Biochar and Soil Microbes: What Happens in the Living Soil?
Soil can look like an inert material, but biologically it is anything but. A healthy soil contains enormously diverse communities of bacteria, fungi, archaea, protozoa and other organisms, interacting with plant roots, minerals, organic matter, air and water.
These microorganisms carry out some of the fundamental processes on which plants depend. They decompose organic material, mineralise nutrients, transform nitrogen, influence phosphorus availability, contribute to the formation of soil aggregates and participate in the cycling of carbon, nitrogen, phosphorus and sulphur.
So when we add biochar to soil, one of the most interesting questions isn't simply “What does biochar give the plant?”
It is:
“How does biochar change the environment in which the soil ecosystem lives?”
The answer involves some fascinating physical, chemical and biological processes.
Biochar Creates a New Physical Environment in Soil
Biochar retains much of the microscopic cellular structure of the plant material from which it was produced.
During pyrolysis, volatile components of the wood are driven off and its carbon structure is transformed. What remains is a carbon-rich material containing an intricate network of pores.
These occur across different size scales, from relatively large channels inherited from the vascular structure of the wood to microscopic pores within the carbon itself.
This gives biochar two important characteristics:
high porosity;
a potentially large internal surface area.
When biochar enters soil, it therefore adds much more than individual particles of carbon. It introduces a new three-dimensional structure containing surfaces, cavities and interfaces where water, dissolved compounds and microorganisms can interact.
The zone immediately surrounding a piece of biochar can develop its own distinctive biological and chemical characteristics. Scientists sometimes call this the “charosphere”, in a similar way to the rhizosphere around plant roots.
Do Microorganisms Actually Live Inside Biochar?
Yes — microorganisms can colonise biochar surfaces and pores.
But the often-used description of biochar as an “apartment block for microbes” needs a little qualification.
Not every pore is suitable for every organism. Pore size matters. Some pores are far smaller than bacterial cells, while larger pores and cracks may provide places where microorganisms can colonise or where water and dissolved nutrients accumulate.
The value of biochar as microbial habitat therefore isn't simply a matter of having lots of holes.
Its importance comes from the combination of physical shelter, moisture, chemical surfaces and interactions with the surrounding soil.
Biochar can also contribute to the development of soil microaggregates. These tiny aggregates can create physically protected microsites where microorganisms are less exposed to environmental fluctuations and predation.
Water Is Part of the Story
Microbial activity is highly dependent on moisture.
As soil dries, the thin films of water surrounding soil particles become increasingly disconnected. Microbial movement and the diffusion of nutrients and substrates become restricted, and microbial activity can decline substantially.
Biochar can modify the way water is held within soil because water can occupy both pores within the biochar itself and pores created between biochar and surrounding soil particles.
This does not mean that every biochar always makes every soil wetter. The response depends strongly on biochar characteristics, application rate and soil texture.
Meta-analyses nevertheless show an overall increase in plant-available water following biochar addition, with some of the largest responses occurring in coarse-textured soils.
Maintaining a more favourable moisture environment can consequently influence microbial activity as well as plants.
Biochar Surfaces Change as They Age
Fresh biochar and biochar that has spent years in soil are not chemically identical.
As biochar ages, its surfaces undergo oxidation and become coated with organic compounds, minerals and microbial products.
Oxygen-containing functional groups — including carboxyl, hydroxyl and phenolic groups — can develop on biochar surfaces.
These contribute to the material's ability to interact with charged ions.
This becomes particularly relevant to something called cation exchange capacity, or CEC.
What Is Cation Exchange Capacity?
Many important plant nutrients occur in soil as positively charged ions, or cations.
These include:
potassium (K⁺);
calcium (Ca²⁺);
magnesium (Mg²⁺);
ammonium (NH₄⁺).
Negatively charged surfaces within soil can temporarily hold these ions rather than allowing them to move freely away with drainage water.
As biochar weathers and develops surface charge, it can contribute additional exchange sites to the soil.
That doesn't mean biochar somehow manufactures these nutrients. Instead, it can affect where nutrients are held and how they move through the soil system.
This is one of the reasons biochar and compost can be so complementary: compost and other organic materials can supply nutrients and biologically available carbon, while biochar can contribute persistent surfaces and pore structure.
Biochar Can Change Soil pH
Another important interaction is pH.
Many biochars are alkaline, although their actual pH varies substantially according to feedstock and pyrolysis conditions.
Adding an alkaline biochar to acidic soil can increase soil pH.
This matters biologically because pH is one of the major controls on soil microbial community composition.
Changing soil pH can therefore favour some groups of microorganisms while making conditions less favourable for others.
This is an important distinction.
It is too simplistic to say:
“Biochar increases good microbes.”
A more scientifically accurate statement is:
Biochar can alter microbial abundance, activity and community composition by changing the physical and chemical environment of the soil.
Studies frequently report increased microbial biomass following biochar application, but responses vary considerably between soils and biochars.
Carbon — But Not Necessarily Microbial Food
There is an apparent puzzle here.
Biochar is extremely carbon-rich, yet most of that carbon isn't readily digestible microbial food.
This is precisely why biochar can persist for so long.
During pyrolysis, carbon becomes increasingly arranged into condensed aromatic structures that are resistant to microbial decomposition.
Biochar therefore behaves very differently from adding fresh plant material or compost.
Compost contains a substantial pool of biologically accessible organic compounds that microorganisms can metabolise.
Biochar contains predominantly much more resistant carbon.
However, fresh biochar can also contain smaller quantities of more readily available organic compounds, and over time its enormous surface area becomes coated with organic substances from the surrounding soil.
So biochar can become an important physical and chemical platform for microbial activity without itself behaving like a conventional microbial food source.
Biochar Can Affect Soil Enzymes and Nutrient Cycling
Soil microorganisms produce enzymes that catalyse the decomposition and transformation of organic compounds.
Research has shown that biochar can alter soil enzyme activity, microbial biomass and microbial community composition.
These changes matter because microorganisms participate directly in nutrient cycling.
Nitrogen, for example, undergoes multiple microbial transformations between organic nitrogen, ammonium, nitrate and gaseous forms.
Phosphorus can be released from organic compounds through microbial activity, while microorganisms can temporarily immobilise nutrients within their own biomass before releasing them again.
Consequently, changing the microbial environment can have effects extending far beyond simply increasing the number of organisms present.
Not All Biochars Behave the Same Way
This is one of the most important things to understand about biochar research.
Biochar isn't one uniform substance.
Its properties depend on:
the original feedstock;
pyrolysis temperature;
heating rate;
residence time;
oxygen availability;
particle size;
mineral content;
pH;
surface area;
pore structure.
The soil matters just as much.
A wood-derived biochar added to acidic sandy soil may produce a very different microbial response from the same biochar added to an alkaline clay soil.
Research reviews have consequently found considerable variation in microbial responses.
That isn't evidence that biochar “doesn't work”. It tells us something more interesting: biochar becomes part of an existing ecosystem, and its effects arise from interactions between the biochar, soil, plants and organisms already present.
Why Combine Biochar With Compost?
Understanding the microbiology also helps explain why we so often recommend combining biochar with compost or other biologically active organic material.
They perform different functions.
Compost provides:
organic matter, nutrients, microorganisms and readily decomposable carbon compounds.
Biochar provides:
persistent carbon, pore structure, surfaces capable of retaining compounds and a long-lived modification to the physical soil environment.
When biochar is mixed with compost before entering the soil, its pores and surfaces can also become associated with water, nutrients, dissolved organic matter and microorganisms before application.
This process is often described as charging or inoculating biochar.
From Feeding Plants to Building an Ecosystem
Perhaps the most important lesson from the science is that biochar asks us to think differently about soil fertility.
Conventional fertilisation can be thought of primarily in terms of supplying particular chemical nutrients to a crop.
Living soil is much more complicated.
Plant roots exist within a biological system where bacteria, fungi, organic matter, minerals, water and gases are constantly interacting.
Biochar doesn't replace that ecosystem.
It modifies the habitat in which it operates.
And because the carbon structure of biochar can persist for a very long time, that physical modification can be far longer-lived than many conventional soil amendments.
That is why we think the most useful question isn't simply:
“What does biochar feed?”
It is:
“What kind of soil are we helping to build?”