Abstract Earthworm cast ageing controls soil ecosystem services such as carbon storage and nutrient availability. To test the hypothesis that biogeochemical changes during casts ageing can be traced by infrared spectroscopy, we collected topsoil aggregates and earthworm casts produced by the anecic earthworm Amynthas khami belonging to five distinct degradation stages in woodland in northern Vietnam. We analysed mid-infrared (MIR) spectra of both sample types using two approaches: (1) calculation of the proportion of aliphatic compounds and the humification index of organic matter (OM) using specific wavenumbers and (2) linear discriminant analysis (LDA) of the most important peaks and shoulders of the entire spectrum. Our results showed that aliphatic compound contribution to OM was similar and the humification index of OM lower (2-fold) in casts as compared to topsoil aggregates independent of their degradation stage. In contrast, LDA showed a continuous shift from fresh casts to aged casts and topsoil aggregates in agreement with biogeochemical changes of cast properties. We thus conclude that MIR analysis of bulk samples and LDA taking into account the absorbance intensity of most important wavenumbers is a promising tool to capture biogeochemical alterations of organic and mineral earthworm cast properties during ageing.

中文翻译：

---

中红外光谱在田间条件下追踪蚯蚓铸件在老化过程中的生物地球化学变化

摘要 蚯蚓铸件老化控制土壤生态系统服务，如碳储存和养分可用性。为了验证可以通过红外光谱追踪铸件老化过程中的生物地球化学变化的假设，我们收集了越南北部林地中属于五个不同退化阶段的厌食蚯蚓 Amynthas khami 产生的表土聚集体和蚯蚓铸件。我们使用两种方法分析了两种样品类型的中红外 (MIR) 光谱：(1) 使用特定波数计算脂肪族化合物的比例和有机物质 (OM) 的腐殖化指数和 (2) 线性判别分析 (LDA)整个光谱中最重要的峰和肩峰。我们的结果表明，脂肪族化合物对 OM 的贡献相似，并且与表土聚集体相比，铸件中 OM 的腐殖化指数较低（2 倍），与其降解阶段无关。相比之下，LDA 显示出从新鲜铸件到老化铸件和表土骨料的连续转变，与铸件特性的生物地球化学变化一致。因此，我们得出结论，大量样品和 LDA 的 MIR 分析考虑了最重要波数的吸收强度，是捕获老化过程中有机和矿物蚯蚓铸件特性的生物地球化学变化的有前途的工具。