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The capacity of forest soils to store carbon is overestimated

Climate models predict that by the end of the century, with increasing CO2 concentrations, plants will grow more than they are today. This will make them absorb carbon dioxide and help slow down the global climate crisis.

Scientists model that increased plant growth also leads to increased soil carbon storage. This is due to a hypothesis from the 1970s in which it was assumed that the carbon in the soil increases proportionally and directly with the plant growth.

However, a study based on empirical data from experiments, in which the researcher participates Cesar Terrer from Stanford University, and a researcher at the Institute of Environmental Science and Technology (ICTA-UAB) during the development of this work, shows that the amount of carbon in plant biomass and in soil do not increase in tandem; in fact when biomass increases, the increase in soil carbon decreases.

“It is difficult to estimate the impact of the finding on climate models. What does seem clear is that the fact that the models do not incorporate this negative relationship between plant and soil carbon is overestimating the carbon storage on forest soils ”, he explains to SINC Terrer.

The fact that the models do not incorporate this negative relationship between plant and soil carbon is overestimating carbon storage in forest soils

Cesar Terrer

This is because plants extract the nutrients they need from the soil to maintain that extra carbon-driven growth. The extraction of the additional nutrients requires accelerating the microbial activity of the soil, which entails the CO2 release back to the atmosphere that would otherwise remain in the I usually.

Therefore, including this relationship correctly would result in a lower capacity of future forest ecosystems to absorb CO2 and, therefore, a more aggressive global warming. “On the other hand, herbaceous ecosystems they seem to have a capacity to absorb carbon in soil much greater than that predicted by the models ”, says the scientist. The study is published in the journal Nature.

Carbon storage in soils

The authors reached this conclusion after analyzing data from 108 experiments Previously published reports on soil carbon levels, plant growth, and high concentrations of CO2 in the air.

They only accumulated more carbon in those experiments where plant growth was fairly stable, despite high levels of carbon in the atmosphere. “It was much more difficult than expected to increase both plant growth and soil carbon,” he said. Rob jackson, co-author and Professor at Stanford University.

It was much more difficult than expected to increase both plant growth and soil carbon

Rob jackson

Plants and soils together currently absorb 30% of the CO2 emitted by human activities per year. Predicting how the underground portion of this carbon sink will change in the coming decades is especially important because carbon absorbed by the soil tends to stay there for a long time.

“When a plant dies, part of the carbon that has accumulated in its biomass can return to the atmosphere, again contributing to global warming. Carbon can be stored in soils for centuries or millennia, ”Terrer explained.

Get to emissions zero

In 2019, the researcher published in the journal Nature Climate Change another study in which it was estimated that doubling atmospheric CO2 from pre-industrial levels, as expected by the end of this century, will increase the plant biomass by 12% on average. This was three times less than previously estimated. In other words, plants are likely to play a much less important role in the carbon reduction than believed.

“The new study focuses on the ability of soils to absorb CO2, while the 2019 study focused on the ability of plants. However, we cannot make an exact estimate of the implications of the new finding for the total capacity of soils to absorb CO2 towards the end of the century, as there is a temporal component (soil carbon increases slowly) that cannot be resolved. with experiments ”, assures Terrer.

In the scientist’s opinion, the main tool to stop the global warming is to curb CO2 emissions to zero. “This has to be complemented by optimizing the capacity of ecosystems to absorb CO2 naturally, including the protection of ecosystems against deforestation, degradation or fire. The objective is that both plants and soils can operate at maximum performance to absorb CO2 ”, says the scientist.

Low emissions together with an increase in the absorption in soils and plants would help to achieve negative emissions

Low emissions together with an increase in the absorption in soils and plants would help to achieve negative emissions. This would ensure that temperatures do not rise above two degrees on average, reflected in the Paris Agreement.

Act on agricultural soils

“In terms of soils more specifically, the greatest opportunity to absorb CO2 is in agricultural soils. Agriculture has removed more than 130 PgC from the soil, which is 13 times all the carbon dioxide that humans release into the atmosphere each year. In fact, it is estimated that by increasing soil carbon by 0.4% annually we could cancel all CO2 emissions and slow down climate change ”, continues the expert.

Ensuring carbon-rich soils is the best strategy to maintain fertile soils and thus be able to feed a growing population

Cesar Terrer

Research indicates that grasslands can absorb unexpectedly large amounts of carbon in the coming decades. In a scenario in which atmospheric CO2 doubles the pre-industrial levels, the researchers estimate that carbon sequestration in grassland soils will increase by 8%, while carbon sequestration by forest soils will remain virtually neutral. This happens despite the fact that CO2 enrichment gives a greater boost to the biomass in forests (23%) than in grasslands (9%), in part, because trees allocate a relatively small portion of the carbon they absorb underground.

More sustainable agricultural practices such as no-till, composting, rotations, cover crops, and increased biodiversity – including legumes that naturally add nitrogen to the soil – would increase soil carbon. “Guaranteeing soils rich in carbon is the best strategy to maintain fertile soils and thus be able to feed a growing population, while also contributing to curbing climate change”, he concludes.

Reference:

J. Rosende et al. “A trade-off between plant and soil carbon storage under elevated CO2”. Nature.

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