Sources and distribution of carbon within the Yangtze River system
Introduction
Riverine input of carbon into the ocean is an important link in biogeochemical carbon cycling between land and ocean. The quantity of carbon transported by rivers is an important component of the global carbon cycle (Meybeck, 1982, Ittekkot, 1988, Ludwig et al., 1996, Meybeck and Ragu, 1996, Lobbes et al., 2000, Sempere et al., 2000, Dagg et al., 2004). Of the 0.9 Gigatons of carbon carried every year by global rivers, about 40% is organic and 60% inorganic (Probst et al., 1994). Although this contribution to oceanic organic matter is small on a global scale, it can be of importance in coastal regions where it constitutes a considerable food source (Smith and Hollubough, 1992, Opsahl and Benner, 1997). Burial of terrestrial organic matter upon the ocean floor is one of the most important mechanisms for its preservation (Degens, 1982).
River carbon fluxes provide essential information on the rate of continental yield and reflect biogeochemical processes within catchment areas. Thus, such fluxes reflect soil properties and extreme climatic and hydrological events that lead to rapid changes in the mass transport of rivers (Spitzy and Ittekkot, 1991, Hedges, 1992). In addition, the erosion of bedrock by river water leads to weathered material being flushed into rivers from their catchment areas. Many studies have demonstrated the strong influence of continental weathering on the geological carbon cycle (Berner et al., 1983, Berner, 1991, Berner, 1992, Galy and France-Lanord, 1999). Anthropogenic influences, such as changes in land use, can also affect the riverine input of carbon (Kao and Liu, 1996). Recent studies have focused on carbon released from carbonate rocks and shale by physical and chemical weathering within a variety of geographic settings (Ludwig et al., 1996, Sempere et al., 2000, Barth et al., 2003). Despite large uncertainties involved in global evaluations, the annual inherited organic yield derived from chemical weathering might well comprise a significant component of the carbon cycle, depending on the initial organic content of the altered rock and on climate parameters (Barth et al., 2003).
Stable carbon and nitrogen isotopes and C/N ratios have been widely used to distinguish between autochthonous and allochthonous sources of carbon. The C/N ratios of terrestrial organic matter vary over a wide range (ca. 12–400) and decrease during diagenesis, whereas the C/N ratios of phytoplankton are less variable (ca. 6–8; Ertel et al., 1986, Hedges et al., 1986, Onstad et al., 2000, Jennerjahn et al., 2004). The isotope fraction is dependent on the photosynthetic mechanism and carbon source, which lead to differences in phytoplanktonic and terrestrial organic matter (Peterson et al., 1985, Saliot et al., 1988, Saliot et al., 2002, Andrew et al., 1998). The stable carbon isotope ratio is also considerably influenced by diagenetic and metabolic fractionation and the assimilation of dissolved inorganic carbon derived from remineralized organic matter (Fry and Sherr, 1984, Goni et al., 1997). Combination techniques have been successfully used to estimate organic carbon budgets, identify biological activity, indicate the decomposition of organic matter, and relate biomarker composition to the source organic material and depositional environment (Peters et al., 1986, Relexans et al., 1988, Thornton and McManus, 1994, Hellings et al., 1999). Nitrogen isotopes have been used to distinguish between natural and anthropogenic sources of nitrate. For instance, δ15N values of nitrate (relative to atmospheric N2) in commercial fertilizer typically range from −2.5‰ to +2.0‰, while organic soil nitrate ranges from −2‰ to +9‰, and human and animal waste ranges from +10‰ to +20‰ (Harrington et al., 1998). These δ15N ranges, however, are not rigorously invariable because isotopic fractionation can occur within both groundwater and soil. Isotopic fractionation is significantly influenced by denitrification within soil and groundwater, the volatile loss of ammonia from manure, and the uptake of nitrate by microbes or algae; this complicates efforts to trace nitrogen sources from δ15N values. Despite the complexity of isotopic fractionation during the nitrogen cycle, numerous studies have demonstrated that nitrogen isotopes of organic matter do, at least in part, reflect their source (Sigleo and Macko, 2002, Thomas et al., 2002). Seasonal variations in nitrogen isotopes within particles collected from the Yangtze Estuary, China, indicate contributions from multiple sources as well as the effects of microbial activity (Wu et al., 2000).
On the basis of hydrologic and geographic criteria, the Yangtze River can be divided into three reaches (upper, middle, and lower) and two sets of tributaries (southern and northern; Chen et al., 2001, Ding et al., 2004). Most northern tributaries are located within the upper reaches and have high sediment loads, including the Minjiang (MJ), Jianglingjiang (JLJ) and Tuojiang (TJ) (Fig. 1, Fig. 3). From the south, only the Wujiang (WJ) enters the mainstream with comparable water discharge, although with lower sediment load. Within the middle reaches, the main tributaries enter from the south, including the Dongting and Poyang Lake systems and the Xiangjiang (XJ) and Yuanjiang (YJ); Hanjiang (HJ) is the only tributary from the north. Most tributaries pass through areas of dense population and rapidly developing industrial zones.
The present study focuses on the wide Yangtze (Changjiang) River Basin because of its economic significance, i.e. changes in vegetation and land use associated with energy projects such as the Three Gorges Dam (TGD) and the South-to-North Water Diversion Project (Zhang et al., 1999, Chen et al., 2003a). The total population (ca. 400 million) living within the watersheds of the Yangtze river is about one-third of the national population, and rice harvests within the watersheds comprise 70–75% of the national harvest. In addition, economic productivity within the Yangtze River Basin amounted to almost half of China's Gross National Product (GNP) during the 1990s.
Such an increasing population and rapid economic growth brought about great changes in land use. Soil erosion has increased dramatically in recent decades, especially within the upper reaches of the Yangtze River, leading to significant changes in the coastal ecosystem and associated community structure (Chen et al., 2003b, Li et al., 2003). However, in terms of biogeochemistry, most recent work has focused on estuarine and coastal areas (Denant et al., 1990, Tan et al., 1991, Qiu et al., 1991, Cauwet and Mackenzie, 1993, Saliot et al., 1998, Zhang, 1999). Relevant information for the entire Yangtze River Basin is limited (Zhang et al., 1999, Zhang et al., 2003, Wu et al., 2000, Liu et al., 2003, Ding et al., 2004). The long-term impact of population increase and climate change on freshwater resources should be assessed within the Yangtze River drainage basin.
The objective of the present study is to obtain information on the nature and quantity of carbon transported by the Yangtze River, especially temporal changes in carbon flux related to intensive human activities. In terms of catchment area and water and sediment discharge, the Yangtze River is the fourth-largest river in the world, yet detailed information is lacking concerning the sources and flux of carbon within the river system. The present study is an effort to fill this gap in our knowledge.
Section snippets
Sampling and sample preparation
Field observations were carried out simultaneously at both northern and southern areas of the drainage basin, followed by excursions along the main stream during April–May 1997 and April–May 2003 when the monthly runoff of the main stream was 800 × 108 m3 and 650 × 108 m3, respectively (http://www.cjw.com.cn). Samples were collected from the river mouth to upstream areas over a distance of 3500–4000 km, covering 15 major tributaries (ca. 50 stations) that together supply 85–90% of the total riverine
Distribution of dissolved and particulate carbon along the main stream
The distribution of dissolved and particulate carbon along the main stream in both 1997 and 2003 is presented in Fig. 2. Average DOC concentrations within the main stream were 105 μM C in 1997 and 108 μM C in 2003 (Fig. 2a). The lowest concentrations were observed in the upper pristine reaches of the river (3000–3300 km from the estuary) for both sampling periods. Once the river enters Sichuan Province (around 2900 km from the estuary), the DOC concentration increases considerably. In the middle
Source and variation of organic matter in the Yangtze River
Higher DOC/TOC ratios (39–78%) were observed in 1997 than in 2003 (10–68%), when a higher discharge rate was recorded (Fig. 2). This indicates that the influence of discharge is greater for POC transport than DOC transport, as also observed for the Rhone River (Sempere et al., 2000). It seems that in the middle and lower reaches, the DIC concentration is diluted by the Dongting and Poyang Lakes System (Fig. 2b). DIC samples from northern tributaries in the upper and middle reaches have higher
Conclusions
Here we report a detailed investigation of the behavior of DOC, POC, DIC and PIC within the Yangtze River drainage basin. We also determined the general features of the organic carbon and nitrogen isotopic composition of plants, surface soil, and river suspended particulate matter. DOC concentrations within the main stream averaged 105 μM C in 1997 and 108 μM C in 2003. A comparison of DOC concentrations in different tributaries in 1997 indicates that higher DOC concentrations (>88 μM) were found
Acknowledgments
This study is funded by the Special Funds from National Key Basic Research Program of P.R. China (2006CB400601, 2004CB720505, 2002CB412405) and of Natural Science Foundation of China (Nos. 90211009 and 40476037, 40476036), Shanghai rising-star project (04QMX1420) and Program for New Century Excellent Talents in University (NCET-04-0424) and the Ministry of Education of P.R. China (No. PCSIRT0427). The authors thank Drs. H.T. Chen, H. Xiong, J.L. Zhou and Z.Y. Zhu for their assistance in field
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