Nitrogen isotope ratios (δ 15 N) in plants and soil are widely known as indicators of the N cycle in terrestrial ecosystem. Recent studies have proposed that the difference between plant and soil δ 15 N (Δδ 15 N) is a better indicator of the N cycle than plant δ 15 N or soil δ 15 N alone. However, the processes of the N cycle indicated by Δδ 15 N are not well understood. The present study compared Δδ 15 N variations between different ecosystems of northern Mongolia and northern Japan (Hokkaido) to associate the Δδ 15 N characteristics with soil N availability. Needles of Siberian larch (Larix sibirica Ledeb.) in Mongolia, Todo-fir (Abies sachalinensis (F.Schmidt) Mast.) in Hokkaido, and mineral soils from both regions were acquired for determination of Δδ 15 N values. Δδ 15 N showed similar large variations (8‰) in the two regions with no significant correlations to climate factors. On the other hand, the relationship between Δδ 15 N and soil δ 15 N was opposite between the two regions with a positive correlation in Mongolia (r s = 0.504) and a negative correlation in Hokkaido (r s = −0.600). Moreover, total inorganic N (total amount of NH 4 + and NO 3 − ) contents were up to 20 times higher in Hokkaido than in Mongolia. Δδ 15 N showed significant correlation with the fraction of NO 3 − relative to total inorganic N in the 0–10 cm soil layer in Hokkaido. These results indicate that Δδ 15 N variation in Hokkaido can be explained by progression of nitrification in soil, which is different in Mongolia where Δδ 15 N variation is explained by microbial N immobilization. Our findings suggest that soil N availability affects Δδ 15 N indicator owing to changes in the N cycle process, which are reflected in the relationships of foliage δ 15 N or soil δ 15 N with Δδ 15 N.
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