NATURAL-ABUNDANCE OF N-15 IN A FOREST SOIL

Because of measurement difficulties, only a few studies on natural N-1 5 abundance (delta(15)N) of inorganic N in forest soil have been pursu ed despite its importance for interpretations of plant delta(15)N sign atures. To investigate stable N isotope ratios in inorganic N, the del ta(15)N values and concentrations of total N, NH4+-N, and NO3--N of fo rest mineral soils in four profiles were measured along a slope (altit ude of 765-870 m) in a coniferous (Japanese red cedar, Cryptomeria jap onica D. Don) forest in Japan. Generally, the delta(15)N values of tot al N, NH4+-N, and NO3--N increased with increasing soil depth. The val ues of delta(15)N ranged from 1.0 to 6.8 parts per thousand, 2.5 to 15 .6 parts per thousand, and -14.8 to 5.6 parts per thousand for total N , NH4+-N, and NO3--N, respectively. Additionally, the delta(15)N value s were different between NH4+-N and NO3--N for each soil depth. Thus, it was concluded that the assumptions about inorganic N used in interp retation of plant delta(15)N values were valid. Moreover, on upper slo pe sites where soil inorganic N was predominantly NH4+-N, the order of delta(15)N was generally total N > NH4+-N > NO3--N for each depth, wh ereas the order of delta(15)N was NH4+-N > total N > NO3--N on lower s lope sites where NH4+-N was less dominant as soil inorganic N and rela tively high net nitrification rates were measured. Our results suggest ed that nitrification plays an important role in regulating delta(15)N in forest-soil N.