Spatial distribution of the denitrification zone in a forested headwater catchment in central Japan: Influences of hydrologic conditions

The spatial distribution of the denitrification process was investigated in tandem with hydrometric and chemical groundwater monitoring in a temperate forest catchment in Japan, to determine the effect of hydrologic conditions on the pathways of nitrogen loss from the catchment and streamwater chemistry, especially the temporal and spatial changes in NO₃^- concentrations in soil and ground water. Field investigations were conducted in an unchanneled headwater catchment (0.59 ha) in the Kiryu Experimental Watershed (KEW), which is covered by mixed stands of secondary broad-leaved deciduous trees and planted coniferous trees. The input-output analysis of nitrogen in the catchment found that approximately 6 kg ha^-1 yr^-1 (average 1995-2001) of inorganic nitrogen was discharged through the stream system, and the amount of denitrified nitrogen was estimated at 4-7 kg ha^-1 yr^-1, suggesting that a significant portion of the nitrogen loss from the catchment was by denitrification. A previous study of the denitrification characteristics in the riparian zone of this catchment showed that denitrification occurred intermittently, corresponding to fluctuations in the groundwater level. The intensity of denitrification depended strongly on the saturation of the near-surface soil. The present investigation showed that the dissolved N₂O concentration, which is an indicator of denitrification intensity, was consistently higher in the shallower portion of the groundwater body than in the deeper portion. Correspondingly, the NO₃^- concentration decreased with depth in the groundwater body. N₂O generated in the shallower portion of the groundwater body was transported by lateral movement of the groundwater, and then released to the atmosphere at the exit point. Therefore, intense, consistent N₂O emissions occurred in the area surrounding the exit point, while emissions were intermittent in the riparian zone. In addition, in the hillside part of the catchment, which was covered by relatively dry forest soils, no N₂O emissions were detected, although the mineralization and nitrification potential were higher than in the riparian zone. These observations indicate that complete saturation by water is one of the critical factors determining the denitrification zone in this catchment.