Measurements of nitrogen isotope composition of plants and surface soils along the altitudinal transect of the eastern slope of Mount Gongga in southwest China

The natural abundances of stable nitrogen isotopes in plants and soils have been viewed as recorders that can be used to reconstruct paleoclimate and ecological processes or to indicate the biogeochemical cycle of nitrogen in nature. This study systematically measured the nitrogen isotope composition (δ¹⁵N) in plants and surface soils along an altitudinal transect of elevation range of 1200 to 4500 m on the eastern slope of Mount Gongga in southwest China. The influences of photosynthetic pathways on plant δ¹⁵N as well as the effects of temperature and precipitation on δ¹⁵N altitudinal trends in plants and surface soils are discussed. Across this altitude transect, the δ¹⁵N values of C₃ and C₄ plants on Mount Gongga range between ?9.87‰ and 7.58‰ with a mean value of ?1.33‰, and between ?3.98‰ and 4.38‰ with a mean value of ?0.25‰, respectively. There is an evident δ¹⁵N difference between C₃ plants and C₄ plants. If, however, you only compare C₄ plants with those C₃ plants growing at the same altitudinal range, no significant difference in δ¹⁵N exists between them, suggesting that photosynthetic pathway does not have an influence on the plant δ¹⁵N values. In addition, we found that C₃, C₄ plants and surface soil (0–5 cm depth) all trend significantly towards more negative δ¹⁵N with increasing elevation. Furthermore, this study shows that the mean annual temperature and the mean annual precipitation positively and negatively correlate with δ¹⁵N in C₃ and C₄ plants, respectively. This indicates that precipitation and temperature are the main controlling factors of the δ¹⁵N variation in plants with altitude. We propose that lower δ¹⁵N values of plants and soils at higher altitude should be attributed to lower mineralization and lower net nitrification rates induced by low temperature and abundant rainfall. Copyright © 2010 John Wiley & Sons, Ltd.