Organic carbon fluxes from the upper Yangtze basin: an example of the Longchuanjiang River, China X. X. Lu, 1,2 Siyue Li, 2 * Min He, 1 Yue Zhou, 1 Li Li 1 and Alan D. Ziegler 2 1 Global Change and Watershed Management Center, Yunnan University of Finance and Economics, Kunming 650221, China 2 Department of Geography, National University of Singapore, Singapore 117570, Singapore Abstract: To investigate the effects of anthropogenic activity, namely, land use change and reservoir construction, on particulate organic carbon (POC) transport, we collected monthly water samples during September 2007 to August 2009 from the Longchuanjiang River to understand seasonal variations in the concentrations of organic carbon species and their sources and the yield of organic and inorganic carbon from the catchment in the Upper Yangtze basin. The contents of riverine POC, total organic carbon and total suspended sediment (TSS) changed synchronously with water discharge, whereas the contents of dissolved organic carbon had a small variation. The POC concentration in the suspended sediment decreased non-linearly with increasing TSS concentration. Higher molar C/N ratio of particulate organic matter (average 77) revealed that POC was dominated by terrestrially derived organic matter in the high flows and urban wastewaters in the low flows. The TSS transported by this river was 2.7  10 5 t/yr in 2008. The specific fluxes of total organic carbon and dissolved inorganic carbon (DIC) were 5.6 and 6 t/km 2 /yr, respectively, with more than 90% in the high flow period. A high carbon yield in the catchment of the upper Yangtze was due to human-induced land use alterations and urban wastes. Consistent with most rivers in the monsoon climate regions, the dissolved organic carbon–POC ratio of the export flux was low (0.41). Twenty-two percent (0.9 t/km 2 /yr) of POC out of 4 t/km 2 /yr was from autochthonous production and 78% (3.1 t/km 2 /yr) from allochthonous production. The annual sediment load and hence the organic carbon flux have been affected by environmental alterations of physical, chemical and hydrological conditions in the past 50 years, demonstrating the impacts of human disturbances on the global and local carbon cycling. Finally, we addressed that organic carbon flux should be reassessed using adequate samples (i.e. at least two times in low-flow month, four times in high-flow month and one time per day during the flood period), daily water discharge and sediment loads and appropriate estimate method. Copyright © 2011 John Wiley & Sons, Ltd. KEY WORDS dissolved organic carbon; particulate organic carbon; C/N ratio; water discharge; sediment loads; Longchuanjiang; Yangtze River Received 29 November 2010; Accepted 11 August 2011 INTRODUCTION Carbon fluxes from worldwide rivers to the oceans were estimated approximately 1 Gt/yr, with approximately 45% being transported as organic carbon (Schlesinger and Melack, 1981; Ludwig et al., 1996). This riverine transportation of organic carbon, composed of dissolved organic carbon (DOC) and particulate organic carbon (POC), from terrestrial ecosystem to marine systems, represents a significant process in the global carbon cycling (Meybeck, 1982; Hedges, 1992; Ludwig et al., 1996). The DOC/POC ratio of the carbon flux was approximately 1.2 at a global scale. The total organic carbon (TOC) is closely related to stream water quality or considered as an indicator of organic contamination. Previous studies reported the positive correlations between certain organic pollutants and TOC (Parks and Baker, 1997; Hinga, 2003; Chen et al., 2006). Therefore, investigation on the geochemical char- acteristics and transport process of riverine organic carbon can help us to interpret global carbon budget and assess organic pollution. Organic carbon is strongly associated with catchment physical characteristics such as soil type, climate, hydrol- ogy and land use (Meybeck, 1993b; Hope et al., 1994). POC in rivers is the result of terrestrially (i.e. soil erosion, allochthonous source) and aquatically (i.e. phytoplankton, autochthonous source) produced organic matter. POC is mainly terrestrially produced in high turbid rivers such as the Yangtze and the Yellow Rivers (Zhang et al., 1992; Cauwet and Mackenzie, 1993; Wu et al., 2007), whereas the contribution of aquatic biomass to POC increases evidently in low turbid rivers (Meybeck, 1993a; Kao and Liu, 1996; Hope et al., 1997; Gao et al., 2007). However, riverine DOC is mostly terrestrial in origin (Raymond and Bauer, 2001; Helie and Hillaire-Marcel, 2006). The C/N ratios have been widely used to decipher autochthonous and allochthonous sources of carbon (Meybeck, 1982). The C/N ratios of terrestrial organic matter have a wide range (~12–400), whereas the C/N ratios of phytoplankton are less variable (~6–8) (Hedges et al ., 1986, 1997). Anthropogenic activities such as deforestation and dams greatly regulate the transports, concentrations and temporal variations of riverine organic carbon (Syvitski et al., 2005; *Correspondence to: Siyue Li, Department of Geography, National University of Singapore, Arts Link 1, 117570, Singapore. E-mail: syli2006@163.com; lisiyue@wbgcas.cn HYDROLOGICAL PROCESSES Hydrol. Process. 26, 1604–1616 (2012) Published online 28 September 2011 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.8267 Copyright © 2011 John Wiley & Sons, Ltd.