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

Fujiyoshi, Lei, Sugimoto, Atsuko, Yamashita, Youhei, Li, Xiaoyang
Ecological indicators 2019 v.101 pp. 1086-1093
Abies sachalinensis, Larix sibirica, ammonium, climatic factors, environmental indicators, leaves, mineral soils, nitrates, nitrification, nitrogen, stable isotopes, terrestrial ecosystems, trees, Japan, Mongolia
Nitrogen isotope ratios (δ15N) 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 δ15N (Δδ15N) is a better indicator of the N cycle than plant δ15N or soil δ15N alone. However, the processes of the N cycle indicated by Δδ15N are not well understood. The present study compared Δδ15N variations between different ecosystems of northern Mongolia and northern Japan (Hokkaido) to associate the Δδ15N 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 Δδ15N values. Δδ15N showed similar large variations (8‰) in the two regions with no significant correlations to climate factors. On the other hand, the relationship between Δδ15N and soil δ15N was opposite between the two regions with a positive correlation in Mongolia (rs = 0.504) and a negative correlation in Hokkaido (rs = −0.600). Moreover, total inorganic N (total amount of NH4+ and NO3−) contents were up to 20 times higher in Hokkaido than in Mongolia. Δδ15N 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 Δδ15N variation in Hokkaido can be explained by progression of nitrification in soil, which is different in Mongolia where Δδ15N variation is explained by microbial N immobilization. Our findings suggest that soil N availability affects Δδ15N indicator owing to changes in the N cycle process, which are reflected in the relationships of foliage δ15N or soil δ15N with Δδ15N.