Influence of soil N availability on the difference between tree foliage and soil δ ¹⁵ N from comparison of Mongolia and northern Japan

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.