Soil also suffers from heat waves: organic waste used to boost its tolerance to 50 degrees Celsius

The successive heat waves that sweep across southern Spain in summer have harmful effects on the entire community that lives there, from humans to the microbes that inhabit the soil. Both share an impressive resilience that has enabled them to survive and adapt, each in its own way, to successive episodes of extreme temperatures. But that adaptability has its limits. And when the temperature exceeds 40 degrees, just as human health suffers, the microorganisms that inhabit the soil—and from there provide a multitude of ecosystem services, such as carbon sequestration and plant nutrition—concentrate more on survival than continuing their work.

A study conducted by the University of Córdoba (UCO) in collaboration with the School of Environmental and Natural Sciences at Bangor University in the United Kingdom has determined the temperature limit that soil in various Mediterranean regions can reach before it begins to degrade. The study also provides insights into what we can do to help the soil. Above 40 degrees, microorganisms' ability to capture carbon diminishes, and it practically "shuts down" at 50 degrees— a temperature to which the calcareous soils of the province of Córdoba are often exposed. The higher the temperature they endure, the lower the soil's phosphorus reserve becomes, which is virtually non-existent when exposed to temperatures above 40 degrees.

To address this issue, the UCO team—comprised of researchers Sana Boubehziz, Antonio Sánchez Rodríguez and Vidal Barrón from the Edaphology group within the Department of Agronomy (DAUCO)— has explored ways to mitigate the damage caused by high temperatures, which they aim to counteract through the use of organic additives that enhance soil resistance. This contribution adds to a comprehensive, multi-stakeholder strategy embodied in the European Soil Monitoring Directive, which aims to achieve healthy soils across Europe by 2030.

Mediterranean soil, at risk

As the principal investigator on this project, Sana Boubehziz, explains, the soil samples were first labeled with radioactive carbon-14 isotopes to monitor the microorganisms' respiration. The goal was to test the resilience of two types of Mediterranean soil—one calcareous from Córdoba and the other more acidic from Badajoz—under different temperature scenarios ranging from 20 to 50 degrees. While the results showed how high the temperature can rise before the soils lose their functionality, they also highlighted the urgent need to find mitigating solutions to slow the degradation of soils that are exposed to increasingly high temperatures year after year.

These mitigation measures could involve incorporating organic amendments rich in organic matter to boost soil resilience. This has been demonstrated in tests using alperujo, or olive pomace, the primary byproduct of olive oil production, and organic waste from the treatment plants of Córdoba's municipal water and waste management companies. Sana Boubehziz explains that after a two-week incubation period, soil samples that received organic additives showed a significant increase in both resistance and phosphorus availability. Specifically, alperujo proved most effective, increasing soil resistance to 50 degrees, once again highlighting the potential of this byproduct from one of Andalusia's main industries to be part of circular economy strategies.

According to the researcher, beyond its practical applications, the study's main contribution is to highlight the specific challenges and needs of a soil type found in Mediterranean areas that is threatened by the effects of climate change. "Each soil is unique," she explains, "and must be managed in a way tailored to its characteristics." For example, in agriculture, "it's been proven that using organic-based fertilizers is healthier for the soil, making it last longer and produce more, making it more profitable in the medium term." This profitability isn't just about growers' bottom lines; it also has a social dimension. Soil, considered a non-renewable resource due to its slow regeneration, plays a key role in carbon sequestration, which makes it a valuable tool against the very climate change that is accelerating its degradation. Taking good care of soil is a way to break the cycle.