中文摘要：

基于一个水文年度的月周期性采样分析，用河流悬浮颗粒物的有机碳（POC）和溶解无机碳（DIC）的同位素信号探讨了桂江径流中碳的生物地球化学循环。桂江悬浮颗粒物中POC含量多介于1．70％～14．27％之间，平均为（4．54±2．94）％；河流POC的△^14C值介于-235．8％o--26．7％o之间，平均为（-135．38±57.27）‰，没有检出“核爆。4C”信号，揭示了较为强烈的流域地表扰动和土壤侵蚀状况。桂江POC的稳定同位素组成（占。C）变化于-29．92％0-24．71％o之间，平均为（-26．86±1．29）‰，这与以c，植物为主的流域生态系统的碳同位素组成一致。桂江颗粒有机质的C／N比多介于5．54～11．53之间，平均为7．97，低于全球河流的平均状况。一方面，土壤有机碳、岩石来源的地质碳及藻类生物量的混合比例决定了桂江河流颗粒有机质的C／N比和△‘4c值；另一方面，微生物群落对水体有机质的代谢分解作用也在一定程度上改变了有机质的元素和同位素比值。桂江河流DIE的F3C值变化于-17．22％o—-10．65％0之间，平均为（-12．95±1．94）‰。冬半年河流DIC（＆^13C值平均为-11．47％o）几乎全部来自碳酸盐矿物的化学风化，夏半年土壤硅酸盐矿物的化学风化对（DIC）^14和^13C值平均为-14．73％o）的贡献达28％。

英文摘要：

The Guijiang River（GJR） is the first order tributary of the Xijiang River system, with the length of its main stream as 438km and drainage basin area as 18790km^2. The GJR drainage basin is mostly mountainous and hilly. Carbonate rock （mainly of limestone）outcrops account for about 51% of the G JR drainage basin, most of which occur in the upper and middle parts of the drainage basin. Modern karst geomorphologic processes have been developed both on the surface and underground, and a deep layer of limestone red earth is distributed on the karst plain. The G]R drainage basin has a typical subtropical monsoon climate,with the average annual temperatures ranging from 19℃ to 21℃ ,and the annual rainfall varying from 1900mm to 2700mm. 80% of precipitation fails is mainly from April to September, and most of the rainfall is in the form of storms deriving from summer monsoon and typhoon. The soils in the GJR drainage basin mainly include red soil, calcareous soil and paddy soil. The zonal vegetations mainly include subtropical evergreen broad-leaved forest, coniferous and broad-leaved forest, shrub and grass. Water samples were collected monthly at the Wuzhou section of the GJR from April 2005 to April 2006, from which particulate organic carbon（POC）samples for A14C and ＆^13C analysis were filtrated through microporous membrane with the pore size of 0. 45 μm. Dissolved inorganic carbon（DIC） samples for ＆^13C analysis were obtained in situ by the direct precipitation methods using saturated NaOH solution and granular BaC12. A 14C of POC were measured on an accelerator mass spectrometer（AMS）, ＆^13C of DIC and POC was measured using GV-IsoPrimeTM II and Finnigan Delta Plus xl mass spectrometers,respectively. The biogeochemical cycles of riverine carbon are discussed based on the radioactive and stable isotopic composition of particulate organic carbon and stable isotopic composition of dissolved inorganic carbon. POC content in the total suspended substance varied mostly from 1.70% to 14. 27%