Analysis of stable carbon (C) isotopic signatures (δC) in various soil C pools provides useful information on soil C sources, transport, and availability. Understanding the extent of deeper soil (below 40 cm) sequestration and transport of plant derived C is of particular interest as this provides a source for microorganisms to drive biological denitrification (as indicated by denitrification enzyme activity, DEA) and hence mitigate the nitrate leaching to groundwater. Meanwhile, studies on deeper soil C sequestration are rare due to methodological constraints. This study was done in deeper vadose zone (0–160 cm) of a sandy agricultural soil in a humid and temperature zone after a C-C vegetation change taking advantage of the marked isotopic differences between C and C plants. It took place in a site previously grown with C crops (beet, barley, grass), but where C crops (maize) were grown continuously for the last 20 years. The other site where C crops were continuously grown was used for comparison. Specifically, the δC signature in top and deeper soil layers was used to distinguish between old C– and newly C-plant derived C in four C pools, i.e., bulk soil C, hot- and cold-water extractable C, and respired CO-C. The δC signature between C soils and C-C shifted soils was similar for bulk soil C but significantly different for water extractable and respired C pools. Hence, we estimated that the contribution of newly derived C to bulk soil C was negligible, whereas the contributions to the other C pools amounted up to 28.4 % along the soil profile. This emphasizes the importance of simultaneously analysing δC signature in various soil C pools to accurately assess C vertical transport and distribution. The concentrations of cold-DOC and values of specific ultraviolet visible absorbance of the wavelengths 254 and 280 nm decreased from 50 to 130 cm soil depths, while they increased below these depths. However, this suggested rise in C chemical quality at the deepest soil depths did not cause an increase in soil respiration activity or DEA, which was attributed to the protective effects of iron and aluminium oxides on C decomposition. Upon the application of labile C and N substrates, the deepest soil layers displayed a significantly increased DEA, suggesting the presence of a relatively abundant population of active denitrifying organisms. Overall, this study documents the presence of plant-derived C in the deeper vadose zone. Meanwhile, this particular C pool might not be an important substrate to drive deep-soil denitrification due to constraints imposed by the protection by metal oxides.

中文翻译：

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沙质农业土壤深层包气带中新植物来源的碳不会刺激反硝化


对各种土壤碳库中稳定碳 (13C) 同位素特征 (δ13C) 的分析提供了有关土壤碳源、运输和可用性的有用信息。了解更深的土壤（40 厘米以下）封存和植物来源的碳的运输程度特别令人感兴趣，因为这为微生物驱动生物反硝化（如反硝化酶活性，DEA 所示）提供了来源，从而减轻了硝酸盐的淋滤地下水。同时，由于方法学的限制，对深层土壤碳封存的研究很少。这项研究是在 CC 植被变化后，利用 C 和 C 植物之间显着的同位素差异，在潮湿和温度区域的沙质农业土壤的更深渗流区（0-160 厘米）中进行的。它发生在一个以前种植 C 作物（甜菜、大麦、草）的地点，但在过去 20 年里一直种植 C 作物（玉米）。另一个连续种植C作物的地点被用来进行比较。具体来说，表层和深层土壤中的 δ13C 特征用于区分四个碳池中旧的碳和新的碳植物衍生的碳，即散装土壤碳、热水和冷水可提取的碳以及呼吸的二氧化碳。 C。对于散装土壤 C，C 土壤和 CC 移动土壤之间的 δ13C 特征相似，但对于水可提取碳库和呼吸碳库而言，δ13C 特征显着不同。因此，我们估计新产生的碳对大块土壤碳的贡献可以忽略不计，而沿土壤剖面对其他碳库的贡献高达 28.4%。这强调了同时分析各种土壤碳库中 δ13C 特征以准确评估碳垂直传输和分布的重要性。 冷DOC的浓度以及波长254和280 nm的特定紫外可见光吸光度值从50到130 cm土壤深度下降，而在这些深度以下则增加。然而，这表明最深处土壤碳化学质量的提高并没有导致土壤呼吸活动或 DEA 的增加，这归因于铁和铝的氧化物对碳分解的保护作用。在施用不稳定的碳和氮底物后，最深层的土壤显示出 DEA 显着增加，表明存在相对丰富的活性反硝化生物种群。总体而言，这项研究记录了更深渗流区中植物来源的 C 的存在。同时，由于金属氧化物保护的限制，这种特殊的碳库可能不是驱动深层土壤反硝化的重要底物。