Abstract. Understanding controls on the persistence of soil organic matter (SOM) is essential to constrain its role in the carbon cycle and inform climate-carbon cycle model predictions. Emerging concepts regarding formation and turnover of SOM imply that it is mainly comprised of mineral-stabilized microbial products and residues, however, direct evidence in support of this concept remains limited. Here, we introduce and test a method for isolation of isoprenoid and branched glycerol dialkyl glycerol tetraethers (GDGTs) – diagnostic membrane lipids of archaea and bacteria, respectively – for subsequent natural abundance radiocarbon analysis. The method is applied to depth profiles from two Swiss pre-alpine forested soils. We find that the ∆¹⁴C values of these microbial markers markedly decrease with increasing soil depth, indicating turnover times of millennia in mineral subsoils. The contrasting metabolisms of the GDGT-producing microorganisms indicates it is unlikely that the low ∆¹⁴C values of these membrane lipids reflect heterotrophic acquisition of ¹⁴C-depleted carbon. We therefore attribute the ¹⁴C-depleted signatures of GDGTs to their physical protection through association with mineral surfaces. These findings thus provide strong evidence for the presence of stabilized microbial necromass in forested mineral soils.

中文翻译：

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森林矿物土壤中的千禧世代GDGT：基于¹⁴ C的微生物坏死稳定化证据

摘要。了解对土壤有机质（SOM）持久性的控制对于限制其在碳循环中的作用并为气候-碳循环模型预测提供信息至关重要。关于SOM的形成和周转的新兴概念意味着它主要由矿物质稳定的微生物产品和残留物组成，但是，支持该概念的直接证据仍然有限。在这里，我们介绍并测试一种分离类异戊二烯和支链甘油二烷基甘油四醚（GDGTs）的方法-分别是古细菌和细菌的诊断膜脂质-用于随后的自然丰度放射性碳分析。该方法适用于两种瑞士高山前森林土壤的深度剖面。我们发现∆ ¹⁴这些微生物标志物的C值随着土壤深度的增加而显着降低，表明矿物底土中千年的周转时间。产生GDGT的微生物的新陈代谢形成对比，表明这些膜脂的低¹⁴ C值不太可能反映贫¹⁴ C碳的异养性获取。因此，我们将GDGT的^14种C耗竭特征归因于其通过与矿物表面结合而提供的物理保护。因此，这些发现为有森林的矿物土壤中存在稳定的微生物坏死提供了有力的证据。