Hydrological influences on spatiotemporal variations of δ15N and δ18O of nitrate in a forested headwater catchment in central Japan: Denitrification plays a critical role in groundwater

To elucidate the internal nitrogen cycle and nitrogen input and output, we measured delta N-15-NO3-, delta O-18-NO3-, and NO3- concentrations in rainfall, soil water from an unsaturated soil horizon, groundwater, and stream water at a small forested headwater catchment. In soil water, the mean NO3- concentration did not decrease concomitantly with soil depth, despite a drastic decrease of delta O-18-NO3- with soil depth, indicating immediate consumption of atmospheric NO3- and production of nitrified NO3-. The spatiotemporal distribution of delta N-15-NO3- and delta O-18-NO3- in soil water indicated that plant uptake contributed mainly to NO3- consumption and that denitrification was negligible. In contrast, denitrification played a critical role in groundwater in decreasing the NO3- concentration. Fluctuations of delta N-15-NO3-, delta O-18-NO3-, and NO3- concentration in groundwater, however, differed from expectations for occurrence resulting from denitrification only. A plausible explanation for the lack of a concurrent increase in delta N-15 and delta O-18 of NO3- is that nitrification and denitrification occur concurrently in groundwater and that denitrification is more important than nitrification. The amount of denitrification in the groundwater body was controlled primarily by groundwater residence time. The combination of hydrological flow path from groundwater to stream water and denitrification in the groundwater critically control the amount of leaching of NO3- and temporal distribution of NO3- concentration in stream water. The possibility exists that denitrification in groundwater strongly influences nitrogen leaching in forests because denitrification occurred even in this catchment, which consists of weathered granitic bedrock and predominantly typic udipsamment, with low organic matter content in the soil.