Nikhil Chari Thesis Defense (Benton Taylor, Advisor)

Title: Deciphering the relationships between climate change, plant root exudation, and the terrestrial carbon sink

Abstract: Soil is the largest terrestrial carbon sink, and soil carbon storage is governed by the movement of carbon from the atmosphere into soil via plants. Most plant carbon inputs to soil – like leaf litter, root litter, or coarse woody debris – must undergo decomposition by soil microbes before they can impact soil carbon stocks, a process that can take months, years, or even decades. However, plants also export low-molecular weight carbon compounds called root exudates directly from live roots into soil. Because root exudates are often recent photosynthate, the production of these compounds can be rapidly impacted by the changes to the plant’s environment. And, because root exudates are non-structural low-molecular weight compounds, they do not need to undergo decomposition before they can rapidly impact soil carbon formation and loss. 

In this dissertation, I sought to determine the magnitude of the root exudate carbon flux, its responsiveness to climate change, and its potential to impact soil carbon stocks. I show that the root exudation is a large carbon flux, comprising an estimated 7-14% of annual terrestrial gross primary productivity. Across multiple climate change experiments, I show that root exudation demonstrates flexible responses to climate change, which vary depending on plant species and climate change drivers involved. Specifically, I show that warming impacts root exudation more strongly than elevated CO2, and that air and soil warming appear to impact exudation rates in opposite directions, with exudation rates increasing strongly with temperature when both factors are combined. 

Root exudate impacts on soil carbon can be positive or negative, because root exudates can form soil carbon by binding to soil minerals but also promote soil carbon loss by enhancing soil microbial respiration rates in a process known as the “priming effect.” Using artificial root exudate experiments, I show that higher rates of root exudation increase rates of soil carbon loss relative to rates of soil carbon formation, which can convert soils from net carbon sinks to net carbon sources. However, I also demonstrate that warming also impacts the microbial community consuming root exudates, reducing their capacity to carry out the priming effect. Overall, I show that root exudation is a large carbon flux which is responsive to climate change and exerts near-immediate impacts on soil carbon dynamics, indicating that it merits incorporation into soil and carbon cycle models as an independent carbon flux. I suggest that increases in root exudation in response to climate warming will reduce the strength of the future soil carbon sink, but this effect could be attenuated by soil microbial community responses to warming.

Committee: Benton Taylor (Advisor), Paul Moorcroft, Noel Holbrook, Ann Pearson, Rich Phillips