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

University Of Vermont & State Agricultural College, Burlington VT

Investigators

Abstract

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.