Soils store three times as much carbon as plants and the atmosphere. Soil organic matter such as humus plays a key role in the global carbon cycle as it stores huge amounts of carbon and thus counters global warming.

Consequently, the Kyoto Protocol permits the signatory countries to count soils and forests against greenhouse gas emissions as so-called carbon sinks.

Exactly why some soil organic matter remains stable for thousands of years while other soil organic matter degrades quickly and releases carbon, however, is largely unknown. The explanatory models used thus far assume that the degradation rate depends on the molecular structures of the soil organic matter.

An international team of 14 researchers headed by Michael Schmidt, a professor of soil science and biogeography at the University of Zurich, has now revealed that numerous other factors affect the degradation rate of soil organic matter in an article published in Nature.

Soil environment determines degradation rate of humus
The degradation speed isn't determined by the molecular structure of the dead plant debris, but by the soil environment in which the degradation takes place," says Schmidt, summing up the new results.

For instance, the physical isolation of the molecules, whether the molecules in the soil are protected by mineral or physical structures and soil moisture influence the degradation rate of soil organic matter. Furthermore, the researchers are able to show that, contrary to the scientific consensus, there is no humic matter in the soil and this should therefore not be used for models.

New experiments and models needed
As Professor Schmidt explains, the findings need to be used for new experiments and models. In doing so, it is not only the first few centimeters of the soil that should be examined, as has been the case up to now, but rather the full top two to three meters.

In their article, the researchers make various suggestions as to how the models for forecasting the response of soils to changes in the climate, vegetation and land use might be improved.

Moreover, the new results cast a critical light on bioengineering experiments with plants containing high amounts of lignin or plant charcoal (biochar), with which more carbon is supposed to be stored in the soil in the long run.

Michael W. I. Schmidt, Margaret S. Torn, Samuel Abiven, Thorsten Dittmar, Georg Guggenberger, Ivan A. Janssens, Markus Kleber, Ingrid Kogel-Knabner, Johannes Lehmann, David A. C. Manning, Paolo Nannipieri, Daniel P. Rasse, Steve Weiner and Susan E. Trumbore: Persistence of soil organic matter as an ecosystem property, in: Nature, 6 October, 2011, DOI: 10.1038/nature10386