Abstract
The spatial and vertical differences in soil nitrogen (N) availability were evaluated in-situ by ion exchange resin (IER) along a slope in a Cryptomeria japonica plantation to clarify the relationship between laboratory rates of N transformations and in-situ N availability. The NC_3^--N dominated IER inorganic N at all positions on the slope, in contrast to the extreme spatial difference of laboratory nitrification to N mineralization ratio. The IER total inorganic N at a depth of 5 cm in the mineral soil was 15.9-57.5 kg-N ha^<-1> year^<-1> and showed a weak negative correlation with the distance from the bottom of the slope (r_s=-0.32, P<0.10), although laboratory N mineralization did not changed with positions on the slope. This may be because of higher nitrification at a lower position on the slope and the greater mobility of NO_3^--N. The IER total inorganic N at a depth of 10 cm or deeper was 9.8-15.6 kg-N ha^<-1> year^<-1> and showed a minor spatial difference at selected four points on the slope. Two-way ANOVA including the result at a depth of 5 cm suggested spatially different vertical change of IER total inorganic N. The IER total inorganic N decreased with soil depth at a lower position on the slope, while it showed less or no decrease with soil depth at an upper position. These results and the vertical distributions of fine roots suggested lower utilization of inorganic N by plants at an upper position on the slope.
