Originally published June 2023, Wildflower News.
“Contrary to large organisms, studies on the impact of microbial invasions are less frequent, despite the fact that microbes have been intentionally released into open environments for a long time.” Panji, Cahya Mawarda et al., University of Groningen, Netherlands, 2020
This is the second in a two-part review of soil-inoculants. The first article (in the May, 2023 issue of Wildflower News) touched on the use of soil inoculants and whether they’re worth the cost. There’s another factor to consider when thinking about augmenting your garden with a bio-inoculant. We shed some light below.
Soil sustains an ecosystem
In any habitat such as a wetland or mixed woodland, living and inorganic forms co-exist in a interdependent way that is sustained over time – typically millennia. Soil is no different. It provides the substrate for a living ecosystem that is the foundation for what’s been called the soil-food-web.
Globally, the impact of introduced macro-species into local environments is well documented – Japanese knotweed, common cord grass, flatworm, zebra mussels, the list goes on. In many cases the impact is neutral, in others interfering with habitats, and at worst degrading natural systems and our urban landscapes.
These photos show fungal mycorrhizae on a decaying alder leaf and a peony root. Mycorrhizae, soil bacteria and other microbes are the foundation for the soil-foodweb, which can be disrupted with the introduction of commercial soil-inoculants. Photos K. Wilson 11
The introduction of non-native soil microorganisms into our yards and gardens through the use of microbial inoculants is underway. Professionals are starting to acknowledge their potential impact. They want to know more, as it’s increasingly recognized that bio-inoculants can modify local soil microbial communities.
Some recent studies
In their 2020 review of rhizobacteria and mycorrhizae, researchers with the National Centre for Appropriate Technology emphasize that attempting to change a soil’s ecosystem by adding a small amount of inoculant – while sometimes dramatically effective—is also inherently unpredictable.
Competition with the vastly greater numbers of microorganisms already in the soil is one consideration. There are also issues of microorganism-plant incompatibility and lack of diversity. Different plants exude different chemicals and release different nutrients into the soil upon decomposition. A soil may lack the necessary plant and microbial biodiversity to respond to an inoculant.
The authors note that 30 plus years of greenhouse-based experiments have been conducted, but few studies are available under field conditions.
In another review of bio-inoculants and their impact on soil microbial communities, authors in north Africa and France presented factors that can change the structure of indigenous soil communities. They cited studies on different combinations of Bacillus strains on various plants that show an inherent complexity and unpredictability.
While the authors note the rhizosphere is “one of the most diverse habitats on the planet and is central to ecosystem functioning”, they acknowledge that a modification in the bacterial community after inoculation could be buffered by ecosystem resilience. Also, the loss of bacterial species may not change soil’s functioning since different bacteria may carry out the same functions. In conclusion, they emphasize the need for further research and that studies on long term impacts have been neglected.
A 2020 literature search from the Netherlands and France showed that microbial inoculants can modify native soil communities and alter soil functioning. In one study, the introduction of Fusarium and Rhizoctonia strains led to a decline in weed population and suppression of the native plant species. The authors noted that while soil inoculation aims to regulate or improve ecosystem processes, the effects can deviate from the intended purposes. They conclude it’s still unclear how long an impact persists over time, and that mechanisms underlying an impact – and how it affects the resident soil microbial communities – are still poorly understood.
There is also potential for plant-soil feedback on invasive plants. Researchers from the western US noted there are many management projects which have attempted to decrease or eradicate invasive species, only to have them recolonize. While this is often attributed to reinvasion through propagules at the site or from the surrounding landscape, it may also may be due to invasive species changing site conditions to favour those same species over native species.
Another factor is the effect of soil inoculants on seed germination of native and invasive species. In a study in 2016, researchers from the University of California tested soil- and lab-grown bacterial inoculants on germination on 19 plant species. The invasive plants showed the highest percentage and speed of germination over the native shrubs. Their study also suggested that lab-grown inoculants increase germination in some native species, whereas soil inoculant does not.
But in another study in 2018, researchers from Ohio and Kansas showed that reintroduction of the native microbiome and native mycorrhizal fungi improves plant diversity, accelerates succession and increases the establishment of plants that are often missing from restored communities, as opposed to commercial fungi.
In their 2018 paper on developing soil microbial inoculants for agriculture, authors from Pennsylvania State University noted that microorganisms disperse widely from things like rain, dust and movement with invertebrates. They conclude that, while industry and academia have boosted investments into bio-inoculants, their efficacy remains unreliable and continued investigation into the long-term fates of microbial products is needed.
To address growing concerns over invasive species, researchers from Germany, Switzerland and the UK have devised four categories to clarify and define the impacts of non-natives in local ecosystems. They concluded that impacts of most non-native species are poorly understood, and hope their definition helps the global community understand the implications for biodiversity and ecosystems. They also hope it helps disentangle which aspects of debate about non-native species are due to conflicting definitions, and which represent true scientific discord.
Something of a conclusion
Soil-inoculants have been used in agriculture for things like nitrogen-fixation and biopesticides for several decades. Starting around 2010, research has started to overlap technologies – genetic, microbial, molecular – to peer into the complexity of microbiome relationships within soil communities.
Meanwhile, professionals and researchers agree that
bio-inoculants have potential for improving crops and natural ecosystems such as grasslands,
there are benefits but also mixed results,
we don’t know the long term consequences, and
non-native soil microbes have potential for interfering with native soil ecosystems. The predominant conclusion is that more research is needed.
Some things to consider
The horticultural world is on the cusp of mass marketing of bio-inoculants for home use. This new frontier is big business, with manufacturers competing for space on store shelves and all that entails, including big advertising.
Meanwhile life forms have a way of ignoring how we want them to behave, expanding beyond their initial conditions. When it comes to local and global markets, consumers are often the same. As a product becomes normalized, people start to use it beyond its intended use and instructions. As you consider whether or how you’re going to apply a commercial bio-inoculant, in your potting soil or your back-forty, it’s good practice to acknowledge the product contains living organisms that are integral to soil-plant and ecosystem functioning.
If you’ve made a commitment to learning about native plants and their place in the aspen parkland, you can continue that commitment by learning about native soil and its inter-relationships.
Here’s a very short list of resources to start:
https://gardening.usask.ca/articles-and-lists/articles-notillgardening/what-lives-in-soil.php
https://ses.uoguelph.ca/research/plant-and-environmental-health
https://soilfoodwebstudent.com/2020/09/07/review-soil-food-web-foundation-courses/