Fast facts
- Soil characteristics, climate and management practices can alter the amount of carbon in soil
- Soil carbon is a balance between inputs (such as plant shoots, roots and leaves) and outputs (such as decomposition and conversion into carbon dioxide)
- Regardless of its potential, the amount of carbon a soil can actually hold is limited by factors such as rainfall, temperature and sunlight, and can be reduced further due to factors such as low nutrient availability, weed growth and disease.
The amount of carbon in a soil is dependent on the characteristics of the soil and the balance between
inputs and losses
- How much carbon can a soil hold
- Factors which affect soil carbon losses
- Factors which affect soil carbon inputs
- Actual soil carbon levels
Many factors, such as rainfall, temperature, vegetation and soil type determine the amount of carbon in
soil.
Some of these factors are fixed characteristics of the soil, some are determined by the climate and
some can be influenced by management practices.
How much carbon can a soil hold – how big is your bucket?
The amount of carbon in a soil can be thought of as a leaking bucket that constantly needs topping up.
The size of the bucket represents the total amount of carbon the soil could potentially hold.
Factors such as clay content, soil depth and soil density will affect the size of the bucket, for example,
the bucket will be smaller for sand than clay soil. Management practices can not influence the size of
the bucket.
Factors which affect soil carbon losses – how leaky is your bucket?
Losses of carbon from soil result from decomposition and conversion of carbon in plant residues and
soil organic materials into carbon dioxide. Processes that accelerate decomposition open the losses
tap further.
Many factors, such as rainfall, temperature, vegetation and soil type determine the amount of carbon in
soil.
The rate of loss is determined by:
- type of plant and animal matter entering the soil
- climate conditions (rainfall, temperature, sunlight)
- soil clay content
Some management practices which reduce carbon inputs and/or increase the decomposition of soil
organic matter can also influence carbon losses.
These include:
- fallowing
- cultivation
- stubble burning or removal
- overgrazing
Factors which affect soil carbon inputs – how much are you re-filling your bucket?
Soil organic carbon inputs are controlled by the type and amount of plant and animal matter being
added to the soil. Any practice that enhances productivity and the return of plant residues (shoots and
roots) to the soil opens the input tap, re-filling the bucket and the amount of carbon in the soil.
The majority of carbon enters the soil as plant residues.
Fire can also contribute by converting plant dry matter into charcoal which enters the recalcitrant
fraction (see Soil carbon: the basics). However, fire itself can lead to carbon losses through release of
carbon dioxide.
Soil organic carbon inputs are controlled by the type and amount of plant and animal matter being
added to the soil. Any practice that enhances productivity and the return of plant residues (shoots and
roots) to the soil opens the input tap, re-filling the bucket and the amount of carbon in the soil.
The majority of carbon enters the soil as plant residues.
Fire can also contribute by converting plant dry matter into charcoal which enters the recalcitrant
fraction (see Soil carbon: the basics). However, fire itself can lead to carbon losses through release of
carbon dioxide.
Plant residue, and thus soil carbon inputs are mainly affected by the:
- type of plants being grown
- amount of dry matter the plants accumulate over the growing season
- environmental factors which govern plant production
A variety of management practices can increase soil carbon levels by increasing inputs.
In theory, maximising productivity also maximises returns of organic residues to the soil. Practices thatincrease productivity include:
- fertiliser application
- improved rotations
- improved cultivators
- irrigation
- crop intensification
Reduced tillage or no-tillage management practices can also increase soil carbon levels as carbon in
the crop stubble is left to return to the soil.
Soil carbon can also be topped-up by direct application of organic materials to the soil. Examples of
these materials include: manure, plant debris, composts, and biosolids, with biochar attracting interest
for its potential in this area.
Although significant changes can occur quickly when moving across extreme differences in
management practices, it is important to note that often decades of constant management are required
to define the ultimate soil organic carbon content that may be reached.
Actual soil carbon levels
Even though a soil may have the potential to store a certain amount of carbon, it is unlikely that this will
be the actual amount that is ever found in the soil.
Limiting factors (such as the availability of water) will affect the attainable amount of soil carbon. While
decreased productivity due to reducing factors (such as low nutrient availability, weed growth, disease,
or subsoil constraints) will further lower soil carbon levels.
Once all these factors have been taken into account the actual soil carbon level which could possibly
be achieved with optimal carbon inputs can be determined.