Increased Plant Growth Driven by Higher CO2 Levels Takes a Steep Toll on Soil

The growth of plants is fueled by carbon dioxide (CO₂) present in the atmosphere. With an increase in carbon levels, it is attractive to imagine supercharged plant growth and huge tree-planting campaigns reducing the CO₂ generated by agriculture, fossil fuel burning, and other human activities.

But a new study published in the journal Nature on March 24^th, 2021, proposes that when increased carbon dioxide levels induce increased plant growth, it has an unexpectedly expensive toll on the soil, which is another huge carbon sink.

According to the authors, one possible explanation for this is that plants mine the soil efficiently for nutrients they require to sustain with carbon-fueled growth. Absorbing additional nutrients necessitates stepping up microbial activity, which eventually discharges CO₂ into the air that might otherwise remain in the soil locked forever.

The results of the study are in contrast to an extensively agreed assumption that the levels of soil carbon and biomass will increase in tandem as more plant biomass tends to fall to the ground and get converted into organic matter.

Investigation of data from 108 previously reported experiments that deal with soil carbon levels, high concentrations of CO₂ in the air, and plant growth by the authors fascinated them to discover the opposite.

When plants increase biomass, usually there’s a decrease in soil carbon storage.

César Terrer, Study Lead Author and Postdoctoral Scholar, Stanford University

Terror, who is also a fellow at Lawrence Livermore National Laboratory, and his collaborators discovered that soils only accumulated more carbon in experiments where plant growth continued to be fairly stable in spite of the high levels of carbon present in the air.

“It proved much harder than expected to increase both plant growth and soil carbon,” stated senior author of the study Rob Jackson, a professor of Earth system science in Stanford’s School of Earth, Energy & Environmental Sciences (Stanford Earth).

According to Jackson, climate projections that are extensively utilized at present do not consider this tradeoff. Consequently, they probably exaggerate the ability of the land to bring down CO₂ from Earth’s atmosphere.

At present, plants and soils collectively absorb approximately 30% of the CO₂ discharged by human activities annually.

Forecasting how the underground part of this carbon sink will vary in the decades to come is particularly essential since carbon absorbed by soil tends to remain there for a longer period.

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.

César Terrer, Study Lead Author and Postdoctoral Scholar, Stanford University

The study is based on research by Terrer, Jackson, and collaborators published in 2019 predicting that a doubling of atmospheric CO₂ from pre-industrial levels—as anticipated by the end of this century—will elevate plant biomass levels by only around 12%.

Put differently, plants will probably play a much less important role in reducing carbon compared to what was anticipated earlier.

Currently, the researchers have discovered that predictions for one more piece of the climate puzzle must also be revised by analyzing how carbon storage functions in plants and soils collectively.

“Soils store more carbon worldwide than is contained in all plant biomass. They need much more attention as we project the fate of forests and grasslands to the changing atmosphere,” stated Jackson, who is also a senior fellow at the Stanford Woods Institute for the Environment.

The study indicates that grasslands might take up unpredictably huge amounts of carbon in the decades to come.

Under a scenario where atmospheric CO₂ increases to twice that of pre-industrial levels, the team predicts that carbon absorption in grassland soils will increase by 8%, while carbon uptake by forest soils will continue to be approximately flat.

This is despite CO₂ enrichment providing a huge boost to biomass in forests (23%) compared to grasslands (9%), partially because trees release a comparatively small portion of the carbon absorbed by them underground.

From a biodiversity point of view, it would be a mistake to plant trees in natural grassland and savanna ecosystems. Our results suggest these grassy ecosystems with very few trees are also important for storing carbon in soil.

César Terrer, Study Lead Author and Postdoctoral Scholar, Stanford University

Rob Jackson is the Michelle and Kevin Douglas Provostial Professor at Stanford Earth. The Co-authors of the study are affiliated with Indiana University, Bloomington; Northern Arizona University; Oak Ridge National Lab; University of Exeter; University of California, Berkeley; Lawrence Berkeley National Lab; ETH Zurich.

Other universities are Swiss Federal Institute for Forest, Snow and Landscape Research WSL; University of Minnesota, St. Paul; Western Sydney University; University of Cambridge; University of Oxford; Washington State University; California Institute of Technology; University of California, Los Angeles; and the University of Antwerp.

The study was financially supported by Lawrence Livermore National Lab, the U.S. Department of Energy, the California Institute of Technology, and NASA.

Journal Reference:

Terrer, C., et al. (2021) A trade-off between plant and soil carbon storage under elevated CO₂. Nature. doi.org/10.1038/s41586-021-03306-8.