Microbial interactions stabilize carbon in the soil

The key to long-term CO2 storage in the soil

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 soil; 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.