Spatial variation of denitrification potential of grassland, windbreak forest, and riparian forest soils in an agricultural catchment in eastern Hokkaido, Japan

Riparian forest soils have a high denitrification potential (DP), but the effect of their spatial distribution on DP within a catchment has rarely been assessed. To provide insights into this effect, we evaluated the spatial distribution of DP and its control factors in the Shibetsu River Catchment (SRC), eastern Hokkaido, Japan. Soil samples from August 2008 were used in the lab to determine the DP (using the acetylene inhibition method) of grassland, windbreak forest, and riparian forest soils in the SRC. Soil DP ranged from 0.001 to 64 mg N kg soil−1 d−1 and followed a log-normal distribution. Soil DP was highest in the surface peat soil of a low-elevation riparian forest. The land-use type and soil depth both significantly affected soil DP, with higher DP in riparian forests and surface soils. Soil DP increased with increasing soil moisture content and water-extractable soil organic carbon (WESOC). The clay, moisture, and WESOC contents in the surface soils decreased significantly with increasing elevation, resulting in increased soil DP in low-elevation riparian forests. The stream NO3− concentration was significantly correlated with the upland (grassland and cropland) proportion (%) of the catchment's area, and the ratio of stream NO3− concentration to this proportion tended to be lower in low-elevation areas, indicating higher NO3− denitrification. We conclude that low-elevation riparian forest soils have a higher DP due to the higher moisture, clay, and WESOC contents of their soils, and that these factors control riverine NO3− export in the SRC.