Spatial patterns of soil nitrate in Japanese forested watersheds: importance of the near-stream zone as a source of nitrate in stream water

Spatial patterns of N dynamics in soil were evaluated within two small forested watersheds in Japan. These two watersheds were characterized by steep slopes (>30°) and high stream NO−3 drainage rates (8·4 to 25·1 kg N ha−1 yr−1) that were greater than bulk precipitation N input rates (7·5 to 13·5 kg N ha−1 yr−1). Higher rates of nitrification potential at near-stream zones were reflected in greater NO−3 contents for soil at the near-stream zones compared with ridge zones. Both stream discharge rates and NO−3 concentrations in deep unsaturated soil at the near-stream zones were positively correlated to NO−3 concentrations in stream water. These relationships, together with high soil NO−3 contents at the near-stream zones, suggest that the near-stream zone was an important source of NO−3 to stream water. Nitrate flux from these near-stream zones was also related to the drainage of cations (K+, Ca2+ and Mg2+). The steep slope of the watersheds resulted in small saturated areas that contributed to the high NO−3 production (high nitrification rates) in the near-stream zone.