海岸带滨海土壤和沉积物中红色黏土层对有机碳的固定机制

The coastal zone is an area that strongly influenced by land-ocean interactions and anthropogenic activities and plays an important role in global carbon cycle due to its high carbon (blue carbon) sequestration rates. Therefore, understanding the carbon sequestration mechanisms and carbon sink capacity in coastal soils and sediments through a holistic view of interactions between land and ocean is of great theoretical and practical significance for adaptation to global climate change and improvement in coastal sustainability. The Yellow River Delta (YRD) is one of the most active land-ocean interaction areas among large river deltas in the world. Our previous work found a wide distribution of special red clay layers (RCL) in the 1 m depth soil profiles of the modern delta. The RCL has much stronger weathering intensity and higher active minerals (such as clay minerals and iron oxides) contents than other soil layers, which shows significant accumulation of organic carbon and other biogenic elements. However, the mechanisms of carbon sequestration in the RCL under land-ocean interactions are still not well understood. In this study, the RCL samples from the YRD soils and sediments and samples from the Yellow River suspended sediments will be collected. The terrestrial and marine sources of organic carbon, distribution characteristics of organic carbon fractions and structures and stability of organic carbon sequestered in the RCL will be thoroughly investigated by using biomarker, carbon isotope and synchrotron radiation techniques. This project aims at providing the scientific basis for revealing the mechanisms and potential of organic carbon sequestration in coastal deep soils and sediments.

海岸带是陆海相互作用和人类活动强烈影响的区域，海岸带系统的高固碳（蓝碳）速率使其在全球碳循环中发挥重要作用，陆海统筹地研究海岸带土壤和沉积物的固碳机制和碳汇潜力，在应对全球气候变化、推动海岸带可持续发展方面具有重要的科学和现实意义。黄河三角洲是世界上大型河流三角洲中最活跃的陆海交互区域之一。我们前期研究发现黄河三角洲现代沉积区1 m土体内广泛分布着红色富含黏粒土层（红黏层）。红黏层的风化程度、黏土矿物和铁氧化物等活性矿物含量要明显高于其他土层，能够显著富集有机碳等生源要素，但目前对陆海相互作用下红黏层的固碳机制还不明确。本项目通过收集黄河三角洲土壤和沉积物红黏层样品、黄河悬浮泥沙样品，结合生物标志物、碳同位素和同步辐射等技术手段，深入研究红黏层固定的陆源和海源有机碳种类、有机碳组分的分配特征、有机碳的结构特征与稳定性，为揭示海岸带滨海深层土壤和沉积物有机碳固定机制和碳汇潜力提供理论依据。

河口三角洲-陆架区域是河流由陆向海输入物质的中转站，有机碳在三角洲土壤、河流沉积物和河口沉积物之间的滞留、交换和源汇等过程是研究海岸带有机碳循环的重要课题。本项目以黄河三角洲土壤红黏层为重要抓手，在对其系统的空间范围和地球化学描述的基础上，运用物理分级、化学提取、同位素分析、结构分析等技术手段，研究黄河三角洲红黏层固定有机碳的来源、结合方式和稳定性，分析红黏层固定的有机碳与无机碳的内在联系，以黄河三角洲为基础评估海岸带大尺度范围土壤有机碳分布规律。主要结果如下：（1）明确了黄河三角洲土壤红黏层的分布范围和元素组成。黄河三角洲1 m土体内的红黏层主要分布在以黄河沉积物为母质发育的潮土区域，组成红黏层的细颗粒物质在迁移沉积成土中，存在氮和重金属过剩、磷亏缺的元素不平衡现象，可能是人类活动影响和营养盐再生过程之外，造成河口区元素不平衡的另一重要因素；（2）揭示了黄河三角洲土壤红黏层的有机碳来源、结合方式和稳定性。红黏层-黄砂层序列土壤有机碳以矿物结合的外源老化（年龄4000～7000 BP）有机碳为主，其中化石来源有机碳占10～24%，黏土矿物、铁矿物和碳酸盐矿物三者可以解释红黏层-黄砂层序列74.3%的有机碳变异；（3）次生碳酸盐是黄河三角洲土壤红黏层中有机碳转化固定的潜在组分。红黏层中无机碳的积累速率可高达197 g m-2 yr-1，该层中无机碳δ13C值均值为-5‰左右，明显低于其他土层，说明植物根系呼吸或有机质矿化产生的CO2可能形成了次生碳酸盐而固定；（4）以黄河三角洲工作为基础，系统评估了我国海岸带大尺度范围蓝碳生境土壤有机碳的储量分布和影响因素。包括黄河三角洲在内的我国海岸带蓝碳生境有机碳埋藏速率范围为6-159 Gg yr-1，其中外源有机碳的平均占比在40%左右。上述研究结果可为科学评估海岸带滨海土壤和沉积物不同层次有机碳的源汇关系提供依据。