Large quantities of soil carbon (C) can persist within paleosols for millennia due to burial and subsequent isolation from plant-derived inputs, atmospheric conditions, and microbial activity at the modern surface. Erosion exposes buried soils to modern root-derived C influx via root exudation and root turnover, thus stimulating microbial activity leading to SOC decomposition and accumulation through organo-mineral stabilization of modern C. With this study we aim to quantify how modern root-derived C inputs impact paleosol C decomposition and stabilization across varying degrees of isolation from modern surface conditions in southwestern Nebraska, USA, where hillslope erosion is bringing a buried Late-Pleistocene-early Holocene paleosol (the “Brady Soil”) closer to the modern surface. We collected Brady Soil samples from 0.2 m, 0.4 m, and 1.2 m below the modern surface and conducted two lab-based incubations. Soils were amended with either (1) a lab-synthesized mixture of low molecular weight compounds (12 atom% C), or (2) C enriched root residues (92 atom% C), in 30-day and 240-day incubation experiments, respectively. We determined microbial responses to synthetic root exudates and residues by partitioning the C label from Brady Soil C, including measurements of total, root, and primed C respiration, microbial biomass C (MBC), microbial C use efficiency (CUE). To assess the capacity of isolated paleosols to accrue modern plant C, we used Nano-scale Secondary Ion Mass Spectrometry imaging. We found that: (1) adding root-derived C inputs primed Brady Soil C across all depths, and was mediated by depth and composition of root additions; (2) root-derived C inputs stimulated microbial biomass C (MBC) growth similarly across depths, but the magnitude of CUE and MBC varied by chemistry of root-derived additions; (3) new particulate organic matter was incorporated into mineral-associated pools over time; (4) material from the added root residues was found in association with bacterial cells and fungal hyphae as well as with soil aggregate and mineral surfaces. Our study shows that paleosols defy expectations of C content and reactivity with depth, and changes in land cover and climate will expose buried paleosols to modern surface conditions, increasing respired C. This work highlights the importance of evaluating the role resurfacing buried soils through landscape change plays in C cycle feedbacks to the climate system.

中文翻译：

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找到问题的根源：美国内布拉斯加州西南部侵蚀山坡上埋藏的古土壤中的土壤碳和微生物对根系输入的反应


由于埋藏以及随后与植物源输入、大气条件和现代地表微生物活动的隔离，大量土壤碳 (C) 可以在古土壤中持续存在数千年。侵蚀使埋藏土壤通过根系分泌物和根周转暴露于现代根系碳流入，从而刺激微生物活动，通过现代碳的有机矿物稳定作用导致 SOC 分解和积累。通过这项研究，我们的目的是量化现代根系碳如何流入。美国内布拉斯加州西南部的山坡侵蚀使埋藏的晚更新世-全新世早期的古土壤（“布雷迪土壤”）更接近现代地表，这些输入对与现代地表条件不同程度的隔离影响古土壤碳的分解和稳定。我们从现代地表以下 0.2 m、0.4 m 和 1.2 m 处收集了布雷迪土壤样本，并进行了两次实验室培养。在 30 天和 240 天的培养实验中，用 (1) 实验室合成的低分子量化合物混合物（12 原子% C）或 (2) 富含碳的根残留物（92 原子% C）对土壤进行改良， 分别。我们通过从布雷迪土壤 C 中分离 C 标签来确定微生物对合成根分泌物和残留物的反应，包括总碳呼吸、根呼吸和启动碳呼吸、微生物生物量碳 (MBC) 和微生物碳利用效率 (CUE) 的测量。为了评估分离的古土壤产生现代植物 C 的能力，我们使用了纳米级二次离子质谱成像。 我们发现：（1）添加根系来源的碳输入可以在所有深度上启动布雷迪土壤碳，并且是由根系添加的深度和组成介导的； (2) 根部来源的碳输入刺激了不同深度的微生物生物量碳 (MBC) 的生长，但 CUE 和 MBC 的大小因根部来源添加的化学成分而异； (3) 随着时间的推移，新的颗粒有机物被纳入矿物伴生池中； (4)发现来自添加的根残留物的材料与细菌细胞和真菌菌丝以及土壤团聚体和矿物质表面相关。我们的研究表明，古土壤违背了对碳含量和随深度的反应性的预期，土地覆盖和气候的变化将使埋藏的古土壤暴露于现代地表条件，增加呼吸的碳。这项工作强调了评估通过景观变化重塑埋藏土壤的作用的重要性在气候系统的 C 循环反馈中发挥作用。