Soil may no longer be able to store carbon dioxide as emissions increase


A rhea bird walks through the smoke in Benjamín Aceval, Paraguay, on Friday, October 2, 2020.

A rhea bird walks through the smoke in Benjamín Aceval, Paraguay, on Friday, October 2, 2020.
Photo: Jorge Saenz (AP)

There are to trope In the world of climate denial, rising carbon dioxide levels is a good thing, actually, because it’s helping plants grow. Putting aside that saying that it ignores all harmful effects on life on Earth, there is also another problem with the tired topic of conversation.

A new study published in Nature on Wednesday suggests that while rising atmospheric carbon dioxide concentrations can They stimulate plant growth, it takes a huge price for another major source of carbon sequestration found only in those plants and trees.: the earth.

Dirt is a vital part of the carbon cycle, yet the impact of increased carbon dioxide in the soil is a notable gap in the literature. The authors of the new study decided to fill that gap. The study notes that there is a widely accepted assumption that soil carbon levels will increase as plants sequester more carbon because when those plants die, they decompose and become soil. But there hasn’t been much evidence to back that up.

“As a scientist, I was perplexed by how little we knew about the effects of [estimated concentrations of carbon dioxide] on soil carbon stocks compared to plant traits, “wrote Cesar Terrer, a fellow at Lawrence Livermore National Laboratory and a Stanford University postdoctoral fellow who worked on the study, in an email.

For the study, Terrer and her colleagues analyzed data from 108 previously published papers that focused on soil carbon levels and plant growth amid increasing carbon concentrations. They found that when carbon levels increase, so do levels of organic matter in the soil. But contrary to what conventional wisdom would have them believe, the researchers found that an increase in soil biomass generally coincided with a decrease in soil carbon levels.

“We were expecting faster plant growth and more biomass to increase soil organic carbon as additional leaves and biomass fall to the forest floor,” wrote Rob Jackson, professor of earth system science at Stanford University. and lead author of the study, in an email. “It wasn’t like that, and that was the biggest surprise in our work.”

In fact, the authors found that the soil only accumulated more carbon in experiments in which, although the atmosphere had high concentrations of carbon, plant growth continued at a constant rate rather than increasing rapidly.

The authors think they know why this happens: As plants grow faster, they need more nutrients, which they extract from the soil. For plants to access more nutrients, soils must grow microbes such as bacteria and fungi faster. That forces them to increase their rate of microbial respiration, which releases carbon into the atmosphere that otherwise it could have remained on earth.

Not all ecosystems, the authors write, will behave in the same way in this regard. According to their meta-analysis, the authors’ modeled the amount of carbon that the soil of various landscapes will absorb as atmospheric carbon increases. They found that if carbon dioxide concentrations reach double pre-industrial levels, the carbon intake rate from forest soils will remain stable, but grassland soils will increase by 8%. That’s probably the case because in grasslands, plants allocate more carbon to their roots rather than to the surface, and studies Show that decaying roots tend to put more carbon into the soil than other parts of plants. This suggests that world leaders should focus on restoration. and conservation efforts in these ecosystems as a form of climate mitigation.

These findings have important implications of how climate scientists account for the amount of carbon that forests, grasslands and wetlands can sequester. Since existing climate projections do not account for the trade-off between soil and plant carbon sequestration, they are probably overestimating the potential of the earth to absorb carbon and mitigate global warming. That means we may not have as much of a margin on carbon pollution as we thought.

“Forests and other lands currently absorb a third of global carbon pollution, about 12 billion tons of carbon dioxide each year,” Jackson said. “We need to understand if this valuable service will continue. “

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