2017, Vol.37
陆地生态系统通过河网水系向河口和近海水域的碳输出过程在区域水质演变和全球碳循环中发挥着重要作用[1~3]。各种形态的碳在河流中的行为并非保守，而是发生着复杂的相互转化[4]。对于来自陆地生态系统的有机质，河流的角色往往很像一个活跃的化学反应器，不断地在分子及元素尺度上改造着被带入河流的含碳物质[5, 6]，并且在流速缓慢的河流下游及河口水域这种碳的生物地球化学循环更为显著[7, 8]。全球陆地生态系统每年向河流水系输送大约2×1015 g C，但能够到达海洋的却不足一半[2]。实际上，河口及海洋中的沉积有机质与陆地有机质在同位素组成及其他生物标志物方面也相距甚远[9]。在几十年甚至上百年时间尺度上化学性质相对稳定的土壤有机质，一旦进入河流水体即可在数周的短时间尺度上发生程度明显的降解[10~12]。在全球许多河流特别是在河口区发现的普遍存在的较高的水体CO2分压(pCO2)也印证了水体微生物对河流有机质的快速分解过程[13~15]。
河流上水库大坝的建设在很大程度上影响了陆地生态系统与河口及海洋之间泥沙及养分物质的供给关系[16~18]。大坝以上水流停滞时间的延长也增加了水体中生物的活动性，进而改变了水体的理化参数，例如使水体的pH值升高、溶解性养分元素减少等[19, 20]。有些研究还发现，由于水坝的修建还导致河流碳的输出通量减少、性质发生明显改变[21~23]。
本文对下游干流筑坝且流域植被覆盖度高的山区河流增江的碳循环过程及碳输出通量做了系统调查，试图为解析自然因素及人类活动对河流碳循环的影响提供一个典型个案
1 材料与方法 1.1 流域概况增江是珠江水系的二级支流，在石滩镇附近排入东江。增江从源头到注入东江口干流全长203km，总落差达484m。位于下游的麒麟咀水文站(QLZ：23°20.734′N，113°50.399′E；6m a.s.l.)控制流域面积2866km2，占增江流域总面积(3160km2)的91 % [24]。流域基岩以硅酸盐类岩石为主，碳酸盐岩地层及含碳酸盐的地层出露较少[25]。流域地貌以山地和丘陵为主，厚层红色风化壳构成主要的成土母质，土壤以湿润铁铝土为主，局部有山地常湿润铁铝土和潜育土-水耕人为土[25]。北回归线穿过流域南部。流域年均气温21.6℃，年均降水量为2188mm(图 1，麻榨站，20m a.s.l.)。地带性植被为南亚热带常绿阔叶林，森林覆盖率达65 % ~70 %。耕地仅占流域总面积的8.7 %。水土流失轻微，河水在多数时间是清澈的，只在洪峰期间变得相对浑浊。流量的年际变化主要受降水量的影响，1954~2009年间，麒麟咀水文站的年流量变化在1.18×109~6.60×109m3之间，年均为3.82×109m3，其中汛期流量(4~9月)约占全年流量的83.3 %。流域内修建了许多小型和中型水库，其中天堂山水库总库容是0.24×109m3[24](图 1)。初溪水利工程(大坝)位于麒麟咀水文站下游16.2km处，2008年3月建成蓄水。蓄水后形成的回水段扩展到大坝以上22km处的小楼断面(XL，23°8.427′N，113°45.171′E)(图 1)。回水区的水位变化减少，水体变得更加清澈。
|
图 1 增江流域地貌与水系示意图 下方嵌入照片是初溪大坝，坝长约380m；另一幅嵌入图指示增江流域在华南地区的相对位置 Fig. 1 Sketch map of the landforms and river system in the Zengjiang River basin. One inset photograph at the lower end of the figure is the Chuxi Water Conservancy Project(CWCP), whose length is about 380m, and another inset shows the location of the study area on the context of South China |
自2008年12月到2010年1月在麒麟咀水文站(QLZ)断面进行每月1次的周期性采样，共14个采样月(表 1)；采样点位于河流中泓线水面以下0.5m深处。2009年7月(汛期)和2010年1月(枯水期)分别在增江干流和支流上选择24个断面(表 1和表 2)进行流域面上采样。每次流域面上采样，自下游向上游顺序进行。
|
表 1 增江下游麒麟咀水文站断面水体理化参数的时序变化 Table 1 Sequential variation of the physical-chemical parameters of the lower Zengjiang River at the QLZ sampling section |
|
|
表 2 增江流域各支流断面与干流断面水体的理化参数(2009年7月与2010年1月) Table 2 Physical-chemical parameters of river water within the Zengjiang basin in July 2009 and January 2010 |
用稀盐酸滴定法现场测试水样的总碱度(TAlk，滴定终点的pH值取4.2)；现场用美国Myron L公司出品的6P型便携式电导率仪测试水温、pH值和电导率。用经酸洗的容积为1L的玻璃瓶装取水样，并滴加3滴饱和HgCl2溶液灭菌。采样一周之内用Whatman GF/F型号的玻璃纤维微孔滤膜(微孔径0.7μm)进行过滤，滤液用作溶解有机碳(DOC)浓度分析，分析仪器是日本岛津公司出产的TOC-VCPH型的TOC分析仪，分析精度优于1.5 %。在麒麟咀水文站断面另外采集1L水样用于叶绿素含量分析。采集时在样品中加入约5mg MgCO3粉末以保持溶液的碱性。采集当天用Whatman GF/F型滤膜过滤获取水体中的总悬浮颗粒物(TSS)，并计算总悬浮颗粒物浓度(CTSS)；用丙酮提取色素，用荧光光度计测定叶绿素a浓度(Ch1-a)[26]。
为获取足量的水体TSS用作其他项目分析，在麒麟咀水文站断面用聚乙烯塑料桶装取约100L水样，并加入约4ml饱和的HgCl2溶液灭菌。在实验室将水样用孔径为0.45μm的醋酸纤维滤膜过滤。一般情况下，过滤在5天内即可完成。小心地将过滤得到的颗粒物从光滑的滤膜表面刮下，用稀盐酸除碳酸盐后再用去离子水反复清洗，之后在50℃的烘箱中烘干、称重。用德国Vario公司出产的EL CHNS-O型号的元素分析仪测定样品的有机碳(POC)和有机氮(PON)含量，用高纯乙酰苯胺及空白样做质量控制，平均分析精度优于0.3 %。
用E. Lewis和D. W. R. Wallace开发的CO2SYS程序(Excel版本)计算水体的pCO2，使用的参数包括总碱度、pH值、水温和溶解硅浓度27[27]。
麒麟咀水文站断面的逐日流量数据取自广东省水文局。
3 结果将所有的现场及实验室分析测试和计算结果列于表 1和表 2。数据表明，增江水体多呈中性偏弱碱性，pH值变化于6.09~9.25之间，平均为7.40±0.59。冬季水温仍在10℃以上，表明增江水体在冬季仍可以保持一定的生产力水平。
麒麟咀水文站断面水体的DOC含量变化于0.76~4.75mg/L之间，平均为2.37±1.31mg/L，明显低于全球河流的平均值5.40mg/L28[28]，但比地处同纬度且气候类型相同的西江水体中的DOC含量(1.20±0.25mg/L，变化于0.47~2.05mg/L29[29])高。2009年7月在增江全流域范围内采集的样品DOC含量变化于1.74~7.47mg/L之间，平均为3.80±1.43mg/L；而2010年1月增江全流域范围内采集的样品DOC含量变化于0.36~2.60mg/L之间，平均为1.50±0.61mg/L。这与麒麟咀水文站断面水体的DOC含量变化规律是一致的，即温暖的月份水体中的DOC含量要高一些(图 2和图 3)。
|
图 2 采样年份内麒麟咀水文站断面水体DOC浓度与水温之间的关系 Fig. 2 Variation of the DOC concentration and water temperature at the Qilinzui(QLZ)sampling section in the sampling period |
|
图 3 增江水系各断面丰水期(2009年7月)水体DOC浓度与枯水期(2010年1月)的对比 Fig. 3 Comparison of DOC concentrations in the flood season(July 2009) with that in the dry season(January 2010) of the Zengjiang River system |
由于流域植被覆盖度高，增江水体相对清澈。麒麟咀水文站断面水体的总悬浮颗粒物浓度(CTSS)与流量呈正相关关系，变化于3.41~59.27mg/L之间，平均值是13.40±14.12mg/L(图 4a)；与之对比，西江梧州断面2005~2006年间水体的CTSS变化于7.13~1029.17mg/L之间，平均为145.10±161.11mg/L(据本课题组60组未发表数据，采样方案与本研究一致)。麒麟咀水文站断面水体的DOC浓度也与流量呈正相关，变化于0.76~4.75mg/L之间，平均值为2.37±1.31mg/L(图 4a)。
|
图 4 麒麟咀水文站断面水体DOC浓度/总悬浮颗粒物浓度及其有机碳含量与流量之间的关系 Fig. 4 Relationships between discharge and (a) the concentra￣tions of total suspended substance (CTSS) and DOC, the data in the dashed oval is not included into the data cluster for the linear regression (to see the explanation in the text), and (b) the content of organic carbon(POC)of TSS at the QLZ sampling section |
14个采样月在麒麟咀水文站断面采集的总悬浮颗粒物中有机碳的百分含量(POC %)变化于2.84 % ~26.61 %之间，平均为(13.40±8.09) % (图 4b)；有机质的C/N比变化于5.34~8.56之间(表 1)，平均为6.43±1.04，明显低于在西江中采集的河流悬浮颗粒有机质的C/N比30[30]。
14个采样月期间，麒麟咀水文站断面总碱度(TAlk)介于0.41~0.95mM之间，平均为0.70±0.14mM，冬天干旱季节水体的TAlk值高于夏天湿润季节。汛期在全流域范围采集的面上样品TAlk值变化于0.03~0.91mM之间，平均为0.41±0.17mM；而枯水季节全流域范围采集的面上样品TAlk值变化于0.06~1.38mM之间，平均为0.65±0.32mM。水体TAlk值与DOC含量呈现相反的变化趋势。
汛期在流域面上采集的样品除天堂山水库中的SKBS样品外，其他样品均表现为CO2相对于大气(大约390 μatm)呈现超饱和状态，变化于584~3991 μatm之间，平均为1710±830 μatm(表 2)。在枯水季节，流域面上样品同样表现为相对于大气CO2的超饱和状态，变化于689~8542 μatm之间，平均为2780±1941 μatm(表 2)。然而，在麒麟咀水文站断面，水体的CO2分压表现为与面上样品不同的特点：14个样品中有6个表现为CO2分压相对于大气呈不饱和状态(图 5中的灰色柱)。这6个样品分别采样于2月、4月、5月、7月、8月和11月，它们的CO2分压变化于34~343 μatm之间，平均仅为147±108 μatm(图 5)。
|
图 5 采样年份内麒麟咀水文站断面水体CO2分压的时序变化及其与总悬浮颗粒物浓度之间的关系 Fig. 5 Monthly variation of the partial pressure of CO2(pCO2)and the relationship with the CTSS at the QLZ sampling section in the sampling period |
在麒麟咀水文站断面，DOC浓度与水温之间表现为中等程度的(p < 0.05) 正相关关系(图 2)。由于华南湿润季风区雨热同期的原因，水体DOC浓度与降水量及径流量之间也呈现这种中等程度的正相关关系。
河流中的DOC一般存在两个主要来源：其一是来自陆地植物碎屑及土壤有机质的淋溶过程和人类生产、生活的排废物，这部分被称为异源(allochthonous)有机质；其二是来源于河流浮游生物的分泌物，这部分被称为自源(autochthonous)有机质[11, 31, 32]。然而，由于研究区的雨热同期现象，河流中的异源与自源有机质的增减是同步的，即汛期雨水的冲刷效应增加了流域范围内异源有机质的贡献，而同期的高温也促进了河水中生物的繁茂，进而增加了自源有机质的含量[28]。两项叠加导致夏季河流中的DOC含量最高，例如6月13日麒麟咀水文站断面水体的DOC含量达到4.75mg/L。
对于那些拥有大面积冲积平原及湖泊的流域，例如亚马逊河流域的下游，水体DOC含量总体偏高，并且峰值出现在汛期洪峰过境期间，揭示了地表径流及潜流对流域DOC贮存库的强烈冲刷效应[33]；另一方面，那些缺乏湿地的山地丘陵流域，例如西江流域，河流DOC的含量总体偏低，且最低值出现在汛期洪峰过境期间，增加的流量对DOC储存库不具有冲刷效应，只对流域表层水体中原有的有机质起到稀释作用[29]。增江河流中较低的DOC含量归因于流域遍布山地丘陵的地貌特征；而河流中最高含量的DOC出现在夏季却与汛期增加的径流对森林与土壤中贮存的DOC的显著冲刷效应有关。数据还揭示，水库生物量在高温下释放出更多的DOC。叶绿素数据也揭示出高温季节藻类更加繁茂(表 1)，当然，在突发性洪水中河流变浑浊的情况例外。在美国的一些森林小流域也发现河流DOC含量在洪水事件中增加的现象[34]。
另外，8~10月份较低的水体DOC含量(图 2)可能与较强紫外线辐射对水体有机质的光化学分解过程有关。水体中有机质的光化学降解现象在河流与湖泊研究中常被关注[35, 36]。当然，水体DOC含量的降低也可能与这一时段的降水偏少有关。
4.2 河流DOC含量的空间变化与靠近流域出口端的麒麟咀水文站断面类似，对流域生物量及土壤层中有机质的冲刷效应也一样存在于全流域面上23个采样断面。如图 3所示，对于23个流域面上采样断面，高温季节的DOC浓度(2009年7月，DOCJul)明显大于寒冷季节(2010年1月，DOCJan)。DOCJul/DOCJan的比值变化于1.20~11.03之间，平均为3.30±2.40。值得一提的是，增江下游干流中DOCJul/DOCJan值较低，例如在STZ、DBX、QLZ、XL、DL和XHT等断面。这主要是因为下游河段冬季接纳的居民生活污水相对较多，使得DOCJan的数值升高所致。ZRD断面的情况例外，下文专门解释。2009年7月，最高的DOC浓度出现在几条支流断面，如PTH、TTZL-3和BSH，这些支流流域中茂密的植被与肥沃的土壤向河流中提供了更多的有机质。
2010年1月，ZRD断面恰好位于珠江口咸淡水交界处[37]，受海水的影响而保持较低的DOC含量(大约1.2~1.4mg/L，参见文献)。2013年4~5月间我们也曾在珠江口测试过62个水样的DOC浓度，平均是1.20±0.19mg/L(未发表数据)。
4.3 河流总悬浮颗粒物浓度及其有机碳含量的变化增江水体的总悬浮颗粒物浓度(CTSS)与流量呈显著的正相关关系(p < 0.0001，图 4a)，洪峰过境时浓度最大。在湿润亚热带，地表径流对土壤的侵蚀作用是河流总悬浮颗粒物的主要来源。然而，流水对有机质的侵蚀作用并非全年一致地作用于土壤表层。雨季来临的第一场较大降水，在华南地区一般发生在3月份或4月初[24]，对土壤造成的相对侵蚀强度最大；而且，在初次降雨过程中发生移动的土壤颗粒的有机质含量也最大。其原因是经过漫长的枯水期的风化分解作用，在土壤表层积累了较多的易于发生迁移的有机质[28, 40]。因此，2009年3月份的样品表现出“中等流量高有机碳含量”的特征(见图 4a中椭圆标注的)。如果把3月份的样品作为特例另行考虑，则其他13个样品中TSS含量与流量之间呈现显著的正相关关系(p < 0.0001，见图 4a)。
增江水体总悬浮颗粒物中有机碳的含量随着总悬浮颗粒物浓度的增加而呈指数趋势降低(图 4b)，这与其他地区的河流是一致的[23, 28, 40]。以下3种机制可以解释上述现象：第一，随着总悬浮颗粒物质的增加，河水的浑浊度变大，限制了水体中光合作用的效率，使得水体颗粒有机质中自源部分的份额降低。这一机制对于总体浑浊度较低的增江是有效的。第二，由于土壤颗粒的粒度及密度的差异，径流的侵蚀过程具有选择性，密度小且富含有机质的细颗粒更易于发生侵蚀。并且，这种侵蚀过程对土壤颗粒的选择性随着侵蚀强度的加深而逐渐变得模糊，因此表现为随着水体浑浊度的增加，其有机质含量降低[41]。第三，随着侵蚀强度的增加，沟蚀揭露出的底层土壤在总悬浮颗粒物中的比例增加，而底层土壤的有机质含量是较低的[42]。对于增江流域，第一种机制可能起主导作用。
4.4 河流有机质的C/N比所揭示的水生生物量的贡献现代河流中来自岩石中的地质有机碳的贡献可以忽略不计[43]。那么，河流中的颗粒有机碳就只有土壤有机碳和水生生物量两种主要来源。水生生物量的组分虽然较为复杂，但有一个共同特征，就是C/N比较低；而土壤有机质的C/N比明显高于水生生物量[44]。因此，河流有机质的成分可以借助C/N比建立一个简单的两端元模型加以区分[45]：
|
(1) |
公式(1) 中，A是C/N比，无量纲；B是贡献率(%)。下标“soil”和“aquatic”分别代表土壤有机质和水生生物量。
对于增江流域，可以假定Asoil=20.7±4.66(珠江流域采集的50个湿润铁铝土样本平均值[30])；Aaquatic=5.34(在麒麟咀水文站断面采集的14个样品的C/N比的最低值，假定这个具有最低C/N比的样品其有机质几乎都来自水生生物量)。两端元模型计算结果表明，麒麟咀水文站断面水生生物量的贡献率平均为93.0±6.5 %，变化范围是79 % ~100 %。
在增江麒麟咀水文站断面，河流悬浮颗粒有机质的C/N比变化于5.34~8.56之间，平均为6.43±1.04，这明显(p < 0.001) 低于在2002年到2003年间按照同样方式采集的样品，即变化于6.84~13.39，平均为9.80±1.66[45]。这表明河流下游大坝的建设使得水生生物量对河流悬浮有机质的贡献率明显增加。
4.5 坝上回水区水体CO2分压的时序变化由于水体中有机质的快速分解，大多数河流及河口区水体中CO2浓度相对于大气呈现超饱和状态[15, 46~48]。增江流域内的支流断面在洪水期及枯水期水体CO2分压相对于大气都表现为超饱和状态。然而，干流下游麒麟咀水文站断面的情况有显著不同，14个月的样品有6个月的水体(2009年2月、4月、5月、7月、8月和11月)CO2分压相对于大气都表现为不饱和状态(图 5和表 1)。
相对较低的水体CO2分压反映出水体中较为强烈的光合作用[20]，这与水体悬浮颗粒有机质具有的较低的C/N比及水体较高的叶绿素a含量的资料是相互印证的(表 1)。上述6个采自大坝以上麒麟咀水文站断面的样品中，水生植物量贡献了其颗粒有机质的大部分，此时这一段水域的流速与浑浊度很接近湖泊的性质。
当麒麟咀水文站断面水体的CO2分压呈现超饱和状态时，流量与水体的颗粒物含量都呈现较大值，TSS浓度通常大于10mg/L，在6月份的洪峰过境时甚至达到60mg/L。洪水为河流带来大量的土壤颗粒，这不但制约了水体光合作用的进行，也促进了水体微生物对有机质的分解过程，使得水体CO2分压与悬浮物浓度呈现显著的(p < 0.001) 正相关关系(图 6)。
|
图 6 麒麟咀水文站断面水体pCO2与总悬浮颗粒物浓度之间的关系 Fig. 6 Relationship between pCO2 and the CTSS in the water body at the QLZ sampling section |
用USGS Load Estimator模型(LoadEst[49, 50])估算增江流域的POC和DOC输出通量(kg·C/day)，并与实测数据进行对比。
LoadEst模型使用流量加权平均浓度并非算术平均浓度来估算POC和DOC的通量[49, 50]。图 7为估算结果。与实测数据相比较，模型估算的通量相对偏差对于POC是13.25 %，对于DOC是23.08 %。
|
图 7 用LoadEst模型计算的增江麒麟咀水文站断面DOC与POC的逐日输出量黑色空心圆代表实测值(根据流量、水体悬浮物浓度及其有机碳含量计算 Fig. 7 Simulated diurnal DOC and POC yields using LoadEst model. The black circles are the measured value, which calculated according to the measured CTSS, POC % and discharge, given that the discharge is constant in the surveying day |
从2009年5月19日到8月19日的103天，麒麟咀水文站断面一直处在洪水位，此间根据LoadEst模型估算的DOC和POC通量分别占全年通量的76.9 %和75.3 %；2009年度增江流域单位面积的DOC和POC输出通量分别为25.08×105g/km2·a和11.58×105g/km2·a。
将增江流域单位面积的DOC和POC输出通量与全球其他不同生物气候带的流域进行对比，结果列于表 3。根据POC输出通量与DOC输出通量的比值关系，以及总有机碳输出通量(TOC=POC+DOC)，我们把选定的流域划分为4类：第一类(Ⅰ)，总有机碳输出通量大，一般接近甚至超过10000kgC/km2·a，并且以POC为主，如西江流域[42]和北江流域[51]，流域地貌类型以山地丘陵为主，缺乏大面积的冲积平原与湿地。第二类(Ⅱ)，仍具有较高的总有机碳输出通量，一般介于2000~10000kgC/km2 ·a之间，但是DOC在总有机碳中占主导，例如亚马逊河流域[33, 52]、奥里诺科河流域[53]、Penobscot河流域[54]，以及位于西伯利亚的Mogot试验流域[55]和位于澳大利亚东北部热带雨林区的Thompson河流域[56]。这类流域一般人口密度较小，要么得益于高温多雨的自然条件，要么得益于低温下较小的蒸发量，流域内常年湿润，生态环境得到较好保护。第三类(Ⅲ)，这一类流域总有机碳的通量一般小于2000kgC/km2 ·a，且以DOC为主，流域地表较为平坦，如冈比亚河流域[57]、法国卢瓦尔河流域[58]、尼日尔河流域[59]、帕拉纳河流域[60]、圣劳伦斯河流域[61]等。因径流量小水力弱，这类流域经流水侵蚀输出的有机碳量较少。第四类(Ⅳ)，这一类流域总有机碳的通量一般也小于2000kgC/km2 ·a，但以POC为主，与第三类流域的明显区别在于流域范围内山地丘陵地貌为主，如位于美国新墨西哥州与德克萨斯州之间的Brazos河流域[62]和马更些河流域[61]等。
|
|
表 3 全球一些流域与增江流域的DOC和POC输出通量对比 Table 3 Comparison of the fluxes of DOC and POC reported from some river basins with that from the Zengjiang River basin |
按照上述划分方案，增江属于第二类流域。但是，增江流域与它附近的一些多山地丘陵的大流域相比，如西江流域[42]和北江流域[51]，在有机碳的输出特点上差别明显。
5 结论增江流域的地貌结构以山地丘陵为主，气候湿润且森林覆盖率高，这些自然地理因素导致增江的河流碳在输出通量及类别组成方面具有鲜明的特色；增江下游大坝回水区水体中颗粒有机碳性质的改变佐证了人类活动对河流碳动力学的影响。
汛期雨水的冲刷效应使得增江水体中异源有机质增加，而同期的高温也促进了河水中生物的生长，进而增加了自源有机质的含量。下游麒麟咀断面水体的DOC含量变化于0.76~4.75mg/L之间，平均为2.37±1.13mg/L，明显低于全球河流的平均值。增江较低的DOC含量归因于流域遍布山地丘陵的地貌特征；而河流中DOC含量最大值出现在夏季，与径流对森林与土壤中贮存的DOC的冲刷效应有关。冬季接纳的居民生活污水相对较多，使得一些下游断面DOC含量升高。
增江水体的总悬浮颗粒物浓度与流量呈显著的正相关关系。随着总悬浮颗粒物的增加，河水中光合作用的效率降低，使得水体颗粒有机质中自源部分的份额降低。总悬浮颗粒物中有机碳的百分含量随着总悬浮颗粒物浓度的增加而呈指数趋势降低。
2009~2010年间，麒麟咀断面总悬浮颗粒有机质的C/N比平均为6.43±1.04，明显低于在2002~2003年间按同样方式采集的样品，即9.80±1.66。表明下游大坝的建设使得水生生物量对河流悬浮有机质的贡献率明显增加。计算结果表明，2009~2010年间，麒麟咀断面水生生物量对POC的贡献率达到93.0±6.5 %，变化范围是79 % ~100 %。有些时段，筑坝还导致回水区水体中的CO2分压低于大气中。
2009年度，增江流域POC和DOC输出通量分别为11.58×105g/km2 ·a和25.08×105g/km2 ·a。对比全球河流，增江的总有机碳输出通量较高，且以DOC为主，类似于南美洲的亚马逊及奥里诺科等河流的碳动力学特征，而不同于纬度带与气候类型都接近的西江与北江。
致谢 感谢审核专家建设性的修改意见!
| 1 |
Aufdenkampe A K, Mayorga E, Raymond P A et al. Riverine coupling of biogeochemical cycles between land, oceans, and atmosphere. Frontiers in Ecology and the Environment, 2011, 9(1): 53-60. DOI:10.1890/100014 |
| 2 |
Cole J J, Prairie Y T, Caraco N F et al. Plumbing the global carbon cycle:Integrating inland waters into the terrestrial carbon budget. Ecosystems, 2007, 10(1): 171-184. |
| 3 |
Glendell M, Brazier R E. Accelerated export of sediment and carbon from a landscape under intensive agriculture. Science of the Total Environment, 2014, 476/477: 643-656. DOI:10.1016/j.scitotenv.2014.01.057 |
| 4 |
陶贞, 高全洲, 黄夏坤等. 桂江河流碳的生物地球化学循环:14C和13 C示踪. 第四纪研究, 2012, 32(3): 465-472. Tao Zhen, Gao Quanzhou, Huang Xiakun et al. Biogeochemical cycle of riverine carbon in the Guijiang River:Tracing with 14C and 13 C. Quaternary Sciences, 2012, 32(3): 465-472. |
| 5 |
Bustillo V, Victoria R L, Sousa de Moura J M et al. Biogeochemistry of carbon in the Amazonian floodplains over a 2000-km reach:Insights from a Process-Based Model. Earth Interactions, 2011, 15(4): 1-29. DOI:10.1175/2010EI338.1 |
| 6 |
Dawson J J C, Adhikari Y R, Soulsby C et al. The biogeochemical reactivity of suspended particulate matter at nested sites in the Dee Basin, NE Scotland. Science of the Total Environment, 2012, 434(1): 159-170. |
| 7 |
Fellman J B, Petrone K C, Grierson P F. Source, biogeochemical cycling, and fluorescence characteristics of dissolved organic matter in an agro-urban estuary. Limnology and Oceanography, 2011, 56(1): 243-256. DOI:10.4319/lo.2011.56.1.0243 |
| 8 |
Unger D, Herbeck L S, Li M et al. Sources, transformation and fate of particulate amino acids and hexosamines under varying hydrological regimes in the tropical Wenchang/Wenjiao rivers and estuary, Hainan, China. Continental Shelf Research, 2013, 57(S): 44-58. |
| 9 |
Hedges J I, Keil R G, Benner R. What happens to terrestrial organic matter in the ocean. ? Organic Geochemistry, 1997, 27: 195-212. DOI:10.1016/S0146-6380(97)00066-1 |
| 10 |
Cole J J, Caraco N F. Carbon in catchments:Connecting terrestrial carbon losses with aquatic metabolism. Marine and Freshwater Research, 2001, 52(1): 101-110. DOI:10.1071/MF00084 |
| 11 |
Raymond P A, McClelland J W, Holmes R M et al. Flux and age of dissolved organic carbon exported to the Arctic Ocean:A carbon isotopic study of the five largest arctic rivers. Global Biogeochemical Cycles, 2007, 21(4): GB4011. |
| 12 |
Raymond P A, Bauer J E. Riverine export of aged terrestrial organic matter to the North Atlantic Ocean. Nature, 2001, 409(6819): 497-500. DOI:10.1038/35054034 |
| 13 |
Battin T J, Kaplan L A, Findlay S et al. Biophysical controls on organic carbon fluxes in fluvial networks. Nature Geoscience, 2008, 1(2): 95-100. DOI:10.1038/ngeo101 |
| 14 |
Ronny L, Jens H, Nils M et al. What controls the spatial patterns of the riverine carbonate system?—A case study for North America. Chemical Geology, 2013, 337/338: 114-127. DOI:10.1016/j.chemgeo.2012.11.011 |
| 15 |
Teodoru C R, Del Giorgio P A, Prairie Y T et al. Patterns in pCO2 in boreal streams and rivers of northern Quebec, Canada. Global Biogeochemical Cycles, 2009, 23(2): GB2012. |
| 16 |
Compton J, Herbert C, Schneider R. Organic-rich mud on the western margin of southern Africa:Nutrient source to the Southern Ocean. ? Global Biogeochemical Cycles, 2009, 23(4): GB4030. |
| 17 |
Emmerton C A, Lesack L F W, Vincent W F. Mackenzie River nutrient delivery to the Arctic Ocean and effects of the Mackenzie delta during open water conditions. Global Biogeochemical Cycles, 2008, 22(1): GB1024. |
| 18 |
Kunz M J, Wueest A, Wehrli B et al. Impact of a large tropical reservoir on riverine transport of sediment, carbon, and nutrients to downstream wetlands. Water Resources Research, 2011, 47(12): W12531. |
| 19 |
Bouillon S, Abril G, Borges A V et al. Distribution, origin and cycling of carbon in the Tana River(Kenya):A dry season basin-scale survey from headwaters to the delta. Biogeosciences, 2009, 6(11): 2475-2493. DOI:10.5194/bg-6-2475-2009 |
| 20 |
Harrison J A, Maranger R J, Alexander R B et al. The regional and global significance of nitrogen removal in lakes and reservoirs. Biogeochemistry, 2009, 93(1-2): 143-157. DOI:10.1007/s10533-008-9272-x |
| 21 |
Parks S J, Baker L A. Sources and transport of organic carbon in an Arizona river-reservoir system. Water Research, 1997, 31(7): 1751-1759. DOI:10.1016/S0043-1354(96)00404-6 |
| 22 |
Wu Y, Zhang J, Liu S M et al. Sources and distribution of carbon within the Yangtze River system. Estuarine, Coastal and Shelf Science, 2007, 71(1-2): 13-25. DOI:10.1016/j.ecss.2006.08.016 |
| 23 |
Zhang S, Lu X X, Sun H G et al. Geochemical characteristics and fluxes of organic carbon in a human-disturbed mountainous river(the Luodingjiang River)of the Zhujiang(Pearl River), China. Science of the Total Environment, 2009, 407(2): 815-825. DOI:10.1016/j.scitotenv.2008.09.022 |
| 24 |
珠江志编篡委员会. 珠江志(第一卷). 广州: 广东科技出版社, 1991. 143~174 Pearl River Water Resources Committee(PRWRC). The Zhujiang Archive(vol. 1). Guangzhou:Guangdong Science and Technology Press, 1991. 143~174 |
| 25 |
Tao Zhen, Gao Quanzhou, Wang Zhengang et al. Estimation of carbon sinks in chemical weathering in a humid subtropical mountainous basin. Chinese Science Bulletin, 2011, 56(35): 3774-3782. DOI:10.1007/s11434-010-4318-6 |
| 26 |
Parsons T R, Maita Y, Lalli C M. A Manual of Chemical and Biological Methods for Seawater Analysis. New York:Pergamon Press, 1984, 101-114. |
| 27 |
Pierrot D, Lewis E, Wallace D W R. MS Excel Program Developed for CO2 System CalculationsORNL/CDIAC-105a. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U. S. Department of Energy. Oak Ridge, Tennessee, 2006. |
| 28 |
Ludwig W, Probst J L, Kempe S. Predicting the oceanic input of organic carbon by continental erosion. Global Biogeochemical Cycles, 1996, 10(1): 23-41. DOI:10.1029/95GB02925 |
| 29 |
Tao Zhen, Gao Quanzhou, Guo Wenping et al. Temporal and spatial variability of dissolved organic carbon concentration in the Xijiang River, South China. Journal of Mountain Sciences, 2011, 8(5): 694-703. DOI:10.1007/s11629-011-2000-3 |
| 30 |
邢长平, 沈承德, 孙彦敏等. 鼎湖山亚热带森林土壤有机质14C年龄初步研究. 地球化学, 1998, 27(5): 493-499. Xing Changping, Shen Chengde, Sun Yanmin et al. Preliminary results of 14C age of soil organic matter in Dinghushan subtropical forest soils. Geochimica, 1998, 27(5): 493-499. |
| 31 |
Spencer R G M, Aiken G R, Wickland K P et al. Seasonal and spatial variability in dissolved organic matter quantity and composition from the Yukon River basin, Alaska. Global Biogeochemical Cycles, 2008, 22(4): GB4002. |
| 32 |
Zhou W J, Zhang Y P, Schaefer D A et al. The role of stream water carbon dynamics and export in the carbon balance of a tropical seasonal rainforest, Southwest China. PloS One, 2013, 8(2): e56646. DOI:10.1371/journal.pone.0056646 |
| 33 |
Richey J E, Hedges J I, Devol A H et al. Biogeochemistry of carbon in the Amazon River. Limnology and Oceanography, 1990, 35(2): 352-371. DOI:10.4319/lo.1990.35.2.0352 |
| 34 |
Raymond P A, Saiers J E. Event controlled DOC export from forested watersheds. Biogeochemistry, 2010, 100(1-3): 197-209. DOI:10.1007/s10533-010-9416-7 |
| 35 |
Mann P J, Davydova A, Zimov N et al. Controls on the composition and lability of dissolved organic matter in Siberia's Kolyma River basin. Journal of Geophysical Research:Biogeosciences, 2012, 117(1): G01028. |
| 36 |
Macdonald M J, Minor E C. Photochemical degradation of dissolved organic matter from streams in the western Lake Superior watershed. Aquatic Sciences, 2013, 75(4): 509-522. DOI:10.1007/s00027-013-0296-5 |
| 37 |
谭超, 邱静, 黄本胜等. 东江下游潮区界、潮流界、咸水界变化对人类活动的响应. 广东水利水电, 2010, 10(1): 36-39. Tan Chao, Qiu Jing, Huang Bensheng et al. Changes of the seawater, tidal current and tidal marks at the lower reaches of the Dongjiang River and their response to human activities. Guangdong Water Resources and Hydropower, 2010, 10(1): 36-39. |
| 38 |
Callahan J, Dai M, Chen R F et al. Distribution of dissolved organic matter in the Pearl River Estuary, China. Marine Chemistry, 2004, 89(1-4): 211-224. DOI:10.1016/j.marchem.2004.02.013 |
| 39 |
Zhang J, Yu Z G, Wang J T et al. The subtropical Zhujiang(Pearl River)estuary:Nutrients, trace species and their relationship to photosynthesis. Estuarine, Coastal and Shelf Science, 1999, 49(3): 385-400. DOI:10.1006/ecss.1999.0500 |
| 40 |
Meybeck M. Carbon, nitrogen, and phosphorus transport by world rivers. American Journal of Science, 1982, 282(4): 401-450. DOI:10.2475/ajs.282.4.401 |
| 41 |
Wang Z G, Govers G, Steegen A et al. Catchment-scale carbon redistribution and delivery by water erosion in an intensively cultivated area. Geomorphology, 2010, 124(1-2): 65-74. DOI:10.1016/j.geomorph.2010.08.010 |
| 42 |
Gao Q, Tao Z, Shen C et al. Riverine organic carbon in the Xijiang River(South China):Seasonal variation in content and flux budget. Environmental Geology, 2002, 41(7): 826-832. DOI:10.1007/s00254-001-0460-4 |
| 43 |
Goni M A, Hatten J A, Wheatcroft R A et al. Particulate organic matter export by two contrasting small mountainous rivers from the Pacific Northwest, USA. Journal of Geophysical Research:Biogeosciences, 2013, 118(1): 112-134. DOI:10.1002/jgrg.20024 |
| 44 |
贾国东, 彭平安, 傅家谟. 珠江口近百年来富营养化加剧的沉积记录. 第四纪研究, 2002, 22(2): 158-165. Jia Guodong, Peng Ping'an, Fu Jiamo. Sedimentary records of accelerated eutrophication for the last 100 years at the Pearl River estuary. Quaternary Sciences, 2002, 22(2): 158-165. |
| 45 |
Gao Q Z, Tao Z, Yao G R et al. Elemental and isotopic signatures of particulate organic carbon in the Zengjiang River, Southern China. Hydrological Processes, 2007, 21(10): 1318-1327. DOI:10.1002/(ISSN)1099-1085 |
| 46 |
Raymond P A, Bauer J E, Cole J J. Atmospheric CO2 evasion, dissolved inorganic carbon production, and net heterotrophy in the York River estuary. Limnology and Oceanography, 2000, 45(8): 1707-1717. DOI:10.4319/lo.2000.45.8.1707 |
| 47 |
Raymond P A, Caraco N F, Cole J J. Carbon dioxide concentration and atmospheric flux in the Hudson River. Estuaries, 1997, 20(2): 381-390. DOI:10.2307/1352351 |
| 48 |
Richey J E, Melack J M, Aufdenkampe A K et al. Outgassing from Amazonian rivers and wetlands as a large tropical source of atmospheric CO2. Nature, 2002, 416(6881): 617-620. DOI:10.1038/416617a |
| 49 |
Lorenz D L, Runkel R L, De Cicco L. River Load Estimation, v0.2.(Available at https://github.com/USGS-R/rloadest.), 2013
|
| 50 |
Runkel R L, Crawford C G, Cohn T A. Load Estimator(LOADEST):A FORTRAN Program for Estimating Constituent Loads in Streams and Rivers, U. S. Geol. Surv. Tech. and Meth., Book 4, U. S. Geol. Surv. Denver, Colo., 69(Chapter A5), 2004. |
| 51 |
Gao Quanzhou. The riverine organic carbon output in subtropical mountainous catchment:The Beijiang River example. Journal of Geosciences of China, 2002, 4(1): 30-37. |
| 52 |
Moreira-Turcq P, Seyler P, Guyot J L et al. Exportation of organic carbon from the Amazon River and its main tributaries. Hydrological Processes, 2003, 17(7): 1329-1344. DOI:10.1002/(ISSN)1099-1085 |
| 53 |
Lewis W M, Saunders J F. Concentration and transport of dissolved and suspended substances in the Orinoco River. Biogeochemistry, 1989, 7(3): 203-240. DOI:10.1007/BF00004218 |
| 54 |
Huntington T G, Aiken G R. Export of dissolved organic carbon from the Penobscot River basin in north-central Maine. Journal of Hydrology, 2013, 476: 244-256. DOI:10.1016/j.jhydrol.2012.10.039 |
| 55 |
Suzuki K, Konohira E, Yamazaki Y et al. Transport of organic carbon from the Mogot Experimental Watershed in the southern mountainous taiga of eastern Siberia. Nordic Hydrology, 2006, 37(3): 303-312. DOI:10.2166/nh.2006.015 |
| 56 |
Bass A M, Bird M I, Liddell M J et al. Fluvial dynamics of dissolved and particulate organic carbon during periodic discharge events in a steep tropical rainforest catchment. Limnology and Oceanography, 2011, 56(6): 2282-2292. DOI:10.4319/lo.2011.56.6.2282 |
| 57 |
Lesack L R, Hecky R E, Melack J M. Transport of carbon, nitrogen, phosphorus and major solutes in the Gambia River, West Africa. Limnology and Oceanography, 1984, 29(4): 816-830. DOI:10.4319/lo.1984.29.4.0816 |
| 58 |
Meybeck M, Cauwet G, Dessery S et al. Nutrients(organic C, P, N, Si)in the eutrophic river Loire and its estuary. Estuarine Coastal and Shelf Science, 1988, 27(6): 595-624. DOI:10.1016/0272-7714(88)90071-6 |
| 59 |
Martins O, Probst J L. Biogeochemistry of major African rivers:Carbon and mineral transport. In:Degens E T, Kempe S, Richey J E eds. Biogeochemistry of Major World Rivers. SCOPE Rep. 42. New York:John Wiley, 1991. 127~155
|
| 60 |
Depetris P J, Cascante E A. Carbon transport in the Parana River. In:Degens E T, Kempe S, Herrera R eds. Transport of Carbon and Minerals in Major Worm Rivers, vol. 3, Mitt. Geol.-Palaont Inst. Univ. Hamburg.SCOPE/UNEP Sonderbed.58.Hamburg:University Hamburg, 1985. 299~304
|
| 61 |
Telang S A, Pocklington R, Nadu A S et al. Carbon and mineral transport in major North American, Russian Arctic, and Siberian rivers:The St. Lawrence, the Mackenzie, the Yukon, the Arctic Alaskan rivers, the Arctic basin rivers in the Soviet Union, and the Yenisei. In:Degens E T, Kempe S, Richey J E eds. Biogeochemistry of Major World Rivers. SCOPE Rep.42. New York:John Wiley,, 1991, 75-104. |
| 62 |
Hart R C. Carbon transport in the upper Orange River. In:Degens E T, Kempe S, Gan W B eds. Transport of Carbon and Minerals in Major World Rivers, vol.4, Mitt. Geol.-Palont Inst. Univ. Hamburg. SCOPE/UNEP Sonderbd. 64. Hamburg:University Hamburg, 1987. 509~512
|
| 63 |
Vidal-Abarca M R, Suárez M L, Guerrero C et al. Dynamics of dissolved and particulate organic carbon in a saline and semiarid stream of Southeast Spain(Chicamo stream). Hydrobiologia, 2001, 455: 71-78. DOI:10.1023/A:1011939418723 |
| 64 |
Malcom R L, Durum W H. Organic carbon and nitrogen concentration and annual organic carbon load of six selected rivers of the United States. US Geological Survey Water-Supply Paper, 1976, 1817F: 1-21. |
Abstract
Geomorphic features, climate characteristics and dam construction are the main factors controlling the cycling and export of riverine carbon to the ocean. The Zengjiang River(ZJR)is a second-order tributary of the Pearl River system, with an area of 3160km2. Silicate rocks dominated the bedrock within the drainage basin. The landforms within the drainage basin are dominated by mountains and hills covered with a thick layer of red weathering crusts on the surface. The soil is mainly of Udic Ferralisols, with parts of mountain areas covered by Peruelic Ferrallisols and Gleysol-Paddy soils. Annual mean temperature and precipitation is 21.6℃ and 2188mm, respectively. The vegetation is southern subtropical evergreen broad-leaved forest, with about 70% of forest coverage. The river water is clear at the usual time due to the low soil erosion rate within the basin, and is relatively turbid only during the bursting flood periods. The averaged annual discharge is 3.82×109m3 from 1954 to 2009, with 83.3% of which was discharged from April to September. A dam was built up and impounded in March 2008 at the lower reaches of the river, which led to a backwater section extended up to 22km. The Qilinzui Hydrological Station(QLZ:23°20.734'N, 113°50.399'E; 6m a.s.l.) controls 91% of the catchment area and located at the backwater section. Water samples were monthly collected at the QLZ section from December 2008 to January 2010. Water samples were also collected in July 2009(flood season)and January 2010(dry season)at other 23 sections on the mainstream or its tributaries. The analyzed water physical-chemical parameters include:the total alkalinity, temperature, pH, electrical conductivity, the concentrations of DOC, POC and chlorophyll-a, and other ions. The partial pressure of CO2 of the surface water was calculated using the CO2SYS program using measured parameters. In the flooding season, a flushing effect by rainfall enhanced the contribution of allochthonous DOC to the riverine carbon in the ZJR, meanwhile the higher atmospheric temperature also promoted the flourishing of autochthonous DOC in the ZJR. The concentration of DOC at the QLZ section varied from 0.76mg/L to 4.75mg/L, with an average of 2.37±1.13mg/L, which is much lower than the global averaged value. The low DOC concentration in the ZJR is a consequence of its mountainous and hilly topography within the drainage basin. However, the higher DOC concentration in the flood season than in other seasons is most likely explained by the significant scouring effect of the DOC pools stored in the forests and soils. The polluted effluents from more densely residential land at the lower reaches of the river basin in winter may be responsible for the higher DOC concentration in those sampling sections. Concentration of total suspended substance(TSS)in the ZJR was positively correlated significantly with the discharge. The photosynthesis in the river became faint with increasing concentration of TSS, which limited the production of autochthonous carbon. The content of organic carbon of TSS decreased exponentially with the concentration of the TSS for the ZJR. In the QLZ section, the C/N molar ratio of the riverine particulate organic matter is 6.43±1.04, which is significantly less than that collected from March 2002 to February 2003 at the same sampling section, i.e., 9.80±1.66, which demonstrates that the contribution from aquatic biomass to total POC had increased after the construction of the dam. The calculation results shown that the contribution from aquatic biomass to the POC from 2009 to 2010 at the QLZ section reached (93.0±6.5)%. Some time, the dam construction also led to a lower partial pressure of CO2 in the backwater area relative to that in the atmosphere. Annual POC and DOC specific yields of the ZJR basin were estimated to be 11.58×105g/km2·a and 25.08×105g/km2·a for 2009, respectively. Compared with other global rivers, the ZJR had a relatively high level of TOC yield, and with DOC being the dominating form of riverine organic carbon, which is similar to the carbon dynamics in the Amazon and Orinoco rivers in South America, and different to that in the Xijiang and Beijiang rivers which with a similar climate and latitude zones to the ZJR.