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

富碳植物碎屑(枯叶、根、茎和四肢)结合到土壤中的过程还不是很清楚。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，这些化合物不会被微生物分解(顽固的土壤碳假设)。最近的研究表明，微生物几乎可以分解所有植物衍生的化合物，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个普遍的现象，它将颠覆顽固的土壤碳的范式，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，从它们留在土壤中的化学指纹可以推断出微生物来源的土壤碳的比例。该项目将使用一种最先进的分析化学方法来表征从10年(从热带到冻土带)分解实验的存档样品上这种微生物指纹的程度。由此产生的数据将被用来确定植物碎屑是否在土壤中产生了大量的不可分解碳库，或者是否基本上所有的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目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描述。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量化教学单元，该单元将开放给更广泛的教育界使用。由于方法和数据无法准确量化分解过程中凋落物化学的变化，阻碍了准确概念模型的开发。该项目将通过量化凋落物化学随时间、跨气候和位置(地上和地下)的变化来解决这一差距，使用固态13C-核磁共振来表征来自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)的存档样品。具体地说，这个项目将解决三个问题，目的是建立一个新的凋落物分解概念模型：(1)随着凋落物的分解，残留的大量凋落物是微生物衍生的还是不可分解的凋落物？(2)凋落物分解在不同位置或不同气候之间的变化是否存在数量上的差异，从而导致土壤中更有效的碳稳定？(3)随着时间的推移，不同类型的凋落物是否由于形成类似的微生物化合物而汇聚到类似的组成上？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示方式。目前的LIDET数据集是地球系统模型的基准，该项目将增加关于分解过程中凋落物化学成分如何在空间和时间上变化的有价值的信息。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.