How soil bacteria contribute to atmospheric CO2

When soil microbes such as bacteria digest plant matter, it can either be used to build their bodies or be respired as carbon dioxide (CO2) into the atmosphere.

Understanding these pathways is crucial for predicting the impact of soil bacteria on climate change.

Soil bacteria and CO2

Soil bacteria play a crucial role in the carbon cycle by breaking down plant matter, a process that significantly influences the amount of CO2 released into the atmosphere.

When plants die, their remains – including leaves, stems, and roots – decompose and become part of the soil. Bacteria in the soil digest this plant material, utilizing it to build their cells or respiring it as CO2.

This microbial respiration is a major source of CO2 emissions from soil, contributing to atmospheric carbon levels. The type of plant matter influences how much CO2 is released.

For instance, lignin, a complex compound found in woody plant parts, results in higher CO2 emissions when decomposed by bacteria compared to cellulose, which is found in softer plant tissues like leaves and stems.

Soil bacteria contribution to atmospheric CO2

A Northwestern University-led research team has, for the first time, tracked how plant waste moves through soil bacteria’s metabolism to contribute to atmospheric CO2.

They found that microbes respire three times more CO2 from lignin carbons (non-sugar aromatic units) compared to cellulose carbons (glucose sugar units).

These findings provide valuable insights into soil carbon cycling and could improve predictions of how soil carbon will affect climate change.

Role of soil carbon and soil bacteria

“The carbon pool stored in soil is about ten times the amount in the atmosphere,” said Ludmilla Aristilde, who led the study. “What happens to this reservoir will significantly impact the planet. As temperatures rise, more organic matter will become available in soil, affecting CO2 emissions from microbial activities.”

Aristilde, an expert in environmental processes, is an associate professor at Northwestern University’s McCormick School of Engineering. The study includes collaborators from the University of Chicago.

Different pathways, different outcomes

The goal of this ongoing research is to understand how soil stores or releases carbon. In the past, researchers tracked how individual compounds from plant matter move through soil bacteria.

Aristilde’s team used a mixture of these compounds, mimicking the natural environment. They tagged individual carbon atoms with isotope labels to track their movement through a bacterium’s metabolism.

“Isotope labeling allowed us to track carbon atoms specific to each compound type inside the cell,” Aristilde explained. “By tracking the carbon routes, we captured their paths in the metabolism. Not all pathways are created equally in terms of producing CO2.”

Sugar carbons in cellulose travel through glycolytic and pentose-phosphate pathways, leading to reactions that convert digested matter into DNA and proteins.

However, aromatic, non-sugar carbons from lignin travel through the tricarboxylic acid cycle, which exists in all forms of life.

“The tricarboxylic acid cycle exists in plants, microbes, animals, and humans,” said Aristilde. “While this cycle also produces precursors for proteins, it contains several reactions that produce CO2. Most of the CO2 that gets respired from metabolism comes from this pathway.”

Expanding the findings

After tracking metabolism routes, the team performed quantitative analysis to determine CO2 production from different types of plant matter.

Soil bacteria and microbes respired three times as much CO2 from lignin carbons compared to cellulose carbons.

“Even though microbes consume these carbons simultaneously, the amount of CO2 generated from each carbon type is disproportionate,” Aristilde said. “That’s because the carbon is processed via two different metabolic pathways.”

In their initial experiments, the researchers used Pseudomonas putida, a common soil bacterium with versatile metabolism. They then studied data from previous experiments and found the same relationship in other soil bacteria.

A new perspective on soil bacteria and CO2

“We propose a new metabolism-guided perspective for understanding how different carbon structures accessible to soil microbes are processed,” said Aristilde. “This will be key in helping us predict what will happen with the soil carbon cycle in a changing climate.”

The study marks a significant step in understanding soil carbon dynamics and their broader implications for climate change.

The insights could pave the way for more accurate climate models and better-informed environmental policies, highlighting the crucial role of soil bacteria in our rapidly changing world.

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