River deltas are often considered as hotspots of atmospheric carbon regulation, but few studies have paid attention to the differences between topsoil and subsoil for carbon sequestration. Here physical and chemical stabilization mechanisms and sources of soil organic carbon (OC) accumulation in different soil layers of a major river delta (Yellow River delta) were studied by fractionation and isotopic analysis. We found that in topsoil, conversion from tidal flat to wetland substantially increases OC by 84%, whereas cotton cultivation decreases OC by 66%. Wetland with a high soil OC content is characterized by substantial accumulation of plant-derived OC and OC in mineral fractions. In subsoil, the OC variation is controlled by the yellow silt layer (YSL)–red clay layer (RCL) sequence. Although the weathering intensity of the RCL is much higher than that of the YSL, the OC age is younger in the RCL, indicating that the highly reactive, secondary minerals and iron oxides are capable of stabilizing younger organic carbon in transportation, deposition and post-pedogenic alteration. ¹⁴C activity demonstrates that modern autochthonous OC is the main component of OC in wetland topsoil. In contrast, millennia-old allochthonous OC is the main component of OC in the subsoil layers, contributing 4–24% of total bulk soil OC. The higher δ¹³C values of carbon fractions (−24.3 to −21.4‰ for the intra-aggregate particulate organic matter fraction, intra-microaggregate silt and clay fraction, and free silt and clay faction) and bulk samples (~ −22‰) are likely to imply the contribution of aged OC in the Yellow River delta. This study offers direct confirmation that soil OC fractions and sources are influenced by different land-use types and pedological properties in characteristic soil layers. These findings suggest that biogeochemical alteration of the soil matrix controls OC stabilization mechanisms in the river sediment–tidal flat–wetland–farmland continuum.

中文翻译：

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三角洲表土和底土层中有机碳成分和来源的变化：本土和异地输入

河流三角洲通常被认为是大气碳调控的热点，但很少有研究关注表土和底土在固碳方面的差异。在这里，通过分馏和同位素分析研究了主要河流三角洲（黄河三角洲）不同土壤层中土壤有机碳 (OC) 积累的物理和化学稳定机制和来源。我们发现在表土中，从滩涂到湿地的转化使 OC 显着增加了 84%，而棉花种植使 OC 减少了 66%。土壤有机碳含量高的湿地的特征是植物来源的有机碳和矿物成分中有机碳的大量积累。在底土中，OC 变化受黄色粉砂层 (YSL)-红粘土层 (RCL) 序列控制。¹⁴ C 活性表明现代本土有机碳是湿地表土有机碳的主要成分。相比之下，千年历史的外来 OC 是底土层中 OC 的主要成分，占总土壤 OC 的 4-24%。较高的 δ ¹³碳组分的 C 值（-24.3 至 -21.4‰ 的内聚集颗粒有机质组分、微聚集内粉砂和粘土组分以及游离粉砂和粘土组分）和大块样品（~ -22‰）可能意味着黄河三角洲老化有机碳的贡献。本研究直接证实了土壤有机碳成分和来源受特征土壤层中不同土地利用类型和土壤学特性的影响。这些发现表明，土壤基质的生物地球化学改变控制了河流沉积物-潮滩-湿地-农田连续体中 OC 的稳定机制。