This is part three of the “Below Our Feet” series, exploring the mysterious workings of the Rhizosphere, which is the region of soil shaped and surrounded by plant roots. In the last article, we met some of the most common soil animals and learned about the vital tasks they perform to maintain healthy ecosystems. In many ways, the Rhizosphere acts as the stomach of the Earth – consuming, digesting, and cycling nutrients and organisms.

To expand on this idea, this article will have a closer look at the Rhizosphere’s function as the stomach of the Earth. Just as the gut microbiome is important for human health, so is the root microbiome for forest health. The root microbiome comprises the lively community of microbes that inhabit the soil directly below our feet. Teeming with myriads of different microbes, the Rhizosphere is also the immune system of the forest, supporting and strengthening trees in various ways.

Soil microbes working undercover

There is more to the forest than just an accumulation of trees and shrubbery. An ecological galaxy exists beneath each footstep you take. A mere handful of forest floor can be home to more than a billion bacteria. These bacteria form the extended family of trees – caring for, nurturing, and protecting them. Soil microbes are known to clean and create soil, fight off pathogens, and reduce stress, just to name a few crucial plant-microbe relationships. Thanks to soil microbes, essential minerals and nutrients otherwise inaccessible to plants are fixed into forms that roots can absorb.

Underground microbial and aboveground plant diversity are inextricably linked and dependent on each other. If the soil microbes are healthy, the trees will stand strong and steadfast. If the soil microbes are unwell, however, trees will suffer, sicken, or even die. Due to their trustworthy ability to reflect forest health and resiliency, soil microbes serve as bio-indicators. They can provide researchers with insight into the state of the ecosystem as a whole.

Nitrogen-fixing bacteria live in the roots of certain plants like Red Alder trees (Alnus rubra). These bacteria transform nitrogen from the air to a usable form so plants can absorb this essential nutrient through their roots.
Picture taken by Oregon State University

The extended family of trees

The depth of relationships between plants and microbes is astounding. Plants pump up to 40% of all the sugars they make through photosynthesis into the Rhizosphere. Now, you might wonder why plants give away almost half of the food they produce. From a competition perspective, this phenomenon seems like an evolutionary mistake. However, it makes complete sense from an Indigenous, reciprocity-centered view. Plants intentionally pump sugars into the Rhizosphere to nurture and expand their support network.

Think of a friend who has recently done you a favour. Did you consider inviting them over for dinner to express your gratitude? Sharing food in exchange for a service is a more common phenomenon than we think. As sedentary organisms, plants rely on their support network just as or even more than us. They require exterior help to digest food, acquire nutrients, and defend themselves against diseases. Therefore, plants exchange sugars with soil microbes in the Rhizosphere. In return, the soil microbes provide services that the plants need.

Networking in the Rhizosphere – plants release complex chemical signals through their roots to communicate with soil microbes.
Picture taken by Dru!

The language of the forest

Plants change the chemical structure of the sugars they pump into the Rhizosphere to encode their requests. Acting as messengers between plants and microbes, these sugar signals can deliver highly complex messages to the root microbiome. In this way, plants communicate their needs to their extended microbial partners. Depending on if a plant requires water, certain nutrients, or help fighting a disease, different soil microbes rush to the scene. The bacteria who respond to a call feed on the sugar signals and grow in numbers until they can fulfill the plant’s request.

As it turns out, plants and microbes use a sophisticated chemical language to communicate. While humans are only slowly starting to understand this language, research has shown that plants selectively surround themselves with beneficial soil microbes, giving plants just what they need exactly when they need it. The greater the diversity of soil microbes they attract and care for, the more likely it is that all a plant’s needs are met. Hence, the root microbiome – the community of microbes living in the Rhizosphere – is the key to ecosystem health and resiliency.

Actinomycetes are bacteria that give soil its typical smell. This ‘earthy’ scent stimulates serotonin production in the brain, improving our mood by acting like an antidepressant. Join one of our habitat restoration events to get your dose of vitamin soil!
Picture taken by fermecrocus

Immune system of the forest

There are incredible similarities between the Rhizosphere and the human gut. Both host a staggering number of bacteria and harbour a microbiome responsible for digestion and biological defense. Plants seem to actively recruit protective microbes to suppress pathogens. Just as our gut bacteria fight off diseases, soil microbes protect plants from pathogens trying to invade their roots. A plant’s immune system is the Rhizosphere.

While the role of the root microbiome on plant health and productivity is gaining attention, the underlying mechanisms are not fully understood and are a major focus of renowned researchers like Suzanne Simard. For example, studies have shown that microbes native to a certain area are more effective at suppressing plant diseases than exotic microbes. As native plant and microbe communities have co-evolved over millennia, they are adept in supporting each other. Plants and associated organisms create an intricate web of life, sharing food and other vital resources with soil organisms for the benefit of all.

Habitat restoration at St. John Point Regional Park. Coarse woody debris attracts beneficial soil microbes that break down complex minerals and nutrients, making them accessible for plant roots.

How you can enrich the root microbiome

If humans upset this fine-tuned underground support network in the Rhizosphere through practices that reduce beneficial soil microbes, plant growth and productivity declines. Harmful practices include the introduction of exotic species, application of synthetic pesticides and fertilizers, and any type of soil disturbance. Where the root microbiome has suffered, ecosystem restoration can help to mitigate and reverse some of the damage done. To attract beneficial microbes to the land you care for, consider following practices:

  • Plant native species who are adapted to the local climate, conditions, and microbial communities. Remove invasive plant species when possible.
  • In gardens, grow a diverse range of plants and use native cover or fill crops (e.g. Blue Wildrye, Seablush, Miner’s Lettuce, or Woodland Strawberry) to increase populations of beneficial soil microbes.
  • Avoid tillage, soil compaction, and artificial fertilizers and pesticides.
  • Add natural nutrient sources such as compost, leaf mulch, green manures, and coarse woody debris to enrich the root microbiome.

To buy native plants or book a free landholder consultation, you can contact us at info@mayneconservancy.ca.

For further reading

The secret world beneath our feet is mind-blowing and the key to our planet’s future

The Rhizosphere – Roots, Soil and Everything in Between

The Influence of Soils on Human Health

Tending your Soil Life


1 Comment

Gin Nielsen · July 28, 2022 at 11:37 am

Thank you for this very informative article, about the Rhizosphere, Gwen!

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