Topsoil C/N ratios in the Qilian Mountains area: Implications for the use of subaqueous sediment C/N ratios in paleo-environmental reconstructions to indicate organic sources

• 208 topsoil C/N ratios are obtained in northeastern margin of Tibetan Plateau.
• Most topsoil C/N ratios (< 12) are as low as those of algae-derived organic matters.
• Our results may contribute to related studies using C/N ratios proxy.

Sediment carbon/nitrogen (C/N) ratios can play an important role in reconstructing lacustrine and coastal paleo-environments, because significantly different C/N ratios result from aquatic phytoplankton versus organic matter derived from terrestrial plants. However, researchers have not yet taken into account the fact that topsoil organic matter is another important terrestrial organic source for lacustrine and coastal sediments. A study of topsoil C/N ratios from the Heihe and Shiyang River drainage basins, located in the Qilian Mountains on the northeast margins of the Tibetan Plateau, shows that topsoil C/N ratios can be as low as those of subaqueous sediments. 80.4% of total topsoils in the Heihe River drainage basin, and 67.7% of total topsoils in the Shiyang River drainage basin, have C/N ratios below 12, while 18.8% of all samples in the Heihe river drainage basin, and 29.2% of all samples in the Shiyang river drainage basin, possess C/N ratios between 12 and 20. Traditionally, researchers have considered that low C/N ratios indicate subaqueous deposition; however, these sediments are terrestrial and most of the organic matter is derived from land plants. Furthermore, topsoil C/N ratios from both the Heihe and Shiyang river drainage basins correlate significantly with altitude and local vegetation. Our results indicate that not all C/N ratios of terrestrial organic matter (including plants and soil organic matter) equal 20 or more. Nor can one distinguish subaqueous sediment organic sources solely on the basis of C/N ratios; low subaqueous sediment C/N ratios may not simply result from a greater proportion of aquatic plants versus terrestrial organic matter, but also from low end-member terrestrial organic matter C/N ratios which are lowered by the addition of soil organic matter with its own low C/N ratios.