Variation of δ¹³C and soil organic carbon under different precipitation gradients in alpine grassland on the Qinghai–Tibetan Plateau

Purpose: Soil organic carbon (SOC) dynamics strongly affect the terrestrial carbon balance, and stable isotopic measurements provide detailed information about SOC cycling. However, the response of δ¹³C and SOC to climatic factors is still not well known in high alpine regions. Methods: In this study, soil δ¹³C and SOC in vertical soil profiles were analysed at alpine grassland sites along four precipitation gradients (350–400, 450–500, 550–600, and 650–700 mm) on the eastern Qinghai–Tibetan Plateau. The β value is defined by the slope of the linear regression relating soil δ¹³C to the logarithm of SOC, and the higher β values represent slower SOC turnover. Results: The SOC concentration tended to decline with increasing soil depth from 0 to 40 cm, while δ¹³C was enriched in alpine grassland on the eastern Qinghai–Tibetan Plateau. The δ¹³C values in the 650–700-mm class were relatively depleted compared with other precipitation classes. Redundancy analysis (RDA) showed that altitude was the most important factor—explaining 20.9% of the variation in ¹³C in surface soil (0–10 cm), followed by mean annual air temperature (MAT) (17.1%) and mean annual precipitation (MAP) (16.9%). MAT and altitude were the main factors controlling δ¹³C and β values. MAP did not significantly affect SOC distribution, but lower δ¹³C and β values occurred with higher precipitation. Correlations between SOC content and MAT increased significantly after removing relationships with MAP. Conclusions: The results indicate that faster SOC decomposition may not necessarily lead to SOC losses on the eastern Tibetan Plateau given current conditions. Future research should consider the effects of interactions between warmer and wetter conditions (predicted under climate change) on SOC content and turnover.