School of Science/Te Aka Mātuatua, University of Waikato, Private Bag 3105, Hamilton, 3240, New Zealand.
Department of Ecology and Environmental Science, Umeå University, 901 87, Umeå, Sweden.
Sci Rep. 2021 Oct 29;11(1):21310. doi: 10.1038/s41598-021-00109-9.
Allophanic tephra-derived soils can sequester sizable quantities of soil organic matter (SOM). However, no studies have visualized the fine internal porous structure of allophanic soil microaggregates, nor studied the carbon structure preserved in such soils or paleosols. We used synchrotron radiation-based transmission X-ray microscopy (TXM) to perform 3D-tomography of the internal porous structure of dominantly allophanic soil microaggregates, and carbon near-edge X-ray absorption fine-structure (C NEXAFS) spectroscopy to characterize SOM in ≤ 12,000-year-old tephra-derived allophane-rich (with minor ferrihydrite) paleosols. The TXM tomography showed a vast network of internal, tortuous nano-pores within an allophanic microaggregate comprising nanoaggregates. SOM in the allophanic paleosols at four sites was dominated by carboxylic/carbonyl functional groups with subordinate quinonic, aromatic, and aliphatic groups. All samples exhibited similar compositions despite differences between the sites. That the SOM does not comprise specific types of functional groups through time implies that the functional groups are relict. The SOM originated at the land/soil surface: ongoing tephra deposition (intermittently or abruptly) then caused the land-surface to rise so that the once-surface horizons were buried more deeply and hence became increasingly isolated from inputs by the surficial/modern organic cycle. The presence of quinonic carbon, from biological processes but vulnerable to oxygen and light, indicates the exceptional protection of SOM and bio-signals in allophanic paleosols, attributable both to the porous allophane (with ferrihydrite) aggregates that occlude the relict SOM from degradation, and to rapid burial by successive tephra-fallout, as well as strong Al-organic chemical bonding. TXM and C NEXAFS spectroscopy help to unravel the fine structure of soils and SOM and are of great potential for soil science studies.
富含水铝英石的火山灰衍生土壤可以固定大量土壤有机碳（SOM）。然而，目前还没有研究对水铝英石土壤微团聚体的精细内部多孔结构进行可视化，也没有研究保存在这种土壤或古土壤中的碳结构。我们使用基于同步辐射的传输 X 射线显微镜（TXM）对主要由水铝英石组成的土壤微团聚体的内部多孔结构进行了 3D 层析成像，并用近边 X 射线吸收精细结构（C NEXAFS）光谱法对<12000 年历史的火山灰衍生的富含水铝英石（含少量水铁矿）古土壤中的 SOM 进行了表征。TXM 层析成像显示，在由纳米团聚体组成的水铝英石微团聚体内部存在着大量曲折的纳米孔网络。四个地点的水铝英石古土壤中的 SOM 主要由羧酸/羰基官能团组成，其次是醌型、芳香族和脂肪族官能团。尽管各地点之间存在差异，但所有样品的组成都相似。SOM 中不包含特定类型的官能团，这表明这些官能团是遗留下来的。SOM 起源于陆地/土壤表面：持续的火山灰沉积（间歇性或突然）导致陆地表面上升，使曾经位于地表的土层被埋得更深，从而与表层/现代有机循环的输入越来越隔绝。存在的醌型碳来自生物过程，但易受氧气和光照的影响，这表明在富含水铝英石的古土壤中，SOM 和生物信号受到了极好的保护，这归因于将遗留的 SOM 隔离起来使其免受降解的多孔水铝英石（含水铁矿）团聚体，以及由连续的火山灰沉降所导致的快速埋藏，以及强的 Al-有机化学键。TXM 和 C NEXAFS 光谱学有助于揭示土壤和 SOM 的精细结构，对土壤科学研究具有巨大的潜力。
