岩溶区地表水体惰性有机碳研究

岩溶作用所产生的岩溶碳汇对全球碳循环有着重要的影响。岩溶水生生物通过CO₂富集机制（CCM）固定岩溶水中的HCO₃^-，将其转化为有机碳。惰性有机碳（RDOC）为水生生物和微生物不能利用和降解的溶解性有机碳。RDOC长时间滞留水体，形成稳定碳汇。多次培养实验发现可以用原位微生物法对岩溶区地表水中的RDOC进行检测。以桂林漓江为对象，研究由外源水补给并流经岩溶峰林平原区的岩溶地表水中的RDOC及其相关因子，结果显示：1）漓江水中Ca²⁺，HCO₃^-浓度从上游到下游逐渐升高，表明外源水进入岩溶区，发生了岩溶作用。漓江水的C/N为6.29~10.50，表明水中有机碳的来源为外源水和水生生物作用。受岩溶作用和水生生物光合作用双重影响，δ¹³C-DIC值从上游到下游逐渐偏正，水生生物优先利用无机碳中的¹²C使得δ¹³C-POC从上游到下游逐渐偏负。C/N与δ¹³C-DIC、δ¹³C-POC的变化以及相关性体现了岩溶地表水体中无机环境和有机环境相关联；2）岩溶区地表水中检测出RDOC且浓度远大于非岩溶区，RDOC浓度的时空变化与水生生物利用岩溶作用所产生的HCO₃^-有关。漓江地表水中DOC、BDOC、RDOC浓度与HCO₃^-浓度变化趋势一致，从上游到下游逐渐升高，下游L6（岩溶区）RDOC浓度为上游L1（非岩溶区）的3倍，在季节上表现出秋季>冬季>夏季>春季，RDOC的含量平均占水体中DOC含量的78%左右，体现了岩溶作用强度和水生生物活动对岩溶地表水中RDOC的影响；3）结合流量数据计算RDOC通量：桂林水文站RDOC的通量为0.75×10⁷ kg/a，阳朔水文站的RDOC通量为1.3×10⁷ kg/a，是桂林水文站断面的1.7倍。

Study of the recalcitrant dissolved organic carbon in karst surface water

Karst carbon sink plays an important role in the global carbon cycle. The aquatic organisms convert inorganic carbon to organic carbon by carbon dioxide concentrating mechanism (CCM). The heterotrophic microorganisms in aquatic system use organic matter produced or remained by other organisms, decompose and deplete the degradable part, which is Bioavailable Dissolved Organic Carbon (BDOC), but leave the Recalcitrant Dissolved Organic Carbon (RDOC) remaining in aquatic system for a long time and forming stable carbon sink. In-situ microbial cultivation method has been demonstrated available to determine RDOC concentration in surface karst aquatic system, which was used in this study. Li River (24°16'~26°21'N, 109°45'~111°02'E), locating at Guilin, Southwest China, originates from non-karst area and flows through karst area, and 6 sites (L1~L6) were selected from the upstream to the downstream of Li River, to find out whether it has RDOC in karst surface water and how much it is, of which L1 and L2 locate in the non-karst area and the others locate in the karst area. Results show:(1) Resulted from the carbonate bedrock weathering in the midstream and downstream of Li River, concentrations of HCO3- and Ca2+ reveal continuous increase trend from upstream to downstream. The highest concentration of HCO3- is 170.8 mg/L at L6, and the lowest is 12.2 mg/L at L1, respectively. And, the highest concentration of Ca2+ is 50.93 mg/L, the lowest is 5.02 mg/L, respectively. The values of δ13C-DIC range between -12.70 ‰~-5.17 ‰, also reveal increase trend upstream to downstream in Li River. On the contrary, the values of δ13C-POC range from -28.43 ‰ to -22.98 ‰, and reveal decrease trend from the upstream to the downstream in Li River. The increase trend of value of δ13C-DIC is induced by carbonate rock weathering which introduces high δ13C-DIC valued DIC participating and consumption of DIC by aquatic organisms which preferentially use 12C-DIC. Otherwise, the value of the δ13C-POC had a decreased trend because of the consumption of DIC by aquatic organisms which preferentially use 12C-DIC. The C/N values of Li River range from 6.29 to 10.50 with the average of 7.84. The C/N value in Li River indicates that the inorganic environment associates with the organic environment in surface karst aquatic system. (2) The DOC concentrations range from 0.59 mg/L to 5.42 mg/L with an average of 2.94 mg/L. The RDOC and BDOC concentrations range between 0.40 mg/L to 4.09 mg/L with an average of 2.42 mg/L and 0.08 mg/L to 2.28 mg/L with an average of 0.71 mg/L, respectively. The concentrations of DOC, RDOC, BDOC reveal continuous increase trend from upstream to downstream of Li River, as well as HCO3-, Ca2+ and δ13C-DIC, but δ13C-POC shows opposite decreasing trend. (3) The concentrations of RDOC accounts for about 78% of concentration of DOC in Li River. The RDOC flux calculated by the RDOC concentration and discharge of Li River, of which is 0.75×107 kg/a at Guilin Hydrological Station in the midstream and 1.3×107 kg/a at Yangshuo Hydrological Station in the downstream. The RDOC flux at downstream is the 1.7 times than RDOC flux at the mid-stream.