It is already known from the permafrost: When the Arctic soils heat up, they release more carbon dioxide into the atmosphere – thus additionally heating up the climate. But this effect is not limited to the Arctic, as a study shows. Thus, the respiratory activity of soils worldwide has already increased by 1.2 percent in the last 25 years. Carbon sequestration is still predominant over CO2 release in most soils – but that could change with continued climate change, the researchers warn.
Soils are an often overlooked factor in the Earth’s climate system. “Globally, soils contain at least twice as much carbon as the Earth’s atmosphere,” explains Ben Bond-Lamberty of the University of Maryland and his colleagues. Part of it remains stored as humus and soil over long periods of time. But because of the soil also the “waste disposal” of nature takes place, a lot is released again. When organic matter such as dead plant matter or dead animals falls to the ground, invertebrates, fungi, and bacteria cause it to be shredded and eventually depleted. The soil microbes decompose the organic carbon compounds and thereby produce large quantities of CO2.
CO2 release has increased worldwide
How much CO2 the soils release worldwide and how this CO2 outflow has changed over time is so far only partially known. “Most previous studies on this topic have studied only one issue at a time,” said co-author Vanessa Bailey of the Pacific Northwest National Laboratory in Richland. “We asked that question on a global scale.” For their study, the researchers evaluated data from the Global Soil Respiration Database, a database of ground-breathing results from more than 1,500 studies worldwide. In addition, they used global data sets and network results to record primary crop production, as well as CO2 and weather data at these sample sites.
The result: Soil respiration has steadily increased over the 25 years of the study – both in absolute terms and in relation to the primary production of the plants in the test areas, the researchers report. The rate at which soil microbes decompose organic carbon and release it as CO2 has risen 1.2 percent globally. At the same time, the share of microbial respiration in total soil respiration increased from 54 to 63 percent. “We’re talking about a tremendous amount of carbon here,” says Bailey. “This is of great importance for predicting the future developments of the climate and carbon cycle.” The increasing trend of soil respiration has been observed in grassland as well as in deciduous and coniferous forests.
Is the climate to blame?
According to the researchers, this global increase in soil respiration is most likely to be explained by the climate changes of the last 25 years. “Soils around the world are reacting to the warmer climate,” explains Bond-Lamberty. Because higher temperatures make soil bacteria more active and accelerate chemical degradation processes, organic matter decomposes faster and more CO2 is released into the atmosphere.
“Depending on how other components of the carbon cycle respond to climate change, these changes in the ground could provide positive feedback that fuels the temperatures even more,” says Bond-Lamberty. “Until now, the soils of the land areas have been regarded as a robust carbon sink. But with rising soil respiration rates this will not last forever. “