Influence of soil N availability on the difference between tree foliage and soil δ15N from comparison of Mongolia and northern Japan

Nitrogen isotope ratios (delta N-15) 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 delta N-15 (Delta delta N-15) is a better indicator of the N cycle than plant delta N-15 or soil delta N-15 alone. However, the processes of the N cycle indicated by Delta delta N-15 are not well understood. The present study compared Delta delta N-15 variations between different ecosystems of northern Mongolia and northern Japan (Hokkaido) to associate the Delta delta N-15 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 Delta delta N-15 values. Delta delta N-15 showed similar large variations (8%) in the two regions with no significant correlations to climate factors. On the other hand, the relationship between Delta delta N-15 and soil delta N-15 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 NH4+ and NO3-) contents were up to 20 times higher in Hokkaido than in Mongolia. Delta delta N-15 showed significant correlation with the fraction of NO3- relative to total inorganic N in the 0-10 cm soil layer in Hokkaido. These results indicate that Delta delta N-15 variation in Hokkaido can be explained by progression of nitrification in soil, which is different in Mongolia where Delta delta N-15 variation is explained by microbial N immobilization. Our findings suggest that soil N availability affects Delta delta N-15 indicator owing to changes in the N cycle process, which are reflected in the relationships of foliage delta N-15 or soil delta N-15 with Delta delta N-15.