20 March 2024

Soil releases more GHGs than expected as drought plays a crucial role

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A U.S. study hypothesizes the existence of an “amplifying cycle” involving drought, soil desiccation and CO2 emissions. A mechanism that climate models do not seem to take into account

by Matteo Cavallito

 

Soil is reportedly releasing more carbon and other greenhouse gases than expected. That is the hypothesis put forward by a group of scientists at TUFTS University in Medford, Massachusetts. The research, published in the journal Environmental Research Letters, consequently suggests the possible acceleration of the climate change beyond the estimates suggested by current models.

“If we don’t consider the interplay of drought, soil desiccation cracking, and CO2 emissions, that could result in significant inaccuracies when modeling and predicting climate change,” explains Farshid Vahedifard, TUFTS researcher and co-author of the study in an article released by the same U.S. university. Morevover, he adds, “Poorer soil health can lead to reduced photosynthesis and lower carbon dioxide uptake and it can compromise the structural integrity of earthen dams that protect against floods.”

The feedback loop between climate change and soil

CO2, the study says, “the predominant contributor to the warming effect of all human-made greenhouse gases (GHGs), accounting for two-thirds of their global heating impact.” However, the authors continue, “While the primary anthropogenic source of increased atmospheric CO2 concentration is the combustion of fossil fuels, the largest terrestrial source of CO2 emissions is soil where 80% of the total terrestrial carbon is stored.” Sixty-two percent of the carbon in the soil is present in its organic form, the remaining portion being the inorganic element.

The study hypothesizes the existence of “an amplifying feedback loop between drought, soil desiccation cracking, and CO2 emission in a warming climate, a critical aspect that has been overlooked in the existing literature.” In addition, the authors “argue that the postulated feedback loop affects the emissions of other GHGs, such as methane (CH4) and nitrous oxide (N2O), from soils.”

Il circolo vizioso clima-suolo. Immagine: Farshid Vahedifard et al 2024 Environ. Res. Lett. 19 031005 DOI 10.1088/1748-9326/ad2c23 https://iopscience.iop.org/article/10.1088/1748-9326/ad2c23 CC BY 4.0 DEED Attribution 4.0 International

The feedback loop between climate change and soil. Image: Farshid Vahedifard et al 2024 Environ. Res. Lett. 19 031005 DOI 10.1088/1748-9326/ad2c23 CC BY 4.0 DEED Attribution 4.0 International

Drought is the main threat

According to the study drought has a crucial role as it “accelerates the oxidation of soil organic carbon (SOC) and, thus, increases CO2 emissions into the atmosphere.” Soil moisture deficits also contribute to reduced rates of photosynthesis in plants and, as a result, less carbon uptake. In its early stage, then, “drought may cause a slight increase in respiration, despite a decline in photosynthesis, leading to increased carbon emissions from the soil.”

The researchers’ attention, moreover, focuses on another phenomenon favored by drought: the spread of desiccation soil cracking “which, in turn, can considerably increase CO2 efflux in soil by exposing deeper and older stores of soil carbon.”

The effects associated with the spread of these cracks in the soil are many. These include increased microbial activity and breakdown of organic matter, resulting in the release of CO2, and loss of nutrients and the ability to support plant growth, resulting in reduced carbon sequestration.

Regulators must promote sustainable soil use

According to the authors, “The problems associated with desiccation cracks are becoming more prevalent as anthropogenic climate change exacerbates the severity and frequency of droughts, heatwaves, and drought-heavy precipitation cycles.” Added to this are the effects of other feedback loops with a significant impact on climate such as melting ice, more widespread fires, and thawing Arctic and subarctic permafrost leading to greater release of carbon dioxide .

In this scenario, Vahedifard recalls, governments and regulators must promote sustainable land use through the adoption of precision irrigation techniques, the use of water conservation practices, and the use of drought-tolerant crops. The use of organic fertilizers and composting, moreover, “can enhance soil organic matter content and improve soil water-holding capacity.” These interventions, in any case, can be helpful only if they are “part of a comprehensive effort to reduce greenhouse gas emissions from all human activity.”