Soil is an important carbon sink, storing more than twice the amount of carbon in the atmosphere globally. The stability of this sink is crucial for regulating atmospheric carbon dioxide. (CO2). The sensitivity of the soil carbon cycle to climate changes, such as warming and changes in precipitation, can lead to the release of CO2 into the atmosphere, creating a positive feedback loop on global warming.

The key concept in the carbon cycle is soil carbon turnover time (τsoil), which determines the timescales of carbon exchange between different reservoirs. τsoil depends on carbon input (e.g. through net primary production of plants, NPP) and carbon output (microbial respiration, erosion, fires). Although τsoil is influenced by various factors including temperature, moisture, soil chemistry and fertility, the precise controlling factors in the tropics and subtropics have been a matter of debate. Most knowledge comes from short-term observations, which limits the understanding of soil carbon dynamics on longer, centennial to millennial timescales.

New study used the geological record – specifically a sediment core from the eastern Mediterranean that received material from the Nile River basin. The scientists analyzed plant lipids (leaf waxes), which serve as biomarkers of higher plants and capture environmental changes on land. Using radiocarbon dating of these lipids, they determined their age at the time of deposition in the sediments. This age was found to reflect terrestrial retention times and changes in the terrestrial carbon cycle. Thanks to the known relationship between the 14C age of n-alkanoic acids and the mean τsoil in the basin, they were able to reconstruct the changes in τsoil in the Nile basin over the last 18,000 years, a period characterized by significant global warming and climate change (the last deglaciation and the Holocene).

The results showed dramatic reduction in soil carbon turnover time during the last deglaciationWhile during the Ice Age (around 18,000 years ago) the mean τsoil was approximately 218 years, during the last 10,000 years (Holocene) it has decreased to 9–22 years (on average 16 years). This means reduction of τsoil by an order of magnitudeSuch a significant decrease in τsoil indicates a substantial increase in the rate of microbial respiration.

The study analyzed the relationship between τsoil and climate reconstructed from the same sediments – temperature (reconstructed from SST in the eastern Mediterranean) and precipitation (reconstructed from hydrogen isotopic composition). They found strong negative correlation between τsoil and temperature (R2 = 0.82). The correlation with precipitation was weaker (R2 = 0.59). These findings led to the conclusion that temperature was the main driver of changes in τsoil in the Nile basin during the last 18,000 years, while the influence of hydroclimate was relatively small. The temperature sensitivity of τsoil was expressed as the Q10 value (the factor of change in τsoil per 10 °C temperature change), which was estimated to be 10.7, which is significantly higher than typical modern values.

In addition to τsoil, the mean age of soil carbon has also decreased significantly (from about 14,000 years to 18,000 years BP to about 1,000 years during the Holocene). This rejuvenation of soil organic matter suggests massive mobilization of older carbon from soils with warming. The reduction of τsoil and the mean age of soil carbon by an order of magnitude implies increasing the flow of CO2 from the soil to the atmosphere similar size. This represents positive feedback on global warmingThe authors suggest that release of older CO2 from (sub-)tropical soils could have contributed to rising atmospheric CO2 concentrations and falling atmospheric Δ14C content during deglaciation, in addition to known sources such as permafrost thaw. Similar results from the Ganga-Brahmaputra River Basin confirm that such drastic decreases in τsoil were likely common in the (sub-)tropics during deglaciation.

An important finding is that simulations using dynamic global vegetation models (DGVMs) underestimate changes in (sub-)tropical τsoil compared to data-based reconstructions. The models suggest only marginal changes, while the data show an order of magnitude decrease. This suggests that the climate feedback caused by accelerated soil respiration in the (sub-)tropics due to deglacial warming is underestimated in the models. This underestimation may be related to different assumptions about temperature sensitivity (Q10) in the models compared to the observed data.

The study provides strong evidence that temperature has dominating influence on soil carbon turnover in (sub-)tropical areas. It also highlights the need for further research and model improvementsto more accurately understand and predict the role of the soil carbon cycle in the future climate system. Spring

The entire study was published in the journal Nature Communications .

Glossary of key terms

• Soil carbon turnover time (τsoil): The average time that carbon remains in the soil before it is released into the atmosphere or transported elsewhere. It is defined as the ratio of the soil carbon pool to the rate of carbon influx or outflow.
• Deglaciation: The period of global warming and glacial retreat that followed the Last Glacial Maximum (LGM). In the context of the article, it refers to the period approximately 18,000–11,000 years before present.
• Radiocarbon dating: A method used to determine the age of organic material based on the content of the carbon isotope 14C.
• Plant-derived biomarkers: Organic molecules produced by plants that are preserved in sediments and can provide information about past vegetation and environmental conditions. Long-chain n-alkanoic acids and n-alkanes from epicuticular leaf waxes are key in this article.
• Reservoir age (reservoir age offset, R): The difference in age (in radiocarbon years) between two carbon reservoirs at a given time, calculated from the ratio of the radiocarbon content of the sample to the atmosphere at the time of its deposition.
• Q10: A temperature sensitivity factor that indicates how much the rate of a biological process (e.g. microbial respiration or soil carbon turnover) changes when the temperature changes by 10 °C.
• Microbial respiration: The process by which soil microorganisms break down organic matter and release CO2 into the atmosphere. It is a major component of soil carbon sinks.
• Soil mean carbon age: The average radiocarbon age of the total soil organic carbon in a given soil profile. It differs from τsoil, which is more influenced by rapidly turning over carbon.
• Positive feedback: The process by which a change in one component of a system leads to a change in another component, which further amplifies the initial change. In this case, climate change (warming) leads to a change in the soil carbon cycle (accelerated turnover), which in turn contributes to further warming.
• Dynamic Global Vegetation Models (DGVM): Computer models that simulate the dynamics of vegetation, soil carbon, and the hydrological cycle at a global scale in response to climate and CO2 changes.
• African Humid Period (AHP): A period of increased rainfall and humid conditions in North Africa during the Holocene (approximately 14,800–5,500 years ago), which led to the emergence of the so-called "green Sahara".
• Amino-bacteriohopanepolyols (amino-BHP): Specific biomarkers for methane-oxidizing bacteria used as indicators of the expansion of methane-producing wetlands.