Biochar and Mycorrhizal Fungi: What Happens Beneath the Soil Surface?

Plants have roots — but for most plants, the system through which they explore the soil doesn't end at the root surface.

Many plants form intimate partnerships with fungi known as mycorrhizal fungi. The fungi colonise or closely associate with plant roots and extend extremely fine filaments called hyphae out into the surrounding soil.

The plant supplies the fungus with carbon compounds ultimately derived from photosynthesis. In return, the fungal network can dramatically extend the volume of soil from which the plant obtains resources.

The relationship between biochar and these fungi is one of the most fascinating areas of biochar biology.

But it is also more complex than the popular idea that biochar simply provides a “home for mycorrhizae”.

What Is a Mycorrhiza?

The word mycorrhiza refers to a symbiotic association between a fungus and a plant root.

There are several different types.

Two particularly important groups are arbuscular mycorrhizal fungi (AMF) and ectomycorrhizal fungi (ECM).

Arbuscular mycorrhizal fungi enter the root cortex and form highly branched structures called arbuscules inside root cells. These structures create a large interface across which nutrients can be exchanged between plant and fungus.

Ectomycorrhizal fungi, which are particularly important in many woodland trees, behave differently. They form a sheath around fine roots and a network between root cells rather than penetrating the cells themselves.

Different plants associate with different fungal groups, so there is no single universal “mycorrhizal network”.

Why Does the Plant Participate?

Plants give away a significant proportion of the carbon compounds they produce through photosynthesis to their fungal partners.

Why?

Because fungal hyphae are extremely fine and can explore soil beyond the depletion zone immediately surrounding a root.

This is particularly important for relatively immobile nutrients such as phosphorus.

Phosphate moves through soil much less readily than nitrate. A plant root can therefore rapidly deplete the phosphorus immediately around itself.

A network of fungal hyphae effectively increases the area of soil that can be explored.

Mycorrhizal associations can also contribute to the acquisition of nitrogen and micronutrients, influence water relations and affect plant responses to environmental stress.

Where Does Biochar Enter the Relationship?

Biochar can influence mycorrhizal fungi both directly and indirectly.

Its pores can be colonised by fungal hyphae. Researchers have directly observed arbuscular mycorrhizal fungi colonising biochar particles in soil.

But physical habitat is only one mechanism.

Biochar can also change:

• soil pH;

• water availability;

• nutrient retention;

• phosphorus availability;

• microbial communities;

• soil aggregation;

• root growth;

• concentrations of compounds that stimulate or inhibit fungi.

Each of these can affect the relationship between a plant and its fungal partners.

Can Fungal Hyphae Actually Grow Into Biochar?

Yes.

Microscopy studies have demonstrated fungal colonisation of biochar pores and surfaces.

The original vascular structure of wood can leave channels of a size accessible to fungal hyphae, creating protected physical spaces within the char.

But again, pore size matters.

Much of biochar's internal microporosity is far too small to accommodate fungi or bacteria physically. Those tiny pores can still affect water and chemical sorption, while larger pores and cracks provide potential biological habitat.

This distinction is important because descriptions such as “biochar is full of holes for fungi” are directionally useful but scientifically incomplete.

Phosphorus Changes the Story

One of the most interesting complications concerns phosphorus.

Mycorrhizal fungi are particularly valuable to plants when nutrients such as phosphorus are difficult to obtain.

If soil suddenly contains abundant readily available phosphorus, the plant has less incentive to invest photosynthetically produced carbon in maintaining an expensive fungal partnership.

Consequently, biochar's effect on mycorrhizae depends partly upon what the biochar does to nutrient availability.

A biochar that changes soil phosphorus availability may therefore alter mycorrhizal colonisation indirectly.

This helps explain why studies don't all produce the same result.

pH Matters Too

Soil pH strongly influences both nutrient availability and microbial ecology.

Because many biochars are alkaline, their addition can raise the pH of acidic soils.

This may alter phosphorus chemistry and the suitability of the soil environment for particular fungal species.

Again, this illustrates why the relationship is not:

biochar → more fungi

but rather:

biochar → changed soil environment → changed plant/fungal relationship.

Biochar, Water and Fungal Networks

Fungal hyphae depend on the moisture conditions within soil.

Biochar can alter soil pore structure and water retention, particularly in coarse-textured soils.

This potentially affects both plant roots and their fungal partners.

There is an interesting complementarity here.

Biochar can influence where water is retained, while fungal hyphae increase the volume of soil explored by the plant-fungal partnership.

Neither process should be exaggerated into a guarantee of drought protection, but together they help explain why biochar-mycorrhizal interactions are of considerable interest in research into plant resilience.

Does Biochar Increase Mycorrhizal Colonisation?

Often — but not always.

This is where it is important to distinguish good science from a marketing claim.

A recent global meta-analysis found an overall increase in root arbuscular mycorrhizal colonisation following biochar application, but the effect was strongly dependent on environmental and experimental context.

Other individual studies have found little or no effect.

So it would be inaccurate to say:

“Adding biochar creates a mycorrhizal network.”

The fungal inoculum must already be present or be introduced, a suitable host plant must exist, and soil conditions must support the association.

A better conclusion is:

Biochar can create physical and chemical conditions that favour mycorrhizal colonisation in some soil-plant systems, and biochar particles themselves can become colonised by fungal hyphae.

The “Wood Wide Web” — A Useful Idea With Some Caveats

Mycorrhizal networks in woodland are popularly described as the “wood wide web”.

It is a wonderful phrase, but it can lead to an oversimplified picture in which trees are imagined consciously sending food and messages to one another through a benevolent underground internet.

The real ecology is considerably more complicated.

Common mycorrhizal networks can connect multiple plants, and carbon, nutrients and signalling effects can occur within these systems. But fungi are organisms pursuing their own ecological interests, and relationships can involve competition as well as cooperation.

What the phrase captures correctly is something important:

a woodland is not simply a collection of independent trees.

Plants, fungi, microorganisms, soil, organic matter, minerals and water form an interconnected below-ground ecosystem.

Why This Matters for Trees and Woodland

This is particularly relevant to tree establishment.

A newly planted tree has a relatively limited root system. Establishing appropriate mycorrhizal relationships can substantially increase the effective soil volume available to it.

Biochar is therefore interesting in woodland restoration, agroforestry, orchards and tree planting not because it substitutes for fungi, but because it may help create a soil environment in which roots, fungi and other organisms can establish.

And this is where the subject becomes especially meaningful for us at Sweethill.

Our biochar originates in our own Devon woodland, where we are gradually working to regenerate surviving areas of ancient woodland and move away from plantation-dominated forestry towards a more diverse woodland ecosystem.

The biochar produced through that management can ultimately return to soil as a stable, porous carbon structure.

Woodland creates wood.

Wood becomes biochar.

Biochar enters soil.

Soil supports fungi.

Fungi associate with roots.

And roots support another generation of plants and trees.

It is not a simple cycle, but a complex living network — and understanding that complexity is exactly why soil biology matters.