Rising temperatures lead to unexpectedly rapid carbon release from soils

How sensitively does organic carbon stored in soils react to changes in temperature and humidity? This question is central to a new study now published in Nature Communications.

The work involved researchers from MARUM—Center for Marine Environmental Sciences at the University of Bremen and from the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research in Bremerhaven.

Globally, soils store more than twice as much carbon as the atmosphere. Therefore, carbon uptake and release by soils constitutes a strong regulator of atmospheric concentrations of the greenhouse gas carbon dioxide (CO₂). In the context of the ongoing anthropogenic climate change, it is thus important to better understand the sensitivity of soil carbon, which is directly related to the release of CO₂ from soils, under a changing climate, such as rising temperatures and/or variations in the hydrological cycle.

Studies have already highlighted the importance of permafrost regions, where rising temperatures lead to the release of carbon from previously frozen soils. However, large amounts of organic carbon are also stored in soils in subtropical and tropical regions. In these regions, it was previously unclear what the main factor for a change in the carbon turnover rate was.

“Microbes that break down organic matter are generally more active under warm and humid conditions, so the carbon content in tropical soils responds very quickly to climatic changes. Some studies report a main influence of changing hydroclimatic conditions, while in others temperature plays the main role,” explains first author Dr. Vera Meyer from MARUM.

Deposits provide a glimpse into the past

To shed more light on these large-scale processes, Meyer and her colleagues chose a rather unconventional approach. Instead of studying soils, they analyzed the age of land-derived organic matter that was transported from soils from the Nile to the Mediterranean and deposited near the river mouth. The Nile transports material from a huge catchment area in the subtropical to tropical regions of north-east Africa to the eastern Mediterranean.

The samples for the study come from a coastal marine sediment core in which age evidence of many thousands of years has been deposited. Such sediment cores therefore allow a much longer look back into times in Earth’s history when the climate was significantly different from today and changed considerably.

“The age of the organic material delivered by the Nile essentially depends on two factors: how long it was in the soils, and how long it took to be transported in the river. The advantage of our approach is that long time scales can be investigated, in this case the last 18,000 years since the last ice age,” says Dr. Enno Schefuß, also from MARUM.

The results surprised the researchers and showed something unexpected: The ages of the land carbon changed only slightly with changes in precipitation and the associated changes in runoff, but reacted strongly to changes in temperature.

In addition, the change in ages due to the temperature increase after the last ice age was significantly greater than expected. This means that the post-glacial warming drastically accelerated the decomposition of organic matter by microorganisms in soils and caused a much stronger outgassing of CO₂ from (sub-)tropical soils than predicted by carbon cycle models.

Co-author Dr. Peter Köhler from AWI Bremerhaven says, “The fact that the models underestimate carbon release from soils so strongly shows us that we need to revise the sensitivity of soil carbon in our models.”

However, this effect not only contributed to the increase in atmospheric CO₂ concentration at the end of the last ice age, but also has far-reaching consequences for the future: the carbon turnover in soils will accelerate with further global warming and could further increase the atmospheric CO₂ concentration via a previously underestimated feedback.