Biochar and Climate Change: Carbon, Water and Resilient Landscapes
Most explanations of biochar and climate change begin — quite rightly — with carbon sequestration.
But that is only one part of the story.
To understand biochar's wider relevance to a changing climate, we need to follow several interconnected cycles: carbon, water, soil and vegetation.
Biochar can potentially contribute to climate mitigation, by slowing the return of plant-derived carbon to the atmosphere.
It can also contribute to climate adaptation, by changing some of the physical properties that determine how soil receives, stores and releases water.
And that leads to a still larger question: what happens to the local movement of energy and water when healthy, vegetated land becomes dry and bare?
First: The Carbon Cycle
Plants remove carbon dioxide (CO₂) from the atmosphere through photosynthesis.
Using energy from sunlight, they convert CO₂ and water into organic compounds, releasing oxygen in the process.
In simplified form:
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
The resulting carbon compounds ultimately become leaves, stems, roots, wood and other plant tissues.
This is biological carbon capture.
The problem is that it is usually temporary.
When a plant or piece of wood dies, decomposer organisms gradually metabolise its organic carbon. Much of it is eventually oxidised and returned to the atmosphere as CO₂.
If biomass burns completely, much of its carbon is also rapidly converted into CO₂.
So although a growing tree removes CO₂ from the atmosphere, the carbon normally remains part of the relatively rapid biological carbon cycle.
Pyrolysis Changes the Chemistry
Biochar is produced by heating biomass under oxygen-limited conditions — pyrolysis.
Because there isn't enough oxygen for complete combustion, the chemical transformations are very different from simply burning wood in an open fire.
As temperature rises, cellulose, hemicellulose and lignin decompose. Water and volatile compounds are driven off, gases and vapours are produced, and a carbon-rich solid remains.
At a molecular level, the remaining carbon becomes increasingly dominated by condensed aromatic structures.
These structures are much more resistant to microbial decomposition than the original plant material.
This is the foundation of biochar carbon sequestration.
We aren't creating carbon.
The tree already captured it from atmospheric CO₂.
Pyrolysis changes the form in which some of that biologically captured carbon exists.
Why Biochar Carbon Lasts
Microorganisms can readily metabolise many of the compounds present in fresh plant material.
The highly condensed aromatic carbon structures in biochar are much harder to break down.
Biochar is not completely inert. It ages, oxidises, fragments and interacts with minerals and microorganisms.
But a substantial fraction can persist for centuries and, under appropriate conditions, considerably longer.
This means that converting appropriate biomass into biochar and putting that stable carbon into a durable reservoir such as soil can delay its return to atmospheric CO₂ dramatically compared with ordinary decomposition.
This is why biochar is studied as a form of carbon dioxide removal.
The Whole System Matters
There is an important qualification.
Making something black and putting it into soil does not automatically make the process climate beneficial.
A proper carbon assessment must consider:
where the biomass came from;
what would otherwise have happened to it;
whether harvesting it reduces existing ecosystem carbon stocks;
energy used in processing;
emissions during pyrolysis;
methane and other emissions;
transport;
how much carbon remains in the biochar;
how stable that carbon is;
what happens to pyrolysis gases and heat.
The strongest climate case therefore comes from sustainably sourced biomass and well-managed pyrolysis, not from cutting down valuable ecosystems simply to manufacture biochar.
Climate Mitigation Is Only Half the Story
Even if global greenhouse-gas emissions fell rapidly, growers would still need to cope with climatic changes already underway.
That means thinking about adaptation as well as mitigation.
For soil, one of the central questions is water.
And this is where the “soil as a sponge” idea becomes scientifically interesting.
What Does It Mean for Soil to Behave Like a Sponge?
Soil isn't a solid mass.
It is a porous system containing mineral particles, organic matter, roots, organisms, air and water.
The arrangement and size of the spaces between those components — the pore-size distribution — strongly affects how water behaves.
Large pores, or macropores, are important for infiltration, drainage and aeration.
Smaller pores retain water through capillary and surface forces.
Biochar itself contains pores, but mixing biochar into soil can also change the pores between soil particles.
Consequently, biochar can alter:
total porosity;
bulk density;
field capacity;
saturated hydraulic conductivity;
plant-available water capacity.
These terms help us move beyond simply saying “biochar holds water”.
Field Capacity and Wilting Point
After saturated soil has drained under gravity, the water remaining is approximately described by its field capacity.
As soil dries, water becomes increasingly strongly held.
Eventually plants can no longer extract enough water to maintain themselves. This is represented by the permanent wilting point.
The difference between these values contributes to plant-available water capacity.
This distinction matters because simply increasing the total amount of water in soil is not enough.
We want more water to remain in a form that plants can actually access.
What Does the Research Show?
Meta-analyses provide good evidence that biochar can improve soil water relations, but the effect is highly dependent on soil texture and biochar characteristics.
One large systematic review found that biochar increased plant-available water considerably more in coarse-textured soils than in fine-textured soils.
Another meta-analysis of 939 observations similarly found the largest increases in field capacity and available water capacity in coarse soils.
This makes physical sense.
Sandy soils contain many large pores through which water drains rapidly. Adding highly porous biochar and changing the distribution of pore sizes can create additional places in which water is retained.
Fine clay soils already contain enormous numbers of small pores, so the effect can be quite different.
Biochar is therefore not a universal sponge that produces the same result in every soil.
Its hydrological effect depends on the interaction between the pore structure of the biochar and that of the soil.
Drought: Holding Water Where Plants Need It
During prolonged dry weather, additional plant-available water can become extremely valuable.
If a soil can retain more of the rain that fell previously, plants may have access to that reservoir for longer as the soil dries.
Biochar can therefore contribute to drought resilience, particularly where water-holding capacity is naturally poor.
This doesn't make plants drought-proof.
Nor does it remove the need for mulching, appropriate planting, increasing organic matter and sensible water management.
It is one component of building a soil system capable of buffering environmental extremes.
Heavy Rain: The Other Side of the Sponge
Climate resilience isn't only about storing water.
It's also about how quickly water enters and moves through soil.
When rain falls faster than soil can absorb it, water begins moving over the surface.
Run-off can transport soil particles and nutrients and contribute to erosion and downstream flood peaks.
Soil structure, compaction, vegetation, root channels, organic matter and pore connectivity all influence infiltration.
Biochar can modify some of these physical characteristics.
Research suggests that its effect on saturated hydraulic conductivity varies with soil texture: in some sandy soils it can slow excessively rapid movement, while in finer soils it can increase conductivity and improve movement through the soil.
That is a much more sophisticated mechanism than simply saying:
“Biochar absorbs floods.”
It doesn't.
Rather, biochar can be one component of soil management aimed at improving the ability of landscapes to receive, retain and release rainfall more effectively.
From Water to Vegetation
This brings us to the wider landscape argument associated with the ideas Walter R. Jenny has promoted.
If soil loses its ability to support vegetation during prolonged dry weather, the consequences extend beyond individual plants.
Vegetation plays an active role in the exchange of energy and water between land and atmosphere.
Plants take water from soil and release water vapour through stomata in their leaves — transpiration.
Water also evaporates from soil and plant surfaces.
Together these processes are called evapotranspiration.
Why Evapotranspiration Cools Landscapes
Turning liquid water into water vapour requires energy.
This is called the latent heat of vaporisation.
When incoming solar energy is used to evaporate and transpire water, that energy is transferred as latent heat rather than simply heating the land surface as sensible heat.
This is one reason moist, vegetated landscapes can remain cooler than dry, bare surfaces.
So there is an important connection:
soil water → plant growth → evapotranspiration → surface energy balance.
If soils dry severely and vegetation becomes sparse, transpiration falls. A greater proportion of incoming energy can then go into heating the ground and the air immediately above it.
What About Reflecting Heat?
This part needs particular care.
Different surfaces have different albedo — the proportion of incoming solar radiation they reflect.
Bare soil, vegetation, water and snow all behave differently.
But it would be misleading to say simply that making land greener prevents it “reflecting heat” and therefore cools the climate.
Reflection and temperature are not the same thing. In some circumstances, a lighter bare surface may actually reflect more incoming short-wave solar radiation than darker vegetation.
The stronger scientific argument for healthy green landscapes concerns evapotranspiration, shading, soil moisture and the partitioning of incoming energy between latent and sensible heat.
So the principle behind the idea is important, but the mechanism is more interesting than the shorthand explanation.
The Soil–Water–Plant–Climate Connection
We can now see a chain of relationships:
Biochar can alter soil pore structure
↓
which can alter infiltration and water retention
↓
which can increase plant-available water in suitable soils
↓
which can help vegetation withstand dry periods
↓
which supports transpiration and biological activity
↓
which affects exchanges of water and energy between land and atmosphere.
This does not mean that adding biochar to a garden will measurably alter regional climate.
It means that at larger scales, maintaining healthy, water-retentive, vegetated soils is one component of creating landscapes resilient to climatic extremes.
Carbon Below Ground, Resilience Above It
This gives biochar two quite different relationships with climate change.
Climate mitigation
Plants remove CO₂ from the atmosphere.
Pyrolysis converts a portion of their biomass carbon into relatively persistent aromatic carbon.
Putting that biochar into soil or another durable reservoir slows the return of that carbon to atmospheric CO₂.
Climate adaptation
Biochar can modify soil physical properties including porosity, density and water retention.
In appropriate soils, this can increase plant-available water and contribute to resilience during dry periods, while changes in infiltration and hydraulic behaviour may also improve the way soils handle intense rainfall.
These are separate mechanisms, but they complement one another.
Why This Matters at Sweethill
For us, the connection between woodland, carbon and soil is tangible.
We produce our biochar from timber arising from the management of our own family-run woodland in Devon, as part of a much longer-term process of restoring and regenerating the woodland.
The tree has already done something remarkable: using sunlight, water and atmospheric CO₂, it has created wood.
Through pyrolysis, we can take a proportion of that biologically captured carbon and convert it into a much more persistent form.
And that carbon-rich material can then be used in soil, where its physical structure can influence water, nutrients and the biological environment in which plants grow.
So the story doesn't end with:
tree → carbon → biochar.
It becomes:
atmospheric CO₂ → tree → wood → biochar → soil → water → microorganisms → plants → resilient landscape.
That is what makes biochar so interesting to us.
It isn't simply a carbon-storage technology.
And it isn't simply a soil additive.
It sits at the meeting point between the carbon cycle, the water cycle and the living soil.