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

富含碳的植物碎屑（枯叶、根、茎和肢）与土壤结合的过程尚不清楚。近一个世纪以来，土壤中的大部分碳被认为是由植物基化合物组成的，它们抵抗微生物的分解（顽固土壤碳假说）。最近的研究表明，微生物几乎可以分解所有植物来源的化合物，土壤碳主要由微生物和它们产生的化合物组成。如果发现植物碎屑的完全分解是一种普遍现象，它将颠覆顽固土壤碳的范式，并有必要改变全球碳通量模式。因为微生物产生的化合物与植物碎屑中的化合物不同，所以微生物产生的土壤碳的比例可以从它们在土壤中留下的化学指纹中推断出来。该项目将使用最先进的分析化学方法来描述从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。结果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。这种精细化的理解将增强当前土壤碳形成的概念框架，并将使土壤碳形成过程在模型中得到更准确的表示。该项目还将开发一个关于土壤碳形成和凋落物分解模型的定量教学单元，该单元将向更广泛的教育界开放。不能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发展。该项目将通过量化凋落物化学随时间、气候和位置（地上与地下）的变化来解决这一差距，使用固体13C-NMR来表征世界上空间最广泛的长期分解实验之一（长期站点间分解小组，LIDET）的存档样本。具体来说，该项目将解决三个问题，目标是创建一个新的凋落物分解概念模型：(1)随着凋落物的分解，剩余的大量微生物产生的还是不可分解的凋落物？(2)凋落物分解随位置或气候的变化是否存在数量差异，从而导致土壤中碳的更有效稳定？(3)随着时间的推移，不同类型的凋落物是否由于形成了相似的微生物化合物而趋于相似的组成？这些问题的答案将改变凋落物分解的概念和预测模型。目前的LIDET数据集是地球系统模型的基准，该项目将为其提供有关分解过程中垃圾化学成分如何跨越空间和时间变化的宝贵信息。该奖项反映了美国国家科学基金会的法定使命，并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，被认为值得支持。

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.