Soils are important stores of carbon. In particular, certain organic compounds such as lignin from wood or vegetable waxes were considered to be stable and difficult to decompose. However, as a field study has now shown, this is a misconception: when the soil warms up, these “stubborn” carbon compounds are broken down more and CO2 is released. In addition, this happens hardly more slowly in deeper soil layers than on the soil surface. Climate change could therefore affect these CO2 sinks more than previously assumed.
Soils are important carbon reservoirs in the earth system. The carbon bound by plants from the carbon dioxide (CO2) in the air is converted by them into organic compounds and stored in their tissues. This carbon reaches the soil surface via roots, dead plant remains and indirectly via animal carcasses. There the organic compounds remain in the litter layer, the humus and the deeper soil layers until they are broken down again by the decomposing activity of bacteria and enzymes and converted back into CO2.
Complex organic compounds in focus
Whether and how quickly this soil carbon is mobilized again depends on the climate and the soil layer, but also on the composition of the organic compounds. For example, certain plant polymers were previously considered to be relatively stable and difficult to decompose. These include the lignin produced by woody plants to stabilize their cells and tissues, and various waxy lipid substances that some plants store in their leaves, stems, and roots to protect against dehydration and pathogens. The so-called pyrogenic carbon left behind by forest fires has also been considered relatively persistent up to now.
So far, it has also been unclear how stable the carbon stores are in the subsoil, the soil layer below a depth of 20 centimeters. “These soil layers contain more than half of all soil organic carbon,” explain Cyrill Zosso from the University of Zurich and his colleagues. “But so far we know very little about the stability of this enormous carbon pool in the course of global warming.” To change this, Zosso and his team conducted a long-term experiment in the forests of California’s Sierra Nevada. To do this, they lowered heating rods to a depth of 2.40 meters into the ground and gradually heated the test areas to four degrees above normal values over the course of four and a half years. This corresponds to the forecasts for largely unchecked warming up to 2100. In the meantime, they investigated how the levels of the various organic compounds are changing.
Rapid loss from the warming ground
“Our results show that all components of the soil humus are equally reduced, simple chemical substances as well as complex components,” reports senior author Michael Schmidt from the University of Zurich. Contrary to expectations, the warming of the soil also caused an accelerated degradation of organic compounds in the deeper layers of the soil, including substances previously thought to be stable. After 4.5 years, the lignin content in this soil zone had dropped by 17 percent, and that of waxes and vegetable fats by 28 percent. The amounts of pyrogenic carbon had even decreased by 37 percent. “These results confirm that there is no such thing as stable soil carbon,” the research team states.
On the one hand, this finding is important for forecasts of CO2 release from warming soils. “If these first observations are also confirmed in long-term field experiments, that would have frightening consequences,” says Schmidt. Because if the forest floor loses massive amounts of soil humus and this carbon is released as CO2, this further accelerates warming.
On the other hand, the knowledge of the lack of stability of the supposedly stubborn organic compounds also influences attempts to use soil and forests as natural carbon sinks. In order to use these, crops with particularly deep roots and cork-rich biomass are being developed and planted. “Until now, it was assumed that CO2 could be retained in the soil,” says Schmidt. But that is apparently not the case.
Source: University of Zurich; Article: Nature Geoscience, doi: 10.1038/s41561-023-01142-1