Elemental and isotopic signatures of particulate organic carbon in the Zengjiang River,southern China

Riverine total suspended sediment (TSS) at the lower reach section of the Zengjiang River, a low‐turbidity river in the southern China, was sampled on a 4‐week basis from March 2002 to February 2003. The gross TSS was divided into sedimentary and suspended fractions (SED and SUS) by the sedimentation method. Organic carbon and nitrogen, ¹⁴C and ¹³C were analysed using an elemental analyser and accelerator mass spectrometer respectively. The results show that particulate organic carbon (POC) yield is 0·8 × 10⁶ g km^?2 year^?1 in the Zengjiang River drainage basin, which is about one‐tenth of that in the Zhujiang (Pearl) River drainage basin. The C/N ratio demonstrates that aquatic biomass is the major contributor to POC in the Zengjiang River. The average share of aquatic biomass in the SUS‐fraction POC and SED‐fraction POC is about 88·89% and 62·76% respectively, with a substantial seasonal variation. δ¹³C values of SUS‐fraction POC (?26·56 to ? 22·89‰) is slightly lighter than that of SED‐fraction POC (?25·05 to ? 22·20‰), indicating that the contribution of aquatic biomass to δ¹³C values is more pronounced in the SUS‐fraction POC than in the SED‐fraction POC. The ‘bomb’–¹⁴C signature is not detected in the POC of Zengjiang River, and the contribution from geological organic carbon is very little. Δ¹⁴C values of the SED‐fraction POC vary from ? 44 to ? 223‰, and the Δ¹⁴C values of the SUS‐fraction POC vary from ? 33 to ? 165‰. For most paired samples, the SED‐fraction POC is generally more depleted in ¹⁴C than that of its counterpart SUS‐fraction POC. Compared with other small mountainous rivers, the ¹⁴C enrichment of POC in the Zengjiang River indicates slight drainage basin erosion. Copyright © 2006 John Wiley & Sons, Ltd.