Hydrological influences on spatiotemporal variations of δ 15 N and δ 18 O 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 δ 15 N‐NO 3 − , δ 18 O‐NO 3 − , and NO 3 − 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 NO 3 − concentration did not decrease concomitantly with soil depth, despite a drastic decrease of δ 18 O‐NO 3 − with soil depth, indicating immediate consumption of atmospheric NO 3 − and production of nitrified NO 3 − . The spatiotemporal distribution of δ 15 N‐NO 3 − and δ 18 O‐NO 3 − in soil water indicated that plant uptake contributed mainly to NO 3 − consumption and that denitrification was negligible. In contrast, denitrification played a critical role in groundwater in decreasing the NO 3 − concentration. Fluctuations of δ 15 N‐NO 3 − , δ 18 O‐NO 3 − , and NO 3 − concentration in groundwater, however, differed from expectations for occurrence resulting from denitrification only. A plausible explanation for the lack of a concurrent increase in δ 15 N and δ 18 O of NO 3 − 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 NO 3 − and temporal distribution of NO 3 − 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.