Carbon isotope and C/N ratios of suspended matter in rivers: an indicator of seasonal change in C4/C3 vegetation

A combination of δ 13C values with C/N ratios in suspended matter has been used to examine the seasonal relationship between C4 and C3 vegetation along the Loess Plateau, NW China. The C isotopic composition of suspended organic matter in rivers, together with C/N ratios can differentiate between soil and plant material, and can be used to estimate the relative contributions of soil organic C and plant litter to the suspended matter. The relationship between C isotopic composition and C/N ratios indicates that the samples are a mixture of two end members: (1) modern soils with relatively constant δ 13C values, low C content and low C/N ratios; (2) plant litter with varying δ 13C values, high C content and high C/N ratios. The results reflect the seasonal distribution of C4/C3 vegetation within the area studied, as part of the Loess Plateau. The abundance of C4 grasses is about 20% for the current summer vegetation ecosystem in the eastern part of the Loess Plateau. Hence, the use of δ 13C values and C/N ratios of suspended matter in rivers and modern soil may be useful for reflecting seasonal distribution of C4/C3 vegetation in catchments. This could be a useful tool for distinguishing between catchments for GIS studies, and long term planning for ecological management of catchment areas.