An international team of scientists have put a big question mark over important elements of the conventional paradigm of soil carbon. They cast doubt on the popular view that temperature increases automatically mean higher rates of Carbon escaping from soil. They cast doubt on the resistance of lignin and biochar to decomposition. They cast doubt on biochar’s capacity to increase soil carbon. And they recommend that scientists study soils at 3m because there is a lot going on down there. For many years, scientists thought that organic matter persists in soil because some of it forms very complex molecular structures that were too difficult for organisms to break down. 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 recent advances, from imaging the molecules in soils to experiments that track decomposition of specific compounds, show this view to be mistaken. For example, the major forms of organic matter in soils are in the forms of simple biomolecules, rather than large macromolecules. The team contends that the average time carbon resides in soil is a property of factors like physical isolation, recycling, or protection of molecules by minerals or physical structures like aggregates, or even unfavorable local temperature or moisture conditions, can all play a role in reducing the probability that a given molecule will decompose.
Current models used to predict how global soil carbon will respond to climate change use simple factors like temperature dependence that indicate acceleration of decomposition in a warmer world. The decomposition-warming feedback predicts large soil carbon losses and an amplification of global warming, but in fact the authors argue this approach is too simplistic. “ 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. 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.
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. “Compounds such as lignin, which we thought were stable, may only last five years in soil, while proteins, which we thought were decomposable, may last more than one thousand years,” says co-author soil scientist Margaret Torn from the Lawrence Berkeley National Laboratory (Berkeley Lab). Paper: Michael W. I. Schmidt, Margaret S. Torn, Samuel Abiven, Thorsten Dittmar, Georg Guggenberger, Ivan A. Janssens, Markus Kleber, Ingrid Kögel-Knabner, Johannes Lehmann, David A. C. Manning, Paolo Nannipieri, Daniel P. Rasse, Steve Weiner & Susan E. Trumbore: Persistence of soil organic matter as an ecosystem property, in: Nature , 6 October, 2011, DOI: 10.1038/nature10386