Altitudinal variations in the bulk organic carbon isotopic composition of topsoil in the Qilian Mountains area, NE Tibetan Plateau, and its environmental significance

Understanding the bulk organic carbon isotopic composition (δ13Corg) of the surface soil in various climates is crucial for past climate reconstruction. Although the relationship between climatic factors (such as precipitation and temperature) and plant δ13Corg are well studied at the global and regional scales, studies relating the surface soil organic matter δ13Corg values and climatic factors remain relatively rare. In this study, 52 surface soil samples were collected from the Shiyang River drainage basin, the northeastern margin of the Tibetan Plateau, where changes in vegetation are sensitive to the climate. The δ13Corg of the surface soils was analyzed to examine their environmental implications. Our results show that soil δ13Corg values change from −26.58‰ to −21.73‰ with an average of −25.20‰ (±1.14‰). Binary regression analysis and linear regression analysis show that the change in the soil δ13Corg values is significantly correlated with altitude; temperature and precipitation are responsible for the altitudinal impact on the soil δ13Corg while air CO2 concentrations have little effect. The sensitivity of the δ13Corg values to the mean annual temperature is +0.066‰/°C and to the mean annual precipitation is −0.3‰/100 mm. We find that extremely low temperatures (above 2960 m a.s.l. in the study area) may result in physiological drought in plants even when precipitation is favorable, which consequently leads to higher soil δ13Corg values. This observation is inconsistent with its direct positive role in the δ13Corg values below 2960 m a.s.l., and the threshold is −1.2 °C in the Shiyang River drainage basin. In addition, the slope of the regression line for the soil δ13Corg values versus altitude with favorable precipitation in our study is +1.2‰/km, which supports that a relationship of +1.1-1.3‰/km for the soil δ13Corg values versus altitude is available on the Tibetan Plateau and the surrounding areas and further indicates that this coefficient in this region maybe potentially be useful for paleoelevation reconstruction.