Assessment of Arbuscular Mycorrhizal Fungi of UK Soils.
What is this about?
This is a project to establish the level of arbuscular mycorrhizal fungi (AMF) in arable soils across a range of locations, soil types and management systems.
The project, led by Dr Tom Thirkell of the Crop Science Centre (CSC), complements work undertaken at CSC into sustainable crop nutrition.
Why is AMF important?
One of the oldest symbiotic relationships in nature is the one plants have with AMF. It’s also currently thought to be the origin of the relationship legumes use to fix nitrogen from the air. Plants feed carbon to the fungi in exchange for nutrients delivered from the soil. Work at CSC is exploring the signalling pathways but very little is currently known about the level and range of AMF in UK soils.
What has the project done?
The Soil Circle is the BOFIN knowledge cluster around this project of those with specialist interest, including scientists who have opted in, share experiences and knowledge, and help shape the project. It currently has 126 members.
Within this is the Soil Squad, 77 farmer members who committed to provide root samples for analysis and gave additional information on soil type, management (organic, regenerative, conventional) and cultivation system (direct drill to plough). They were provided with packs, postage paid envelopes and a sampling protocol drawn up by BOFIN and CSC.
Farmers were asked to choose a first wheat crop, and most of the samples provided came from crops of KWS Extase or RGT Skyfall. RAGT Seeds and KWS UK have provided additional samples from their variety screens. These have all been analysed by staff at CSC.
What were the samples like?
CSC received 41 root samples from BOFIN Soil Squad members. Of the root samples provided, every single one had been correctly taken, treated and packed according to the protocol provided. What’s more every single sample was accompanied with full cropping history details. This means the Soil Squad has provided a really valuable and thorough dataset for analysis.
How does AMF relate to farming practice?
Details of cultivation practice were drawn from the mycorrhizal root survey Soil Squad members completed at the time they sent in their samples. In addition, details of predominant soil type and how participants describe their farming system were drawn from their original registration submission. Generally speaking, the results show what you might expect:
Direct drilling brings the most root colonisation, followed by shallow non-inversion tillage, while there is least root colonisation in deep tillage systems.
Organic performs best (although note small sample size), followed by regenerative agriculture, while AMF is lowest in conventional farming systems.
Colonisation tended to be highest in medium soils, followed by heavier soil types with lowest levels found in light soils.
Only four of the samples came from plough-based systems, two of which were from organic farms. It is therefore difficult to draw conclusions from these samples.
How does variety choice affect AMF?
The study was supported by plant breeders RAGT Seeds and KWS UK, who also provided samples from small-plot trials near Cambridge. A range of KWS varieties, both winter and spring-sown, all grown in the same field with the same management practice were compared.
In terms of AMF colonisation, there was no difference between winter and spring-sown varieties, but for individual varieties, there was huge variation, from about 20% to almost 50%.
KWS Extase was the highest at 48.1%, Palladium, Zealum and Ladum were much lower at around 25-30%. Ladum had the highest number of arbuscles, at 14.5%, with Palladium at 6%.
The results suggest Palladium doesn’t think it needs mycorrhizas in quite the same way as Extase. The nature of the relationship is that the wheat plant decides when and to what extent it trades resources.
This would be an interesting area of further study – it might be that those varieties that have really high colonisation are allowing it because they get more benefit from that interaction. It may be that root architecture has a big impact, or there may be other, as yet unknown factors.
How do cover crops influence AMF?
These were the focus of trials at RAGT where they’re testing different cover cropping mechanisms and approaches. We sampled the site in June of a crop of Skyfall winter wheat that followed one of five cover crop mixes or fallow as the control.
Where the wheat crop followed a cover crop, the results indicate you get almost three times as much AMF colonisation than if it follows fallow. CC4 in the chart is a biofumigant mixture that didn’t perform as well as two Maxicover seed mixes (CC1 and CC3).
This supports anecdotal evidence that the longer you have living roots in the soil, the longer you have carbon being provided to the mycorrhizae and it can stay in its growing, foraging growth habit.
Where you have a fallow period with no living roots, the fungi retreat into spores and become dormant, waiting for another root to come along. In summary cover cropping is generally good for mycorrhizae.
How does AMF colonise roots?
AMF spores in the soil germinate under favourable conditions and the fungi grow hyphae, which are thin, thread-like structures. These hyphae grow through the soil and search for suitable host plant roots. The fungi and plants communicate through biochemical signals, allowing the fungi to recognize compatible host roots.
Upon finding a suitable host, the fungal hyphae penetrate the plant’s root. This penetration typically occurs either between epidermal cells or directly through an epidermal cell.
Once established, the fungi can spread within the root system and even to adjacent plants. The network of fungal hyphae can extend far into the soil, effectively increasing the root surface area and thereby the plant’s ability to access resources.
The symbiotic relationship is mutually beneficial. The plant provides the fungi with lipids (fatty acids) carbohydrates (like glucose and sucrose), which are products of photosynthesis and used by the fungi to lay down its carbon-based structure. In return, the AMF enhance the plant’s uptake of water and nutrients, particularly phosphorus, and also other minerals like nitrogen, potassium, and calcium from the soil.
AMF are carbon auxotrophs – the only way the fungi can get carbon is from the host plant. This process is regulated by the plant’s perception of its environment, and when it’s optimal to engage with the AMF. Activity is particularly significant in nutrient-poor environments where the enhanced nutrient uptake provided by AMF can be vital for plant survival.
This symbiotic relationship plays a crucial role in soil health and plant growth, influencing plant diversity and productivity in various ecosystems. What’s more, fungal carbon is very long lived in the soil – much longer than plant carbon. So AMF may also provide an effective way to sequester carbon in your soil.
What do arbuscles do?
The arbuscles (from Latin ‘small tree’) are highly branched fungal structures inside the plant root, which provide enormous surface area over which the fungi can trade mineral nutrients (mostly phosphorus and nitrogen) for plant carbon, in the form of lipids and sugars.
The number of arbuscules gives a hint to the amount of nutrient exchange that’s going on, but this is only a hint. At the time your samples were collected, the levels of nutrient trade between plant and fungus are probably near their peak. More in-depth work in controlled conditions will let us see in more detail how much nutrient trade is taking place.
What do vesicles do?
The mycorrhizal fungal vesicles in the roots are balloon-like structures that allow the fungi to store large quantities of lipids inside the root. Across all the samples we received, there are very few vesicles, and this is not hugely surprising. The flow of carbon to the fungi would still have been high at the timepoint we sampled.
Instead of storing this carbon, the fungi are using it to make arbuscules, and sending it out to the growing mycelium in the soil so that it can forage for mineral nutrients.
In later growth stages, the carbon delivery to the roots will slow and eventually stop altogether. Shortly after flowering, the fungus will switch from arbuscule production to vesicle production. Rather than building or maintaining mycelium out in the soil, the fungus will store carbon in the vesicles.
This allows it to ‘over-winter’ while it has no carbon supply, and wait for another growing root to be close enough that it can colonise. If samples were collected at dough development/senescence, we would expect to see far more vesicles and fewer arbuscules.