Soils play a major role when it comes to the long-term storage of CO2 and the resulting reduction of this gas in the atmosphere - therefore they can contribute to slowing down climate change. In order to gain a better understanding of these mechanisms, it can be helpful to look at the microscopic level of soil microorganisms. An international and interdisciplinary group of researchers has examined how microorganisms interact with each other to contribute to the decomposition and storage of carbon in terrestrial ecosystems.
Carbon is the most important element for all life forms on earth; it is circulating between the atmosphere, oceans and land ecosystems in the so-called carbon cycle. While a single carbon atom (as CO2) remains in the air for an average of three years before being chemically bound and converted to biomass by plant photosynthesis, it takes 23 years on average for a carbon atom in the soil organic matter to be released into the atmosphere as CO2 through microbial decomposition of dead biomass.
This microbial decomposition, however, leaves a part of the carbon in the soil, where it can be bound for a very long time - researchers estimate that they can stay in deep soil layers for hundreds, possibly thousands of years. The mechanisms responsible for this highly efficient "retention" have recently become subject to great public interest and soil scientists from all over the world are performing intensive research in this regard.
An interdisciplinary consortium of experimental scientists from the fields of soil science and mathematical modelling lead by Johannes Lehmann from Cornell University has presented a new concept of soil-based carbon stabilization that acknowledges the fact that microorganisms live inside a highly complex environment on a miniscule scale. On several occasions, Johannes Lehmann was received as a guest at the Institute for Advanced Study of the Technical University of Munich (TUM-IAS) in the course of his Hans Fischer Senior Fellowship. The consortium’s findings were published in "Nature Geoscience".
"The amount of carbon that microorganisms can decompose or turn into biomass for long-term storage not only depends on the amount of carbon and the sum of all microorganisms in the ground; the probability with which a microorganism and an organic carbon compound even cross paths in this microscopic environment in the soil is another very important factor. If there is a highly uneven spatial distribution of microorganisms and carbon in the soil, chances increase that a carbon molecule ends up isolated - it will therefore not be decomposed," explained Ingrid Kögel-Knabner from TUM, the final author on the paper.
"Soil conditions are undergoing continuous change," said Johannes Lehmann, the first author on the study. "Even though there may be a high supply of carbon as biomass residues, microorganisms starve, especially when they have to adapt to constantly changing conditions within a maze," he added.
In addition, the chemical diversity of dead biomass in the soil plays an important role, as microorganisms need to produce suitable enzymes for digesting every type of molecule they want to metabolize.
If there is a great variety of molecules to be decomposed and each of these types of molecules only exists in small numbers, microorganisms may find that would not make sense to invest energy in the production of various enzymes if not all of them promise to create "benefit" in the form of a net gain of energy. The efficiency of an investment, however, is of great importance to soil microbes as they are in heavy competition with many other soil microorganisms.
Sometimes, this leads to alliances being established between microorganisms, enabling them to digest resources more efficiently as a group. These diverse interactions between microorganisms can lead to emergent behavior, a kind of "self-organization" within the microbial community - this, in turn, affects the decomposition and storage of carbon.
"Soil offers a highly complex ecosystem in which many different types of microbes and microorganisms mostly interact in soil pores - a kind of tunnel system on the microbial level," explained Christina Kaiser from the University of Vienna; she is also performing research in this area in her own project supported by the European Research Council (ERC). "These social interactions between soil microorganisms have an effect on the entire system and thereby also affect the metabolic cycles in the soil," she added.
The new concept therefore postulates that the "functional complexity" of the soil, based on the spatial distribution of microbes and decomposable carbon compounds as well as the chemical diversity of these compounds, heavily affects the long-term stabilization of carbon in the soil. The microbes’ ability to self-organize their ecosystem is another factor that is just as important.
In the future, this new perspective could lead to a better understanding of the mechanisms of storing carbon in the soil. This understanding will, in turn, not only improve the development of climate prediction models with higher precision; it would also serve as a foundation for the advancement of "climate relevant" soil management practices. A targeted effort to preserve the functional complexity of the soil ecosystem could contribute to the carbon remaining stored in the soil for the long term.
Johannes Lehmann, Colleen M. Hansel, Christina Kaiser, Markus Kleber, Kate Maher, Stefano Manzoni, Naoise Nunan, Markus Reichstein, Joshua P. Schimel, Margaret S. Torn, William R. Wieder, Ingrid Kögel-Knabner (2020), Persistence of soil organic carbon caused by functional complexity, Nature Geoscience. DOI: 10.1038/s41561-020-0612-3