Soil will release more CO2 as plants hit a growth spurt

As increasing levels of atmospheric carbon dioxide accelerate plant growth, the soil may not be able to keep up with the pace. Researchers at Stanford University found that when elevated CO2 levels give plants a boost, the soil begins to lose its ability to store more carbon. 

One likely explanation is that plants effectively mine the soil for nutrients they need to keep up with carbon-fueled growth, according to the study authors. The extra nutrients are made available to plants through increased microbial activity in the soil, which releases CO2 that would otherwise remain locked away.

Scientists have widely accepted the idea that plant biomass and soil carbon will increase simultaneously. However, the researchers found that this is not the case. 

“When plants increase biomass, usually there’s a decrease in soil carbon storage,” said lead author César Terrer.

The team analyzed data from 108 previously published studies on high CO2 concentrations, soil carbon levels, and plant growth. The results showed that soils only accumulated more carbon in experiments where plant growth remained fairly steady. 

“It proved much harder than expected to increase both plant growth and soil carbon,” said senior study author Professor Rob Jackson. He explained that climate projections do not account for this tradeoff, which means they likely overestimate the potential of the terrestrial carbon sink. 

Each year, plants and soils collectively absorb an estimated 30 percent of the CO2 emissions associated with human activities. It is particularly important to understand how the underground part of the terrestrial carbon sink will change in the coming decades, considering that carbon absorbed by the soil tends to remain stored for a very long time. 

“When a plant dies, some of the carbon that accumulated in its biomass may return to the atmosphere. In soils, carbon can be stored for centuries or millennia,” said Terrer.

In 2019, the Stanford team determined that as CO2 levels continue to rise, plants will likely play a far less significant role in drawing down carbon than previously predicted. The new research indicates that the soil will also fall short as a carbon sink as plant biomass increases. 

“Soils store more carbon worldwide than is contained in all plant biomass,” said Professor Jackson. “They need much more attention as we project the fate of forests and grasslands to the changing atmosphere.”

The study is published in the journal Nature.

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By Chrissy Sexton, Earth.com Staff Writer