Edaphic rather than climatic controls over 13C enrichment between soil and vegetation in alpine grasslands on the Tibetan Plateau

1. Soil organic carbon (SOC) dynamics is crucial for evaluating ecosystem carbon balance and its feedback to climate warming. However, it is difficult to detect statistically significant changes in SOC stock over short-time period due to its large pool size, slow turnover time and huge spatial heterogeneity. Stable isotopic measurements, such as Delta delta C-13 (i.e. the difference of natural abundance of C-13 and C-12 (delta C-13) between surface soils and source plants) and its variations along environmental gradients provide an alternative approach inferring soil carbon dynamics over broad geographical scale. However, current isotopic evidence is primarily derived from temperate and tropical regions, with very limited measurements available in alpine regions. 2. Here, we examined spatial variations of Delta delta C-13 in alpine grasslands on the Tibetan Plateau, using large-scale isotopic measurements obtained from consecutive field samplings. We aimed to test whether previously observed isotopic patterns in temperate and tropical regions still hold true in alpine regions and whether climatic or edaphic variables regulated large-scale patterns of C-13 enrichment between soil and vegetation in alpine ecosystems. 3. Our results showed that topsoil stable carbon isotope composition in alpine steppe and meadow ranged from -26.1 parts per thousand to -19. 7 parts per thousand and from -25.7 parts per thousand to -22.2 parts per thousand, with an average of -23.7 parts per thousand and -24.1 parts per thousand, respectively. As previously observed in temperate forests, soil delta C-13 exhibited linear increases with plant delta C-13 in alpine grasslands. 4. In contrast to earlier findings, our results revealed that edaphic rather than climatic factors regulated spatial variability of the Delta delta C-13 in high-altitude regions. Moreover, edaphic controls over Delta delta C-13 exhibited meaningful differences between alpine steppe and meadow. The Delta delta C-13 exhibited an initial increase and a subsequent decrease with soil carbon content in alpine steppe, but was negatively associated with silt content and carbon: nitrogen ratio in alpine meadow. 5. Our results confirmed the association between the delta C-13 of surface soils and vegetation across contrasting ecosystems, but revealed that edaphic rather than climatic variables were better explanations of C-13 enrichment between soil and vegetation at high altitudes. Changes in soil texture and substrate quality could therefore induce soil carbon dynamics in alpine ecosystems.