Graduate Defense: Megan Kelly-Slatten

Dissertation Information

Title: Harnessing The Power Of C4 Perennial Grasses To Understand Soil Carbon Dynamics

Program: Doctor of Philosophy in Ecology, Evolution, and Behavior

Advisor: Dr. Marie-anne de Graaff, Biological Sciences

Committee Members: Dr. Kevin Feris, Biological Sciences; Dr. Leonora Bittleston, Biological Sciences; and Dr. Julie Jastrow, Biological Sciences

Abstract

Soil organic carbon (SOC) is the world’s largest terrestrial carbon sink, and plays a critical role in supporting ecosystem functions, food production, and climate change mitigation. However, traditional land management practices have depleted SOC. Thus, increasing SOC levels has become a key strategy to improve human welfare and ensure both sustainable food production and effective climate regulation. Plant roots serve as the primary pathway for transferring atmospheric C belowground. However, a fundamental paradox in terrestrial biogeochemistry is that root C input to the soil can either provide a net C sink by promoting soil C stabilization, or it can stimulate the loss of existing soil C through enhanced microbial degradation of SOC. While we know that root biomass can impact soil C dynamics, we do not fully understand how the variability in morphological and chemical root traits interact with the soil microbial community and impact the balance of root-C retention and decomposition of existing SOC. Our study aims to gain a better understanding of the mechanisms at the root-soil-microbe interface so that we can better develop models and management practices to increase SOC.

The planting of native C4 perennial grasses, such as switchgrass (Panicum virgatum) and big bluestem (Andropogon gerardii), has demonstrably increased SOC accumulation. This is attributed to two key factors: the extensive root systems of C4 grasses that enhance soil C inputs, and their low resource requirements, which necessitate minimal management interventions. These characteristics position C4 grasses as promising candidates for climate change mitigation strategies. Both species and cultivar variations in C4 grasses significantly impact SOC accumulation. This inherent variability makes them ideal candidates for elucidating the mechanisms influencing SOC dynamics. By leveraging this natural diversity, we can identify key plant traits that govern SOC accumulation and retention within the soil.

We used an ongoing study at Argonne National Laboratory to investigate how differences in C4 grasses impacted SOC dynamics after 10 and 14 years of plant growth. Chapter 1 investigates how root structure and soil chemical properties influence the accumulation of SOC after 10 years of plant growth. This research explores how root morphology affects the quantity and chemistry of C inputs and implications for SOC retention. Chapter 2 examines how bioavailable C and microbial community attributes (size, structure, and function) impact SOC accumulation patterns in soil fractions. This research focuses on how microbial communities impact decomposition of plant material and contribute to SOC formation and loss. Finally, chapter 3 measures the retention of SOC between 10 and 14 years of plant growth. This research investigates how the chemical composition of roots mediates the decomposition and loss of plant material within the soil. In entirety, this research will use C4 grasses as model species to improve the understanding of SOC dynamics.