OPUS: MCS - What Remains? Quantifying the First Steps of Soil Organic Carbon Formation

富碳植物碎屑（枯叶、根、茎和枝）融入土壤的过程还不清楚。近世纪来，人们认为土壤中的大部分碳是由植物化合物组成的，这些化合物能抵抗微生物的分解（柠檬酸盐土壤碳假说）。最近的研究表明，微生物可以分解几乎所有的植物衍生化合物，土壤碳主要由微生物及其产生的化合物组成。如果植物碎屑的完全分解被发现是一种普遍现象，它将颠覆土壤碳的模式，并有必要改变全球碳通量模型。由于微生物产生的化合物与植物碎屑中的化合物不同，因此可以从它们在土壤中留下的化学指纹中推断出微生物来源的土壤碳的比例。 该项目将使用最先进的分析化学方法来表征来自10年分解实验（从热带到苔原）的存档样本上的微生物指纹的程度。由此产生的数据将用于确定植物碎屑是否在土壤中产生大量不可分解碳，或者是否基本上所有植物碎屑都转化为微生物生物量。这一精确的理解将增强目前土壤碳形成的概念框架，并使土壤碳形成的过程能够在模型中更准确地表示。该项目还将开发一个关于土壤碳形成和凋落物分解建模的定量教学单元，该单元将开放供更广泛的教育界使用。无法准确量化分解过程中凋落物化学变化的方法和数据阻碍了准确概念模型的开发。该项目将通过量化垃圾化学如何随时间变化来解决这一差距，跨越气候，并与位置（地上与地下），使用固态13 C-NMR来表征世界上空间最广泛的长期分解实验之一的存档样本（长期站点间分解团队，LIDET）。具体而言，本项目将解决三个问题，建立一个新的概念模型的凋落物分解的目标：（1）作为凋落物分解，剩余的质量微生物来源的或不可分解的凋落物？(2)凋落物分解如何随位置或气候而变化，从而导致土壤中更有效的C稳定，是否存在定量差异？(3)随着时间的推移，不同类型的垃圾是否由于形成相似的微生物化合物而聚集在相似的组成上？这些问题的答案将改变凋落物分解在概念上和预测模型中的表现方式。目前的LIDET数据集是地球系统模型的基准，该项目将增加有关垃圾化学成分在分解过程中如何在空间和时间上变化的宝贵信息。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估来支持。

The processes by which carbon-rich plant detritus (dead leaves, roots, stems, and limbs) is incorporated into soil are not well understood. For nearly a century, most of the carbon in soils was thought to consist of plant-based compounds resistant to decomposition by microbial organisms (the recalcitrant soil carbon hypothesis). More recent research indicates that microbes can decompose virtually all plant-derived compounds, and that soil carbon consists largely of microorganisms and compounds they produce. If complete decomposition of plant detritus is found to be a widespread phenomenon, it would upend the paradigm of recalcitrant soil carbon, and necessitate changes in global models of carbon flux. Because microbes produce compounds distinct from those in plant detritus, the fraction of soil carbon that is microbially-derived can be inferred from the chemical fingerprint they leave in the soil. This project will use a state-of-the-art analytical chemistry method to characterize the extent of this microbial fingerprint on archived samples from a 10-year decomposition experiment (form the tropics to the tundra). The resulting data will be used to determine if plant detritus generates a significant pool of undecomposable carbon in the soil, or if essentially all plant detritus is transformed into microbial biomass. This refined understanding will augment the current conceptual framework for soil carbon formation, and will enable the process of soil carbon formation to be more accurately represented in models. This project will also develop a quantitative teaching unit on soil carbon formation and litter decomposition modeling, which will be open access and available to the broader educational community.The development of an accurate conceptual model is hindered by methods and data that fail to accurately quantify changes in litter chemistry during decomposition. This project will address this gap by quantifying how litter chemistry changes over time, across climates, and with position (above- vs. belowground), using solid-sate 13C-NMR to characterize archived samples from one of the most spatially extensive long-term decomposition experiments in the world (Long-term Intersite Decomposition Team, LIDET). Specifically, this project will address three questions with the goal of creating a new conceptual model of litter decomposition: (1) As litter decomposes, is remaining mass microbially-derived or undecomposable litter? (2) Are there quantitative differences in how litter decomposition varies with position, or among climates, that lead to more efficient C stabilization in soils? (3) Over time, do different types of litter converge on a similar composition due to the formation of similar microbial compounds? The answers to these questions will transform how litter decomposition is represented conceptually, and in predictive models. The current LIDET dataset is a benchmark for Earth System Models, and this project will augment it with valuable information on how litter chemical composition changes across space and time during decomposition.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.