“Carbon is fantastic chemical element: not much metal, not much non-metal, not too light, not too heavy. An element which pairs nicely with hydrogen, oxygen or nitrogen to form carbohydrates, lipids, proteins, nucleic acids and other molecules of life. However, to sustain life, carbon needs to cycle between planet´s atmosphere, ocean and soil.
On planet Earth, an ape-like species evolved to discover the wheel. Several thousands of years later, they discovered how to dig fossils (remnants of dead plants and animals) and burn them to move wheels and other things. Many representatives of the ape-like species were delighted by this process and invested a great amount of time and energy in it.
After about hundred-fifty years of fossil-burning and massive release of carbon into the atmosphere, Earth became so heated that all frozen water and soil melted, causing irreversible changes to the climate so that life on the planet became impossible”.
This could be an entry about carbon and life on planet Earth from an imaginary alien encyclopedia.
We hope that this scenario will stay only in domain of science-fiction.
How carbon cycles in the living systems?
As most of us learned in school, in presence of sunlight, carbon (in the form of gas, CO2) is converted to sugar by plants, algae and certain bacteria – in the process called photosynthesis. In this way carbon is captured from the atmosphere and enters living organisms. Other organisms which can´t do photosynthesis (such as we are), get their carbon by feeding on photosynthetic organism (such as plants) or other organisms who fed on photosynthetic organisms (such as animals).
What happens with organic carbon in the soil?
Same question can be also asked in a different way: what happens with all carbon content of dead organisms?
Basically, there are two possibilities: one is that organic carbon is used by soil microorganisms in the process known as aerobic respiration and returned to the atmosphere as CO2. This is what we also do when we breath and, as we know, this process needs oxygen. Another option is that organic carbon is bound or transformed by microorganisms and stays trapped in the soil.
Estimates are that soil releases approximately three times more CO2 in a year into the atmosphere than all fossil fuels burned by humans. In regard to our current excess CO2 emissions, keeping carbon bound in the ground is clearly a preferred option than giving it back. However, as we will see, this is not a trivial task.
How climate change affects carbon cycle in the soil?
In theory, if there is more CO2, photosynthetic organisms will grow better and more CO2 will be captured. However, up to a certain level. Studies and models have shown that after initial increase in number of microorganisms in the soil (due to more food from more decaying plants and other organisms), in a longer term, more carbon will be released back into the atmosphere as CO2. Further, due to overgrowth of soil microbes, other elements, such as nitrogen, will be depleted and soil will ultimately become infertile. Similar thing happens with increased warming of the soil: initially more growth, than breakdown of the system in a longer run.
What are challenges in studying soil microbiomes?
Microbial communities in the soil are adapted to perform under different conditions. For example, rotting leaves of an oak tree or needles of a pine create very different chemical environment. Second, conditions vary greatly between different types of soil, for example between a tropical forest or a sandy soil. Micro-environments also vary with size of soil particles, interaction with plants roots and between all members of soil microbiomes: bacteria, fungi, protists and viruses. Further, external conditions, such as humidity and temperature, change with weather or season even in the same type of soil. Adding increased atmospheric CO2 and climate change on top of it, poses a next-level challenge in studying soil microbiomes.
To measure how well a microbial community (i.e. microbiome) can cope with external challenges, (not only in soil but in any environment in general) scientists agree on measuring three things: resistance, resilience and functional redundancy. In short, resistance shows how community changes under stress, for example, which microbes survive better than others. Resilience tells how quick the community recovers after the stress. Functional redundancy shows how many functions (for example, which biochemical reactions) community can perform as a whole. This means if, for example, a certain microbial species dies, other members of the community can still compensate for that function.
How soil microbiomes can mitigate imbalances in the carbon-cycle in the future?
In our previous blog serial, we explained how microbes can bioremediate oil spills, pesticides and other pollutants by accumulating or transforming them. In a similar fashion, carbon in the soil can be transformed and inactivated by microbes outside of their cells or it can stay bound inside of them.
Although majority of soil microorganisms are not cultivable in the laboratory, we need to dig deeper not only into the genetic structure and organismal composition, but into soil biochemistry and metabolic pathways which are activated or deactivated under given external conditions and climate change. Integration of all “omics” data with existing models of climate change and across scientific disciplines such as geology, meteorology and others, is crucial to get a comprehensive picture of the roles of soil microbiomes in the process. Having this knowledge, we may get a better idea of possible intervention in the cycling of carbon and other elements crucial for life on our wonderful blue planet.
Soil Microbiomes Under Climate Change and Implications for Carbon Cycling. Dan Naylor, Natalie Sadler, Arunima Bhattacharjee, Emily B. Graham, Christopher R. Anderton, Ryan McClure, Mary Lipton, Kirsten S. Hofmockel, Janet K. Jansson. Annual Review of Environment and Resources 2020 45:1, 29-59. https://dx.doi.org/10.1146/annurev-environ-012320-082720
Varney, R.M., Chadburn, S.E., Friedlingstein, P. et al. A spatial emergent constraint on the sensitivity of soil carbon turnover to global warming. Nat Commun 11, 5544 (2020). https://doi.org/10.1038/s41467-020-19208-8