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 degrees) and high stream NO3- drainage rates (8. 4 to 25.1 kg N ha(-1) yr(-1)) that were greater than bulk precipitatio n N input rates (7.5 to 13.5 kg N ha(-1) yr(-1)). Higher rates of nitr ification potential at near-stream zones were reflected in greater NO3 - contents for soil at the near-stream zones compared with ridge zones . Both stream discharge rates and NO3- concentrations in deep unsatura ted soil at the near-stream zones were positively correlated to NO3- c oncentrations in stream water. These relationships, together with high soil NO3- contents at the near-stream zones, suggest that the near-st ream zone was an important source of NO3- to stream water. Nitrate flu x from these near-stream zones was also related to the drainage of cat ions (K+, Ca2+ and Mg2+). The steep slope of the watersheds resulted i n small saturated areas that contributed to the high NO3- production t high nitrification rates) in the near-stream zone. (C) 1998 John Wiley & Sons, Ltd.