchments. The regression slope, which we assumed to reflect the impact on water quality, was 24% lower for the Akkeshi catchment (0.012 ± 0.001) than for the Shibetsu catchment (0.016 ± 0.001). In the Akkeshi catchment, there was a significant negative correlation between the proportion of wetlands in the drainage basins and stream NO₃–N concentration. Stream dissolved organic nitrogen (DON) and carbon (DOC) concentrations were significantly higher in the Akkeshi catchment. Upland and urban land uses were strongly linked to increases in in-stream N concentrations in both catchments, whereas wetlands and forests tended to mitigate water quality degradation. The denitrification potential of the soils was highest in wetlands, medium in riparian forests, and lowest in grasslands; and was significant in wetlands and riparian forests in the Akkeshi catchment. The solubility of soil organic carbon (SOC) and soil moisture tended to determine the denitrification potential. These results indicate that the water environment within the catchments, which influences denitrification potential and soil organic matter content, could have caused the difference in stream water quality between the two catchments.

Abbreviations: DOC, dissolved organic carbon • DON, dissolved organic nitrogen • PON, particulate organic nitrogen • SOC, soil organic carbon • SON, soil organic nitrogen • TDN, total dissolved nitrogen • TN, total nitrogen • WESOC, water-extractable soil organic carbon • WESON, water-extractable soil organic nitrogen