Subsoil is one of the most important sources of carbon dioxide in the atmosphere as well as the largest carbon store. Global warming accelerates the decomposition of soil humus. It also affects waxy and woody compounds that were previously thought to be stable, which help plants store carbon in leaves and roots. These are the results of a study conducted in the Sierra Nevada National Forest by researchers from the Faculty of Geography at the University of Zurich.

About a quarter of the world’s carbon emissions are absorbed by forests, meadows and pastures. Plants use photosynthesis to store carbon in their cell walls and soil. About half of the carbon in the soil is in the deeper layers, which are more than 20 centimeters deep. But even these lower layers are warming due to climate change.

Loss of vital carbon sinks

A warming climate is causing a significant loss of organic compounds that help plants store carbon in their leaves and roots. Previously, scientists assumed that complex polymers with a more stable molecular structure could withstand natural decomposition longer and thus store carbon in the soil.

However, a study led by UZH showed that the lignin compound, which gives plants rigidity, was reduced by 17%, while waxy compounds called cutin and suberin, which protect plants from pathogens and are found in leaves, stems and roots, were decreased. Even pyrogenic carbon, an organic compound of 30% left over after a forest fire, was present in greatly reduced amounts.

The experiments were conducted in the forests of the Sierra Nevada in California. The one meter deep soil was artificially heated at 4°C for 4.5 years after daily and seasonal cycles. This amount of warming is in line with projections to the end of the century under a working-as-usual climate scenario.

Consequences of land use in combating global warming

These findings have important implications for using soils and forests as natural carbon sinks, one of the key strategies in the fight against global warming. As part of this strategy, cultivars with particularly deep roots and rich fungal biomass are grown. “Until now, it was thought to capture CO2. Michael W. Schmidt, professor of geography and last author of the study, says.

“But our results show that all components of soil humus, both simple chemical compounds and polymers, will degrade at the same rate. If these initial observations are confirmed in long-term field experiments, the implications will be alarming.” If the forest floor loses humus on a large scale and carbon is released in the form of CO₂global warming will accelerate. “Our goal should be to stop emissions at source,” says Schmidt.