Elucidating biotic and abiotic controls over soil carbon stabilization using isotope geochemistry
Creators: name: Serach, Lily Rose
name: 0000-0003-0024-3523
Publisher: name: The University of Texas at Austin
Year: 2021
Subjects: Soil carbon
Carbon cycle modeling
Biogeochemistry

description: The stability of soil organic carbon (SOC) is one of the largest sources of uncertainty in global carbon cycle models and is central in identifying potential feedbacks to a warming climate. Vertical SOC profiles are used to investigate the spatial and temporal processes involved in the soil carbon cycle. Interpretations of the vertical distribution of SOC concentrations, radiocarbon (¹⁴C), and stable carbon isotopes (δ¹³C) provide key information on carbon cycling processes in the soil. When these profiles are considered alongside flux of CO₂ out of the soil surface into the atmosphere, vertical trends in soil pore space CO₂ concentrations and the associated isotopic values (both stable and radiogenic), one can develop empirically-derived narratives on soil carbon dynamics at a slice of time in a soil. We can use models to the simulate development of these vertically resolved trends through time and therefore investigate the cumulative effect of different soil carbon cycle processes that ultimately result in the trends we observe. The goal of this dissertation research is to elucidate the key processes involved in SOC stabilization in forest soils using high resolution profile data and different numerical modeling techniques. The first chapter of this dissertation focuses on advancing the understanding of how carbon is transported to deeper depths in the soil, which is important for elucidating how stable SOC forms and persists through time, and what type of data are necessarily for reducing uncertainty in the model parameters used to represent various soil carbon cycle processes. The second chapter this dissertation focuses on the relationship between observed δ¹³C and C concentration profiles in well-drained forest soils. The third and final chapter of my dissertation involves another 1D reactive transport model to estimate the contribution of anaplerotic CO₂ fixation to observed trends in δ¹³C, Δ¹⁴C, and soil respiration rate in soils from 3 terrestrial biomes: boreal, temperate, and tropical. This chapter aims to quantify a potential Δ¹⁴C reservoir effect due to anaplerotic soil CO₂ fixation and determine the extent that the direct assimilation of soil CO₂ into microbial biomass contributes to the formation of soil organic matter as well as the formation of more stable SOC. Overall, the research and results that comprise this dissertation highlight the utility of soil profile data in understanding the long term cycling of carbon through the soil, and underscore the importance of key processes such as vertical transport of less labile SOC, the PCO₂ effect on C₃ plants, and anaplerotic soil CO₂ fixation in our understanding of how soil organic matter forms. A better understanding of these processes and the extent that they occur in different soils will inform interpretations of Δ¹⁴C and δ¹³C data and could help to predict the response of stable SOC to a changing climate, particularly in high-latitude soils