Raphaella Hull is a visiting PhD student from the University of Cambridge, where she researches arbuscular mycorrhizal symbiosis in plants. She is interested in how to grow with soil fungi in mind.
Making good soil is key to building resilience for the future. Here, I talk about the ways in which we can invite mycorrhizal fungi into our gardens and rebuild the soil, including methods that are being used here at the Botanic Garden.
Broadly, soils are made up of solid matter, organic matter, air, and water. The solid matter component can be sand, silt, or clay, whilst the highly sought-after loam is a mixture of solid matter types. The organic matter fraction is made up of plant and animal derived residues at various stages of decay. We focus on increasing the organic matter fraction during soil regeneration.
At any given time, soil organic matter is a mixture of the living tissues of plants and animals and their exudates; the partially decomposed plant and animal residues (known as detritus); the substantially decomposed mixture of materials (known as humus); and resistant organic matter, for example charcoal.
Soil organic matter contains twice as much carbon as the atmosphere and terrestrial vegetation combined, making the conservation and regeneration of soils essential for the mitigation of climate breakdown.
Most roots in the soil are intricately connected to one another by fine webs of mycorrhizal fungi. These ecosystem-creators spend their whole lives belowground, making it difficult for humans to know they are there. Plants, however, can sense mycorrhizal fungi and invite them into their roots, forming a symbiotic relationship that is as old as plants themselves.
When plants first moved from the ocean onto land, some 500 million years ago, they lacked roots for taking up nutrients. To overcome this problem, early plants formed a symbiotic relationship with fungi that were better equipped to forage for food. In return, plants provided mycorrhizal fungi with carbon fixed by photosynthesis. This intimate partnership, in which fungi act as an extended plant root system, has been maintained across the entire plant kingdom and is essential for soil aggregation, nutrient distribution, and ecosystem resilience.
The habitats conserved by the Botanic Garden, in particular the wildflower meadows, grasslands, and woods, provide space for a rich diversity of mycorrhizal fungi. The areas of the Botanic Garden in horticultural cultivation likely also support an abundance of soil biota, due to the regenerative methods used by the horticultural team.
To promote mycorrhizal fungi, the soil environment must be kept as hospitable as possible. This means there must be living roots in the soil, sufficient organic matter, minimal fungicide and fertiliser use, and minimal soil disturbance.
Provide living roots
Mycorrhizal fungi require living roots in the soil because they depend on plant-derived lipids for their survival. Perennial planting, continuous cropping, and areas with a permanent herb layer have the potential to increase mycorrhizal associations, as living roots are maintained in the soil. In contrast, annual planting often results in patches of bare soil between growing seasons and involves significant soil disturbance. To continue the provision of living roots between annual plantings, we can grow plants called ‘green manures’ that grow reasonably quickly whose main purpose is to cover bare soil.
Green manures not only provide living roots for mycorrhizal fungi belowground, thus acting as key entry points for carbon into the soil, but also offer food for pollinators aboveground. They can also be used as organic mulch when cropped or incorporated into the soil when planting. Finally, green manures provide essential shade for the soil to prevent water loss during hot spells and retain heat during cool spells.
Green manures are used in three main ways at the Botanic Garden: as short-term summer ‘catch crops’, for overwintering, or as long-term manures.
Catch crops are sown when a patch of land becomes vacant and are cropped a few weeks to a couple of months after sowing. The green manure can then be left on the soil surface as a mulch or dug in during planting. Cropping is generally done before the plant has flowered and set seed. However, some catch crops have attractive flowers for pollinators, and some plants can be left for this reason. The concept is similar with overwintering crops, but these plants are winter hardy and can therefore be sown in late autumn and used in the following spring. Long-term manures are used for a piece of land that will not be in cultivation for some time.
There are many different green manure plant species. The most common catch crop is white mustard Sinapis alba, which is very fast growing and particularly effective at increasing soil carbon due to its woody content. White mustard is used to fill spaces between planting on the Broadwalk at the Botanic Garden. Phacelia tanacetifolia is another fast-growing leafy crop, but one that has beautiful flowers. It is killed by the first hard frost and is best left on the soil surface as a mulch. You will see it at the Botanic Garden as more green manures are incorporated into planting regimes.
Add organic matter
After providing living roots, the second way to promote soil fungi is to add well-rotted organic matter, as this provides soil available nutrients and helps to improve the soil structure. Organic matter is anything with animal or vegetable origins; animal manure, compost, straw, seaweed, green manure, or woodchip are typical examples. These sources of organic matter can be spread on the surface as a mulch or worked into the soil when planting. If used as a mulch, the worms will incorporate the organic matter into the soil for you and you can avoid disturbing the soil profile by digging.
Incorporating organic matter into the soil produces a nutrient-balanced substrate that grows healthier and more nutritious plants, holds more moisture, and supports annual plant growth for longer. Furthermore, application of organic matter may stimulate photosynthesis and high-quality plant carbon inputs into the soil, resulting in more efficient microbial carbon use, therefore locking in more carbon belowground.
Virtually any bulky, soil-improving manure can be used as a mulch, provided it is well enough rotted to spread on the ground and around plants. The most common mulch used at the Botanic Garden is bark chip, which is an effective weed suppressant and holds in moisture, as well as providing a source of organic matter as it slowly rots. Bark chip is not commonly used to mulch vegetables, as it can deprive them of nitrogen during early stages of its decomposition.
Occasionally, unusual mushrooms are spotted growing on the bark mulch at the Botanic Garden. For a few years, the edible wine cap mushroom Stropharia rugosoannulata (non-mycorrhizal) was found growing on the bark. In fact, it is possible to cultivate wine caps below taller vegetable plants on bark mulch, as these saprophytic fungi break down woody debris and release nutrients back to the plants.
Thirdly, we can promote mycorrhizal fungi and enrich the soil by avoiding the use of agrochemicals. Typically, the effects of agrochemicals on mycorrhizal fungi have not been explored or have been found to be rather varied. However, the herbicide glyphosate has been shown to decrease the presence of mycorrhizal fungi in plant roots. In addition, it is widely known that the use of synthetic fertilisers inhibits the relationship between plants and fungi and possibly leads to mycorrhizal fungi strains that are not beneficial to plant growth. Rather than supply nutrients through artificial fertilisation, it is more beneficial to plants and the wider ecosystem to provide nutrition via the enrichment of the soil.
Reduce soil disturbance
The way soil is managed also has profound effects on the abundance of soil fungi. Digging and mechanised tillage practices physically break fungal hyphae and have disastrous effects on mycorrhizal networks, being a major factor in reducing fungal abundance and diversity. Instead, no-dig approaches, perennial planting, and conservation of boundary areas that have permanent vegetation cover reduces soil homogenisation and aeration. These non-invasive soil management regimes support the presence of complex networks of mycorrhizal fungi and likely increase root carbon inputs into the soil.
Add mycorrhizal fungi directly?
With a growing awareness of the benefits of mycorrhizal fungi, it is unsurprising that there is also a growing interest in directly inoculating plants with mycorrhizal spores during cultivation. This is already facilitated through the sale of commercial mycorrhizal mixes. Cultivating plants with their mycorrhizal partners has the potential to boost plant growth and improve soil structure. However, there is a need for caution and more research in this area, as there are many potential detrimental consequences associated with inoculation of mycorrhizal fungi.
The trading agreements of plants and fungi are complex and not all interactions will result in positive effects. In some scenarios, mycorrhizal inoculation can lead to decreased growth and survival of target plant species. In fact, the introduced fungi can instead lead to increased fitness of invasive weeds. Furthermore, there is the potential for introduced mycorrhizal fungi to reduce the abundance and diversity of native fungi and to invade non-target habitats. Thus, like all species introductions, the necessity and methodology of mycorrhizal inoculation requires careful consideration.
The main consideration before mycorrhizal inoculation is whether it is necessary. Mycorrhizal fungi are widespread and abundant, even in many agricultural systems. As a result, inoculation is generally not necessary to establish mycorrhizal symbiosis. Rather, mycorrhizal spores present in the soil can be brought out of dormancy by regenerative soil practices. However, if soils have been sterilised by poor land use, then inoculation could be necessary for successful restoration of these sites. Ideally, native species and local strains would be used and there would be a consideration of the potential consequences on wider ecology.
Growing with fungi
Mycorrhizal fungi are essential in our soils, where they form intimate connections with plants to weave networks of reciprocity that hold the soil together. In soils stripped of these support systems, the reciprocal exchange of nutrients and carbon is lacking. However, we can use regenerative practices that promote mycorrhizal networks and rebuild the soil to welcome reciprocity back into the system.
Harnessing the potential of nature-based solutions for mitigating and adapting to climate change https://www.science.org/doi/10.1126/science.abn9668
Mycorrhizal Fungi as Mediators of Soil Organic Matter Dynamics https://www.annualreviews.org/doi/epdf/10.1146/annurev-ecolsys-110617-062331
The promise and the potential consequences of the global transport of mycorrhizal fungal inoculum https://slunik.slu.se/kursfiler/BI0898/40019.0910/Mykorrhiza_2,_Schwartz_et_al_2006.pdf
Grassland soil carbon sequestration: Current understanding, challenges, and solutions https://www.science.org/doi/10.1126/science.abo2380