US-based cover crop supplier Green Cover Seed have recently hosted a fascinating 4-part webinar series with Dr. Christine Jones. We definitely encourage you to watch them, but to get you started we’ve pulled together some of the key insights from these videos, with a focus on what these teachings mean for farmers on a regenerative journey.

Our main takeaway is that growing a diversity of living plants is the most important focus on any farm aiming to build healthy soil. This is a topic that has been discussed in the regenerative farming community for a while, but Christine shares how soil science is beginning to explain the mechanisms occurring between diverse living roots and the soil microbiome (the makeup of fungi and bacteria). 

It is clear that diversity aboveground is directly linked to diversity belowground, and diversity belowground is linked to the health and carbon storage capability of the soil as well as pest and disease resistance of any plants communing with that soil – there’s no doubt that these outcomes are of real significance for any farmer! 

So how does this all work, and what are the key messages for growers learning to regenerate their piece of earth? We’ve pulled out three main areas that Christine shared about:

1. The fungal energy pathway and the microbiome

Christine cites some of the papers presented at the Wageningen Soil Conference we wrote about here, emphasising that the early model of the soil food web is beginning to be replaced by a new, more dynamic model. Previous models of the soil carbon pathway were fairly linear, with carbon entering the soil through breakdown of above ground matter and detritus by larger organisms and fungi, propelled by a chain of soil organisms eating another. It is now understood that this ‘decomposer’ food web pathway is only responsible for small amounts of carbon entering soils. We are beginning to replace this conventional soil food web diagram with a new model, with fungi at the forefront. 

It is now understood that the vast majority of carbon entering soils does so through the ‘fungal energy channel’, which Christine also refers to as the liquid carbon pathway. Essentially, living plants use sunlight and atmospheric CO2 to photosynthesise, creating sugars (carbon) which are channelled down into the roots and released in the form of root exudates to the surrounding soil. These exudates are consumed by a multitude of fungal and bacterial communities, which transport carbon compounds around the soil. 

A healthy microbiome (dominated by saprotrophic and symbiotic fungi) will stabilise the majority of this carbon within the soil. It is this process of moving carbon from the air into stable soil compounds which is referred to as the fungal energy channel (1). This fungal network is also responsible for supplying energy to bacterial communities producing plant-available phosphorus and fixing nitrogen into the soil. 

The health of your soil is a key determinant of how well the fungal energy channel works, and a simple way to observe this pathway in action is by looking for rhizosheaths, as evidence of fungal hyphae feeding off sugars exuded from plant roots (and fixing nitrogen). You can keep a record of your farm’s score for rhizosheaths at set sample locations in your Soilmentor app – this test is one of our Regen Indicators! 

The diversity of plants growing in the soil is core to improving your fungal pathway. This is true for a number of different reasons:

1. Structure: A variety of different leaf structures increases opportunity for photosynthesis with more light interception, increasing the rate at which root exudates draw down carbon through the fungal pathway.
2. Microbe sharing: Plants from different functional groups cooperate with each other, and are able to recruit microbes from each other’s microbiome, as long as the roots are able to mingle near each other. E.g. If you grow a grass alongside a drought-tolerant herb in a low rainfall scenario, the grass can signal to microbes alongside the roots of neighbouring plants that have drought-tolerant characteristics. The grass may then ingest these microbes as endophytes (so they become part of the plants internal microbiome), which ‘switch on’ certain genes in the plant to thicken cell walls for water retention, making the grass more drought tolerant. When drought pressure subsides, the grass can then expel the endophyte and the genes are switched off again (2).
3. Fungi thrive: The microbial makeup of your soil more or less determines the likelihood that carbon levels will build up in the soil rather than being respired away. A higher proportion of carbon is stored in a stable form in fungally dominated soils, which allows for high activity in the fungal energy pathway. The fungi community thrive when there is greater diversity of plant functional groups present!

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Christine sharing about a diverse companion cropped field

The message from Christine is to prioritise diversity, without necessarily focusing on mycorrhizae. Mycorrhizal associations are amazing – but they represent just one part of the fungal story, and as long as you include one mycorrhizal plant in a cover crop mix, you’ll allow for a mycorrhizal network to build below ground. Other cover crop species without mycorrhizal associations may be equally important in the fungal energy channel. 

2. The importance of signalling chemicals – the language of the microbiome

You can think of soil as a complex network of microbial life. This microbial community is fed by the aboveground world through plant roots and their exudates, and these microbes send out signalling chemicals to interact with each other, and with roots in the soil (3). Christine refers to this complex network of connections as the ‘soil sociobiome’. 

One way to activate certain elements of this soil sociobiome is by adding signalling chemicals to the soil ecosystem. Christine defines a biostimulant as a substance that activates dormant microbes in the soil. We learn that the majority of soil microbes exist in a dormant state until they are activated by biochemical signalling.

Christine’s advice is that it is best to apply biostimulants to the seed before sowing (which we’ve heard re-iterated by Nicole Masters and John Kempf), as the most important feature of biostimulants is the signalling chemicals they contain. Ideally, a germinating seed should be forming a strong relationship with soil microbes from the beginning of its life, and establish a healthy ‘endosphere’ – the microbiome within the plant itself. Adding biostimulants as inoculants for crop planting will maximise these associations at the beginning from crop germination.

We also learn that biostimulants produced through a fermentation method will contain many more signalling chemicals than one from aerobic production methods (e.g. vermicompost, korean natural farming, bokashi). These chemicals have been observed to persist in the soil across multiple generations of plants, so the effects on the soil microbiome can last across multiple harvests (4). 

N.B. Vermicompost is considered a fermented product due to processes in the worm’s gut causing fermentation.

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Christine sharing a microscopic image of fungal hypae on roots

3. Reduced pest & disease pressure

Increased plant diversity also increases crop resistance to pests and diseases. Christine refers to expanses of monoculture as a “recipe for disaster” in terms of pest pressure. 

The buffer against pest pressure is not always about eliminating the pest or disease, but about supporting the crop to be more resilient, and equipped to fight infection and remain productive. There are more microbial cells in plants than there are plant cells, and this ‘endosphere’ of bacteria, archaea & fungi moving around among the cells of the plants are capable of supporting the plant to resist pest and disease damage. 

Plants ‘call for help’ when they are under attack from pests or diseases, and free living microbes in the soil can be ingested to support a defence against this attack (2). This call for help will go unanswered if the soil microbiome lacks sufficient health to respond. The solution is to ensure diverse cover crop species are planted alongside crops to support proper functioning of the soil microbiome, so soil microbes can be internalised by distressed crops through roots when needed. 

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Figure from a paper (Gopal & Gupta) – the sociobiome

Key actions to take from these webinars

• Keep ground covered with diverse living roots for as much of the year as possible. If your soil is left bare, your soil microbiome will deteriorate and carbon will be released from your soil. 
• Try to include at least 4 different functional groups in cover crop mixes & maximise the opportunity for these crop mixes to interact with each other and with your main crop below ground. One case study mix that Christine mentions is of four species – radish (brassica), oats (grass family), sunflower (aster family) and phacelia (borage family). The key here is that each of these plants belong to different functional groups, allowing for maximum benefit in the soil sociobiome. Despite the fact that none of these plants are legumes, this mix still allows for nitrogen fixing and availability, and outperformed mixes of only legume species in field trials. 
• Application of biostimulants to seeds before planting will support the fungal pathway (e.g Johnson-Su compost, vermicompost or bokashi). 
• Fungicides are the most detrimental agrichemical to the fungal energy pathway. Experiment with reducing / removing fungicide spray rounds across your farm, alongside biostimulant application & diverse cropping. 

References

1. Re-visioning soil foodwebs – Editorial by Mark Bradford, published in the Journal of Soil Biology & Biochemistry (2016)
2. Enhancement of drought stress tolerance in crops by plant growth promoting rhizobacteria (2016)
3. Microbial Signaling in Plant—Microbe Interactions and Its Role on Sustainability of Agroecosystems (2017)
4. Crying out for help with root exudates: adaptive mechanisms by which stressed plants assemble health-promoting soil microbiomes (2019)