When the biomass of plants increases in response to rising carbon dioxide levels, the amount of carbon the soil is able to store decreases, according to an analysis of over 100 experiments published in Nature. Current models of terrestrial carbon sinks do not account for this trade-off, so future projections may need to be revised.
Terrestrial ecosystems remove about 30% of the carbon dioxide emitted by human activities every year. Plants sequester carbon dioxide as they use photosynthesis to fuel their growth, whereas soils lock carbon away as biomass decays. However, as carbon dioxide emissions continue to rise, it is unclear how this carbon sink may respond.
One hypothesis suggests that rising levels of atmospheric carbon dioxide will increase the ability of both plants and soil to sequester carbon, but César Terrer and colleagues show that this may not be the case. Analysing data from 108 elevated carbon dioxide experiments, the authors reveal an inverse relationship. When plant biomass increases as a result of elevated carbon dioxide levels, storage of carbon in the soil declines. In their experiments, elevated carbon dioxide levels resulted in an increase in soil stocks of carbon in grasslands (by around 8%), but not in forests. This was despite an increase in forest biomass of around 23%.
The authors suggest that this trade-off could be related to the way that plants acquire their nutrients. As plants grow, their roots mine the soil for nutrients. This may decrease the ability of the soil to sequester carbon, the authors conclude.
After the embargo ends, the full paper will be available at: https://www.nature.com/articles/s41586-021-03306-8
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