There is a too much in the world of climate negation, that rising levels of carbon dioxide are good, in fact, because it helps plants grow. Leaving aside this saying that ignores all the harmful effects on life on Earth, there is also another problem with the point of tired talking.
A new one to study published in Nature on Wednesday suggests that while atmospheric carbon dioxide concentrations are rising tin stimulate plant growth is a great loss another important source of carbon sequestration found only in these plants and trees: the sun.
Dirt is a vital part of the carbon cycle, but the impact of the increase in carbon dioxide on the soil is a notable gap in the literature. The authors of the new study decided to fill this gap. The study notes that there is a widely accepted assumption that carbon levels in the soil 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 it up.
“As a scientist, I was perplexed by how little we knew about the effects [estimated concentrations of carbon dioxide] on soil carbon reserves compared to plant traits, ”wrote in an email Cesar Terrer, a member of the Lawrence Livermore National Lab and a postdoctoral fellow at Stanford University who worked on the study.
For the study, Terrer and colleagues analyzed data from 108 previously published papers focusing on soil carbon levels and plant growth amid rising carbon concentrations. They found that when carbon levels increase, so does the levels of organic matter in the soil. But contrary to what conventional wisdom led them to believe, the researchers found that an increase in soil biomass typically coincided with a decrease in soil carbon levels.
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“We were expecting faster plant growth and more biomass to increase soil organic carbon as additional leaves and biomass fall to forest floor,” Rob Jackson, a professor of system science, wrote in an email. Earth at Stanford University and lead author of the study. “He didn’t, and that was the biggest surprise of our work.”
In fact, the authors found that the soil only accumulated more carbon in experiments where, although the atmosphere had high concentrations of carbon, plant growth continued at a steady pace rather than increasing rapidly.
The authors believe they know why this is so: as plants grow faster, they need more nutrients to extract from the soil. To give plants access to more nutrients, soils need they cultivate microbes such as bacteria and fungi faster. This requires them to increase the rate of microbial respiration, which releases carbon into the atmosphere otherwise it may have remained on the ground.
The authors do not write that all ecosystems will behave the same way in this regard. Based on their meta-analysis, the authors’ modeled to the amount of carbon that the soil will absorb from various landscapes as atmospheric carbon increases. They found that if carbon dioxide concentrations reach double pre-industrial levels, the carbon input rate of forest soils will remain flat, but grassland soils will increase by 8%. This is likely to be the case because in the prairies, plants spend more of their carbon on their roots rather than exceeding them and studies show that decaying roots tend to introduce more carbon into the soil than other parts of plants. This suggests that world leaders should focus on restoration and conservation efforts of these ecosystems as a form of climate mitigation.
These findings are important implications for how climate scientists explain the amount of carbon that forests, grasslands, and wetlands can sequester. Since existing climate projections do not explain the balance between soil and plant carbon sequestration, they probably overestimate the earth’s potential to absorb carbon and mitigate global warming. This means that we may not have as much leeway with carbon pollution as we might have thought.
“Forests and other lands currently absorb a third of the world’s carbon pollution, about 12 billion tons of carbon dioxide each year,” Jackson said. “We have to understand if this valuable service will continue ”.