AMF colonization of roots-purple threads are hyphae and the round structures are vesicles, or fungal storage units. Photo credit Kaida Penny

I stopped by a garden centre this weekend to pick up a bag of soil. Looking around, I noticed that all the potting mixes listed arbuscular mycorrhizal fungi (AMF) on their packaging. AMF are microscopic, soil-dwelling fungi that inhabit the roots of almost all plants. Their ability to improve plant nutrition, protect against pathogens and increase stress tolerance have made them popular soil additives, or ‘bio fertilizers’.

“Why wouldn’t you want AMF in your soil?” the clerk asked, incredulous. “They’re amazing!”

AMF are amazing – In many ways, they are keystone species for both managed and natural systems. These invisible plant symbionts are responsible for so many ecosystem services: beyond increasing biomass of their hosts, they can reduce the need for fertilizers and pesticides, and make crops more drought-tolerant. Though the mechanism isn’t completely understood, there is evidence that root infection with AMF improves a plant’s nutritional value, independent of its nutrient status.

The influence of AMF extends beyond their plant hosts to the ecosystem. While they are well known to improved soil nutrient cycling, they can also reduce nutrient loss through leaching and stabilize soils, reducing soil erosion.

AMF have been shown to promote plant biodiversity, and even influence above-ground plant herbivores and pollinators. AMF hyphae in the soil provide food and habitat for an extensive soil food web, called the mycorrhizosphere.

So, I am glad that the agricultural industry—the ‘sustainable’ and ‘organic’ market in particular—is showing interest in AMF. As an agricultural researcher, I think that this is a positive development—we should do all we can to reduce the footprint of agriculture. But, leaving the store, I wondered if commercial inoculants were the best way for growers to embrace AMF management. As an ecologist, I have so many questions, so few answers.

Thick organic layers of the soil profile, shown in this soil core taken from beneath ancient cedar stands in southern BC. Photo credit Dylan Zaitsoff

Here are some questions a grower should ask before using commercial AMF inoculants:

• What do you want it to do?

Ask a biologist what AMF do, and you will get either a grimace or a treatise. They do many things, and we’re still figuring out all the different roles they play in ecosystems. More importantly, we do not know who does what—or when. There are likely thousands of species, and they do not behave in predictable ways. While an isolate might promote growth in one plant, it decreases growth in another. This is true for any measurable effect you might be interested in (such as nutrient content, root lesions, seed set, etc.). The bottom line is that have differential effects on plants depending on the identity of the fungus, the identity of the plant, soil conditions, climate…ad infinitum. In my career, I have yet to find a universally beneficial AMF and I have no illusions that one exists.

Research: Fungi are cultured in the greenhouse using host plants. Photo credit Miranda Hart.

• Will it establish in your system?

So, say you’re not sure you need it, but you still want to add AMF, as ‘insurance.’ It is very difficult to determine whether an AMF inoculant has successfully established. In my research, I have found that some commercial inoculants fail to establish, period. But to know this, I spend thousands of dollars designing molecular probes. This is a Herculean effort for my lab; it is impossible to do as a layperson. So your ‘insurance’ policy might be a dud—but you’ll never know.

• What if it establishes too well…

OK, you still think AMF inoculants are a good idea; it feels good to be adding biodiversity rather than removing it—doesn’t it? Besides, what’s the worst that can happen? I wonder, did the Australian government ask themselves this before introducing the cane toad? The spread of invasive species is one of the biggest threats to biodiversity across all continents and ecosystems. This is an unfathomable problem for soil ecosystems because we don’t know what a ‘native’ soil community looks like, or how many of the world’s soils have been compromised by invasive soil organisms. The fate of a single isolate of AMF might seem pretty inconsequential—we can’t even see them! But we have no idea if added AMF inoculants will invade natural systems and we do not know what will happen to native soils if they do.

Despite my reservations, the enthusiastic garden centre employee was right—I do want AMF in my soil! For productive and sustainable agro-ecosystems, it is important that growers do all they can to foster healthy soil communities, including AMF. In some cases, this might mean using commercial inoculants. But we have much to learn before I can unequivocally recommend a specific inoculant. Such research is underway, in my lab and in many labs around the world, and the answers are coming, just not as fast as growers are applying it to their soils.