Test stable carbon isotopic composition of soil organic matters as a proxy indicator of past precipitation: Study of the sand fields in northern China

Understanding the stable carbon isotopic composition (δ13C) of surface soil organic matter under various climates is crucial for quantitative reconstruction of past climatic changes. Although the relationship between precipitation and plant δ13C is well understood at global and regional scale, studies relating δ13C of surface soil organic matter to precipitation in northern China remain controversial. In this study, 90 surface soil samples were collected from the desert/loess transition zone of Northern China, where vegetation is sensitive to climatic changes. The stable carbon isotopic composition of organic matter in these soil samples was analyzed in order to examine its palaeoclimatic implications. Our results show that the δ13C varied from −26.56 to −19.41‰ among the investigated sites, with an average of −23.54‰. There was a strong negative correlation between the δ13C of surface soils and the mean annual precipitation (MAP) (R2 = 0.52, p < 0.05). Variation in the δ13C of C3 plants was the main factor that determined the δ13C of soil organic matter. The C4/C3 biomass ratio may also contribute to the δ13C changes, but this effect was relatively minor in our study area. Our findings suggest that a general transform function between the proxy index δ13C value and climates may not be applicable at one specific site, and that a comprehensive investigation is needed when reconstructing the past climate change using the δ13C of buried soils.