Abstract: The light fraction (LF) of soil is sensitive for identifying changes in soil due to human activities or environmental changes. To explore the distribution and influencing factors of soil LF content on the central and eastern Tibetan Plateau, we measured the LF, heavy fraction (HF) and its carbon and nitrogen content and C/N ratio of topsoil at different vegetation types. We further analyzed the relationships between the carbon and nitrogen content, C/N ratio of LF and environmental factors. The results showed that the contribution of LF to total soil organic matter was small (2.11%). The content of LF in topsoil at different vegetation types followed the order of forest land > alpine meadow > alpine steppe > desert. The average contents of soil LF organic carbon (LFOC) and HF organic carbon (HFOC) in all sampling sites were 27.23% and 2.39%, respectively, with highest content in the forests (31.73% vs. 2.64%). LFOC was the lowest in desert (24.63%), whilst HFOC was the lowest in alpine meadow (2.30%). The average contents of LF total nitrogen (LFTN) and HF total nitrogen (HFTN) were 1.22% and 0.12%, respectively, being highest in alpine meadow (1.28% vs. 0.12%) and lowest in forests (1.14% vs. 0.10%). The average value of the LF C/N ratio and HF C/N ratio was 27.67 and 11.59. C/N ratio in both LF and HF (36.95 vs. 15.15) was the highest in the forests, whilst was the lowest in LF of alpine meadow (25.45) and in HF of alpine steppe (10.03). These results imply that humification degree of forests is high and the available nitrogen content is low, while an opposite pattern was found for alpine meadows and alpine steppe. The contents and distribution of the LF in topsoil on the central and eastern Tibetan Plateau significantly varied across vegetation type, while soil pH is the most important environmental factor affecting LF carbon and nitrogen contents.

Key words: Tibetan Plateau, topsoil, light and heavy fraction, carbon and nitrogen content, distribution characteristics.