There’s a trope in the climate denial world that rising carbon dioxide levels are good, actually, because it’s helping plants grow. Putting aside that saying that ignores all the deleterious effects on life on Earth, there’s also another issue with the tired talking point.
A new study published in Nature on Wednesday suggests that while increased atmospheric carbon dioxide concentrations can stimulate plant growth, it takes a huge toll another important source of carbon sequestration which lies just those plants and trees: the soil.
Dirt is a vital part of the carbon cycle, yet the impact of rising carbon dioxide on soil is a notable gap in the literature. The authors of the new study decided to fill that gap. The study notes there is a widely accepted assumption that carbon levels in soil will increase as plants sequester more carbon because when those plants die, they decompose and turn into soil. But there hasn’t been much proof 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,” Cesar Terrer, a fellow at Lawrence Livermore National Lab and postdoctoral scholar at Stanford University who worked on the study, wrote in an email.
For the study, Terrer and his colleagues analysed data from 108 previously published papers focused on soil carbon levels and plant growth amid increasing carbon concentrations. They found that when carbon levels rise, levels of organic matter in the soil also increase. But unlike what conventional wisdom would have them believe, the researchers found that an increase in soil biomass usually coincided with a decrease in soil carbon levels.
“We expected faster plant growth and more biomass to increase soil organic carbon, as extra leaves and biomass fall to the forest floor,” Rob Jackson, a professor of Earth system science at Stanford University and the study’s senior author, wrote in an email. “It didn’t, and that was the biggest surprise in our work.”
In fact, the authors found that soil only accumulated more carbon in experiments where, even though the atmosphere had high concentrations of carbon, plant growth continued at a steady pace instead of rapidly increasing.
The authors think they know why this is: As plants grow taller faster, they require more nutrients, which they extract from the soil. To give plants access to more nutrients, soils must grow microbes like bacteria and fungi more quickly. That requires them to increase their rate of microbial respiration, which releases carbon into the atmosphere that may have otherwise remained in the earth.
Not all ecosystems, the authors write, will behave the same way in this regard. Based on their meta-analysis, the authors modelled how much carbon various landscapes’ soil will absorb as atmospheric carbon increases. They found that if carbon dioxide concentrations reach double pre-industrial levels, forest soils’ rate of carbon intake will remain flat, but grassland soils will increase 8%. That’s likely the case because in grasslands, plants allocate more of their carbon to their roots rather than aboveground, and studies show that decomposing roots tend to put more carbon into the soil than other parts of plants. This suggests that world leaders should focus restoration and conservation efforts on these ecosystems as a form of climate mitigation.
These findings have major implications for how climate scientists account for the amount of carbon that forests, grasslands, and wetlands can sequester. Since existing climate projections don’t account for the tradeoff between soil and plant carbon sequestration, they’re probably overestimating land’s potential to suck up carbon and mitigate global warming. That means we may not have as much leeway with carbon pollution as we may have thought.
“Forests and other lands currently absorb one-third of global carbon pollution, about 12 billion tons of carbon dioxide each year,” said Jackson. “We need to understand whether this valuable service will continue.”