Forest fungi and the dance of carbon sequestration

The Day after Halloween: a spooky day to tackle a spooky subject. The subject of climate change.

Yesterday, COP26 - the UN Climate Change Conference - began. This summit aims bring the smartest, most powerful, and/or most political people on the Earth together to try to bring the world closer to meeting the goals set out in international agreements on climate change like the Paris Agreement and the UN Framework Convention on Climate Change. In honour of this event, and in answer to a great question asked Scott Bleackley, today I have a climate change-themed article for you - with a special focus on soil. The lyrical title of today's post is also borrowed, with permission, from Scott.

Why soil? Well, soil is the basis of agriculture, and agriculture is not only a contributor to climate change - think, for example, of the methane produced by livestock, the nitrous oxide emissions (indirectly) produced by use of fertilizers, and the deforestation in the name of producing new pastures - but is also impacted by climate change. Going forward, humans will need to figure out not only a way to keep on producing enough food in regions with increasingly unpredictable weather, but to do so without blasting out ever more greenhouse gasses, tearing down more and more and more forested land, and increasingly depleting the oceans. It's a tricky task to be sure.

But it's not all bad news - scientists already have ideas on how to tackle this issue. As these authors put it, "Climate-smart and sustainable agriculture, that is, conceiving agriculture to be resistant and resilient to a changing climate while keeping it viable in the long term, is probably the best solution." And one of the possible ways to make agriculture more climate-smart and sustainable tackles this big problem with a microbial approach: fungi. And in particular, one specific type of fungi has been getting some attention for their potential climate change-combatting actions.

Arbuscular mycorrhizal (AM) fungi are a big group of related fungi that are best known for being BFFs with plants, including food crops. The name gives us a clue to what these fungi look and act like. 'Arbuscular' means branched, like the many arms of the fungus, and 'mycorrhizal' literally means "fungi-plant root". 'Myco' refers to fungus, and 'rhiza' is just a fancy name for a root. What this refers to, is the great reputation that AM fungi have for helping plants get their nutrients.

These friendly fungi basically funnel essential nutrients like nitrogen, phosphorus, and zinc up through the soil, and pass them on to their leafy pals. In exchange, the plants kindly pass down to to their fungal friends that sweetest of molecules that is both necessary for all life on earth and key in climate change: carbon.

That's right, these AM fungi help with that sought-after moderator of climate change: of getting carbon out of the air, and into the ground.

But let's back up a bit. Why do we want carbon to be funnelled down into the ground anyways?

There are two concepts that explain the focus on carbon in discussions on climate change: 1) Carbon dioxide, a gaseous form of carbon, is a greenhouse gas, and 2) Although it's totally normal for carbon dioxide to be floating around in the Earth's atmosphere, there is currently a lot more of it than is ideal.

Let's tackle that first point, first. Carbon dioxide - which is a gas composed of one part carbon and two parts oxygen - is a greenhouse gas. Simply put, it acts as a greenhouse over the Earth, helping to keep in the heat of the sun, and thereby warming up the land and sea. A little bit of greenhouse gas is a good thing, and keeps the Earth toasty warm enough for wimpy humans like myself to survive. But, like many things in life (and immunology! And microbiology!), a good balance of different gasses in the Earth's atmosphere is necessary to keep everything running smoothly. Too little greenhouse gas would plunge the Earth into an ice age. Too much greenhouse gas sets the Earth on track for rising temperatures and erratic climate (think floods, droughts, and hurricanes). In other words, too much carbon equals weird, hostile weather. That's bad news for wimpy humans like me.

Now, for the second point. Again, it is normal to have carbon dioxide in the atmosphere. Carbon dioxide is what we humans emit with each exhale, and trees rely on the presence of carbon dioxide in the air to grow. However, carbon dioxide is also what we emit with each burning of fossil fuels. And ay, there's the rub. We need carbon, and we use carbon - but the scientific consensus is that we have been using too much of a good thing.

On that note, it's also important to understand that some fluctuation in greenhouse gas levels is totally normal. The main problem is simply that we're not within the range of normal fluctuations in carbon levels anymore. By measuring carbon that gets stuck in glacier ice - carbon that got stuck in the ice a long time ago is deeper down, whereas carbon that got stuck in the ice yesterday is at the very top of the ice sheet - scientists have been able to track those normal fluctuations, and can clearly see how abnormal the current amount of carbon in the atmosphere is.

So how do we deal with this problem of an over-abundance of carbon in the air? Well, the current belief in the scientific community is that the approach must be two-pronged. We have to both reduce the amount of carbon that we keep pumping into the air, for instance by weaning ourselves off of fossil fuels, and simultaneously increase the amount of carbon that we get back into the ground. In other words, we need to sink carbon. And AM fungi might be a great ally to us in that.

As we already discussed, AM fungi have a very healthy, two-way relationship with many plants, in which the fungi provide an array of nutrients to the plants, and the plants provide carbon to the fungi. That is arguably the most direct way in which fungi can act as a 'carbon sinker' - by literally helping it sink into the ground.

As with most living things (including you!), carbon is one of the main ingredients necessary to make a fungus. Carbon molecules are the building blocks of life - if you (or a fungus) were made up of Lego pieces, most of that Lego would be carbon. Other stuff, like nitrogen, phosphorus, zinc, and iron are the cherries on top that help the carbon along. In other words, the carbon that the trees send down to the AM Fungi BECOMES the fungi.

But, that's not the only offering on the table from these supportive subterranean microbes. As well as embodying the carbon that we want to sink, AM fungi can also help structure the soil in a way that helps keep carbon in the ground.

At its best, soil is full of carbon because it is full of the remains of things that used to be living. And remember, all living things have a lot of carbon. You may have heard about soil being graded on its 'organic matter' content. In scientific terms, something is 'organic' simply if it contains carbon. That brings me to a fun fact as a side note: from a chemist's perspective, all of the fruit and veg that you eat is organic, whether it is grown with pesticides or not, because all vegetables and fruit are made up of carbon. I've tried and failed to make that fact a joke several times, but it requires very specific background knowledge. Anyways, back to the soil.

Although soil should ideally be rich in carbon, carbon can also be leached out of soil by, for example, different kinds of microbes. Like us humans, some microbes that eat up carbon in the soil also release some of it as carbon dioxide (or other greenhouse gasses, but we won't get into that here). These microbes more or less breathe in the same way as us. The only difference is that, while we eat carbon in the form of lettuce and legumes, or meat and potatoes, these so-called heterotrophic microbes eat up carbon in the soil in the form of decaying plants, animals, or other microbes.

Now, fair play to those greenhouse gas-exhaling little critters. After all, we humans go about our days doing pretty much the same thing. But again, reaching a balanced amount of heterotrophic microbes in the soil is a good thing, and we don't want these little guys to over-grow. And as you may have guessed by this lead-up, AM fungi likely help keep the amount of heterotrophic soil microbes at a nice, balanced level. The most likely explanation for how the fungi accomplish this is simple: they out-compete the other microbes. They just take up the carbon and the physical space that would otherwise be claimed by these other heterotrophic microbes.

Aside from being lost via the 'exhales' of heterotrophic soil microbes, soil erosion is another way that carbon can leach from soil. Soil erosion can occur, for example, by over-tilling of soil, or any other excessive disruption of soil that exposes that soil to air. When carbon-rich soil is exposed to air, the carbon 'oxidizes' (basically, gets stuck to oxygen), naturally becoming CO2: carbon dioxide, a gas. In other words, carbon is lost from the soil and released into the air. It basically burns off. But AM fungi can likely help protect against this type of carbon leaching as well, again in a simple but elegant way. AM fungi give the soil structure. Their branching fungal arms help keep soil compact, and keep it from getting excessively disrupted. This limits the contact of the carbon in lower levels of soil with oxygen, and that gaseous escape of carbon from the soil as a result.

These are probably just a few of the climate change-combating skills of AM Fungi, and there is still more research to be done to fully understand how these friendly fungi support their ecosystems. But one thing's for sure - they are a valuable natural resource that's worth looking in to.

Until next week,

Give those 'shrooms some love!

~ Alex