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 (δ(15)N) 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 δ(15)N as well as the effects of temperature and precipitation on δ(15)N altitudinal trends in plants and surface soils are discussed. Across this altitude transect, the δ(15)N values of C(3) and C(4) 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 δ(15)N difference between C(3) plants and C(4) plants. If, however, you only compare C(4) plants with those C(3) plants growing at the same altitudinal range, no significant difference in δ(15)N exists between them, suggesting that photosynthetic pathway does not have an influence on the plant δ(15)N values. In addition, we found that C(3), C(4) plants and surface soil (0-5 cm depth) all trend significantly towards more negative δ(15)N with increasing elevation. Furthermore, this study shows that the mean annual temperature and the mean annual precipitation positively and negatively correlate with δ(15)N in C(3) and C(4) plants, respectively. This indicates that precipitation and temperature are the main controlling factors of the δ(15)N variation in plants with altitude. We propose that lower δ(15)N 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.