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Module 01 >

Monitoring Soils

Module 02 >

Interpreting Soil Results

Module 03 >

Soil Fertility and Nutrient Management

Module 04 >

Common Soil Constraints

Module 05 >

Soil Carbon Capture

Module 06 >

Digital Agriculture for Soils

Module 07 >

Using Biologicals to Build Soil Organic Matter and Resilient Soils

Module 08 >

Managing Irrigated Soils in the Riverina Region of NSW

05

Soil Carbon Capture

Module 01 >

Monitoring Soils

Module 02 >

Interpreting Soil Results

Module 03 >

Soil Fertility and Nutrient Management

Module 04 >

Common Soil Constraints

Module 05 >

Soil Carbon Capture

Module 06 >

Digital Agriculture for Soils

Module 07 >

Using Biologicals to Build Soil Organic Matter and Resilient Soils

Module 08 >

Managing Irrigated Soils in the Riverina Region of NSW

Section 5A

Introducution

Soil carbon capture, also known as soil carbon sequestration, is the process of transferring CO2 from the atmosphere into the soil in a stable form. It’s a natural part of the carbon cycle, where plants absorb atmospheric CO2 through photosynthesis and convert it into organic compounds. These compounds, along with decomposing plant material and microbes, become part of the soil organic matter.

What is soil carbon?

Soil carbon refers to both the carbon in organic matter, which plays a critical role in the chemical, physical, and biological health of soils, and the inorganic carbon that occurs as carbonate minerals (such as limestone) or charcoal in soils.

The importance of soil carbon

Soil carbon is crucial for soil health, improving productivity, profitability, and resilience against climate change. By enhancing soil carbon, we can increase soil’s ability to retain water and nutrients, thereby supporting better crop yields and biodiversity. Moreover, soil acts as a significant carbon sink, storing more carbon than the world’s biomass and atmosphere combined, which makes it a key player in mitigating climate change.

Sustainable land management practices, such as reduced tillage, cover cropping, and organic farming, can increase the soil’s organic carbon content. These practices not only help in capturing CO2 but also improve soil structure, reduce erosion, and increase farm sustainability.

Figure 1. Overview of how particulate and mineral-associated organic matter form and function.
Source: Jocelyn Lavallee, Colorado State University. CC BY-ND.

Let’s delve into more details

Soil Organic Carbon (SOC)
Soil Organic Matter (SOM)
CO2
Soil Inorganic Carbon (SIC)

Soil Organic Carbon (SOC)

Soil organic carbon is an important indicator of soil quality. SOC measurements indicate levels of soil organic matter (SOM), which is an important determinant of soil health. SOC derives from the interaction of ecosystem processes such as photosynthesis, respiration, and decomposition. It includes relatively available carbon as fresh plant remains, plus relatively inert carbon in materials derived from plant remains such as humus (rich organic material) and charcoal. Through SOM, it helps regulate nutrient supply, microbial activity, and soil moisture content.

Soil Organic Matter (SOM)

SOM influences a range of soil functions and properties

  • storage and supply major plant nutrients such as nitrogen, phosphorus, and sulfur
  • Store and release plant nutrients like calcium, magnesium and potassium reducing leaching
  • maintain soil structure by binding soil particles together into aggregates
  • increasing water infiltration and soil water holding capacity
  • improves soil bulk density and prevents soil compaction
  • provides energy for soil microorganisms
  • increases soil pH buffering capacity, resisting changes in pH preventing soil acidity, alkalinity, and soil salinity
     

Soil Inorganic Carbon (SIC)

Soil inorganic carbon consists of mineral forms of carbon originating from

  • Weathering: Carbon from the breakdown of parent material
  • Reaction with CO₂: Soil minerals react with atmospheric carbon dioxide (CO₂)

In drier climates, carbonate minerals (such as calcium carbonate) are the dominant form of soil inorganic carbon Soil inorganic carbon influences soil pH and mineral availability.

Section 5B

The Global Carbon Cycle

The amount of carbon in the world today is the same as it was millions of years ago. However, human activities such as burning fossil fuels and deforestation has affected the distribution of carbon between the atmospheric, terrestrial, and marine pools of global carbon.

Photosynthesis

Plants remove carbon from the atmosphere as carbon dioxide (CO₂) through photosynthesis and convert it into plant biomass, which then becomes soil organic carbon. Plant and microbial respiration and decomposition releases some of that carbon back to the atmosphere. However, soil carbon remains the largest terrestrial carbon sink in the global carbon cycle. Thus, soil carbon helps to regulate atmospheric carbon (CO₂) - the primary greenhouse gas and helps to mitigate global warming and climate change. Unfortunately, human activities such as intensive agriculture cause significant losses of soil organic carbon.

Did you know?

Soil carbon remains the largest terrestrial carbon sink in the global carbon cycle!

The fast carbon cycle

Fast Carbon Cycle
Figure 2. The fast carbon cycle showing the movement of carbon between land, atmosphere, and oceans in billions of tons (gigatons) per year. Yellow numbers are natural fluxes, red are human contributions, and white are stored carbon.
Source: The United States Department of Energy, Biological and Environmental Research Information System.

Remember

Healthy soil with adequate carbon content is essential for sustainable agriculture, ecosystem resilience, and climate stability.

Section 5C

Soil Carbon Capture

Soil carbon capture involves transferring carbon dioxide (CO₂) from the atmosphere into the soil in the form of organic carbon. 

What is soil carbon Capture?

Soil carbon capture involves transferring carbon dioxide (CO₂) from the atmosphere into the soil in the form of organic carbon.

How Soil Carbon Capture Begins

Soil carbon capture begins with photosynthesis, where plants convert atmospheric CO₂ into organic compounds, which are then incorporated into the soil through plant residues and root exudates. Soils are composed, in part, of broken-down plant matter, which means they inherently contain a significant amount of carbon absorbed from the atmosphere during the plants’ lifetimes. Thus, soils can store more carbon than the atmosphere and plants combined.

Section 5D

Soil Carbon Sequestration

Soils store, cycle, and emit different forms of carbon in the carbon cycling processes. 

What is Soil Carbon Sequestration?

Soil carbon sequestration is using land management practices aiming to increase the soil organic carbon content, resulting in a net removal of CO₂ from the atmosphere. It is estimated that agricultural soils have the potential to sequester over a billion additional tons of carbon annually.

Forms of Carbon

The different forms of carbon emitted may be very stable and stay in the soil for thousands of years. Especially in colder climates where decomposition is slow, soils can store—or “sequester”—this carbon for extended periods.

Without soil’s intervention, this carbon would return to the atmosphere as CO₂. Unfortunately, the conversion of natural ecosystems (such as forests and grasslands) into farmland disrupts soil structure, releasing much of the stored carbon back to the atmosphere.

Did you know?

The rate of conversion of plant biomass to SOC limits the rate at which carbon is sequestered in soils.

Section 5E

Carbon Farming

Carbon farming refers to deliberate agricultural practices that increase carbon stored in soil and vegetation while reducing greenhouse gas emissions.

What are Carbon Farming Basics?

  • Awareness: Growers should understand their production system’s carbon footprint.
  • Mitigation: Reducing emissions through efficient practices benefits both the farm and the planet.
  • Tracking: Growers can participate in carbon accounting to measure their impact.
  • Incentives: The Emissions Reduction Fund offers financial incentives for carbon sequestration efforts.

Improving SOC

Growers can improve SOC by managing organic matter, promoting microbial activity, and enhancing nutrient cycling by embracing carbon farming practices such as no-till cultivation, cover cropping, perennial crops with deep roots, and improved grazing management.

Carbon capture practices that enhance CO₂ removal from the atmosphere benefit both growers and the environment by improving soil health, productivity, and sustainability.

Remember

SOC is the indicator of soil quality.

Section
5F

Carbon Flow in the Farm

Carbon flow in a farm refers to the movement and transformation of carbon within the farming system. This process is integral to the cycle of life on a farm and has significant implications for agricultural productivity and environmental sustainability.

Let’s have a look at the carbon flow in a typical agricultural farm. 
Figure 3: Carbon Flow in the Farm. Source: Carbon Farming, Carbon Cycle Institute. CC-By-ND.
Source: Carbon Farming, Carbon Cycle Institute. CC-By-ND

Soil carbon inputs:

  • Crop/plant residues.
  • Manure and compost.
  • Root exudates.
  • Soil microbial biomass.

Soil Carbon losses:

  • Decomposition by microorganisms releasing CO2
  • Leaching
  • Erosion

What is mineralisation?

Mineralisation makes some of SOC remain as stable organic matter i.e. humus. Some of the mineralized carbon may remain stable in the soil for years or even centuries (carbon sequestration).

Section 5G

Carbon Capture and Soil Carbon Sequestration in the Farm

Ways to increase soil carbon levels in the farm vary according to land-use, soil type and climate.

Carbon enters the farming system primarily through the process of photosynthesis, where plants absorb carbon dioxide (CO2) from the atmosphere. This carbon is then stored in the soil, crop roots, wood, and leaves; a process known as carbon sequestration.

To increase SOC by 0.1% in the top 10cm of the soil requires a large amount of plant biomass, with sufficient nutrients. Farmers can enhance carbon storage in agricultural soils through adopting practices that can help boost soil carbon inputs and minimise losses.

Farming systems also lose carbon as gases, primarily in the form of CO2 and methane (from animal digestion and respiration), contributing to greenhouse gas (GHG) emissions. Therefore, the ability of soil to absorb and store carbon, known as its carbon sink capacity, is crucial for both farm productivity and climate change mitigation.

Section 5H

Monitoring Soil Carbon Levels

Effectively sequestering carbon as SOC through changes in land management practices requires a knowledge of the initial levels of SOC at a given locality and the expected levels after the changes take effect. The measurement of SOC is a critical requirement in the management of soil carbon.

Measuring soil organic carbon

The amount of organic matter in soil is difficult to measure directly. Laboratory tests measure SOC which makes up about 58% of total soil organic matter. It is important to know the purpose for which the measurements are being made.

What are the different types of soil organic matter?

  • Soil organic matter is made up of several ‘pools’ which last for different lengths of time in soil and affect different soil functions. Organic matter and residue deposited in, or on the soil, is the most active pool, but may be rapidly lost.
  • The three major SOC fractions are particulate-C, humus-C, and char-C.
  • Humus, made up of decayed organic residues from plants, soil organisms and microbes, is more resistant to breaking down than particulate or active organic matter and is a more stable slow pool.
  • Charcoal is very stable, but is not biologically active, and therefore is an inert or passive pool.

Various measurements of SOC

SCaRP) recommends measuring variations in soil organic carbon (SOC) content and composition in the 0 - 30 cm layer of soil. Most (but not all) changes in soil organic carbon occur in the top 30 cm of the soil profile.

If the measurements are being made to assess soil health or soil condition, the measurement of soil carbon percentage as grams of carbon per 100g of oven dry soil is sufficient (usually expressed as SOC % in a soil test report). The cost of this is low and could be done as a part a standard soil test.

However, if the object is to trade in carbon and receive payment for sequestering carbon it is necessary to measure SOC stocks (tonnes C/ha/depth). This is a costly as it requires the measurement of soil bulk density and soil sampling strategies that meet higher standards. This must be conducted professionally. Procedures and costs may vary from one service provider to another.

Remember

Accurate soil carbon testing is crucial for effective carbon auditing.

Rough Estimation of Soil Organic Carbon Stocks

1. Collecting Samples

  • Define the project area and identify sampling locations.
  • Ensure random sampling without bias to estimate changes in carbon stocks.
  • Seek guidance from agronomists or horticulturists for site-specific planning.
  • More on Developing a Sampling Plan.

2. Sample Preparation and Analysis

  • Dry soil samples at 60°C to remove moisture, and weigh.
  • Combustion of soil samples at around 360°C in a furnace to burn organic material.
  • Weigh the remaining soil After combustion.
  • Convert weight loss (before - after combustion) into an estimate of organic carbon content using conversion factors.

* Use a consistent soil carbon estimation technology within each estimation area and sampling round.

3.  Laboratory Analysis

Laboratory analysis may use dry combustion or chemical methods to measure soil organic carbon and account for soil volume by measuring bulk density of soil.

4. Calculation of Organic Carbon Stock

  • Calculate changes in SOC stocks over time within each carbon estimation area (CEA).
  • Use the measured data to estimate carbon sequestration or changes due to management practices.
  • Remember to use a consistent soil carbon estimation method within each estimation area and sampling round.
If local soil conditions are known, it may be possible to use SOC% to obtain an estimate of the initial soil carbon conditions and stocks to assess the feasibility of a proposed carbon sequestration project. Local knowledge can help identify what levels of SOC are achievable based on soil type and climate.

Section 5I

Developing a Carbon Farming Project

For a proposed carbon farming project to successfully sequester carbon as SOC, the initial levels of SOC must be less than those expected after the the changed land management systems and practices adopted. Therefore, the changed and management practices and operations must be backed by strong evidence for sequestering carbon as SOC.

General Requirements for a Carbon Farming Project

The general requirements for a carbon farming project are:

  • The proposed land management systems must be clearly defined, including the land management operations that are to be implemented.
  • There must be strong evidence that the land systems and land management operations will sequester carbon as SOC for the specific climate and soil types.
  • It is essential to measure the initial levels of SOC to clarify the capacity of the proposed land management systems to sequester carbon as SOC.
  • A rigorous accounting system is required to be able to demonstrate changes in SOC where carbon trading and rewards for improving SOC are expected.
  • A rigorous field sampling methodology must be employed to assess trends in SOC over time. This must adequately account for field and seasonal variability, most particularly if carbon trading and rewards for improvements in soil carbon are expected.
Figure 4: Developing a soil carbon farming project
Source:  Louisa Kiely, Carbon Farmers of Australia.

Potential land management scenarios for soil carbon capture

Soil organic carbon levels in ecosystems are controlled by a range of factors, namely climate, soil, vegetation, and time, but reaches an environmental equilibrium after a while. The change in the storage of soil organic carbon is controlled by the balance between carbon inputs and losses. Theoretically, the difference in soil organic carbon between the environmental equilibrium levels and the current depleted level is the soil organic carbon sequestration potential of that ecosystem. This is the potential carbon sink, that can be restored to the soil.

Did you know?

The change in the storage of soil organic carbon is controlled by the balance between carbon inputs and losses.
The possibility of increasing SOC levels will be better where soils currently have low SOC due to overcropping and/or overgrazing but also have a high potential to produce biomass.

Examples of such areas are:

  • Areas of higher rainfall and moderate climate favour plant growth and potentially offer more opportunities to sequester carbon by changing land management as there are larger differences between soil carbon levels due to land management.
  • In lower rainfall areas, the potential to sequester SOC is more limited, other than on degraded sites.
  • Land with better quality soils and higher land use capability provides improved chances of responding to climate potential by improving biomass and therefore increasing SOC.
  • Land which has been left bare and burnt due to land management history and erosion and is currently unproductive will respond to increased biomass if the moisture regime is improved.

What do natural limitations mean for carbon sequestration?

Land management where strong adoption of improved and appropriate land management practices consistent with sustainable agriculture has already occurred may have limited ability to sequester additional levels of SOC. In these cases, carbon sequestration has reached a threshold which is governed by natural limitations, thus not suitable for carbon farming projects.

Section 5J

Government Policies and Incentives

There are several government policies and programs aiming to incentivize carbon sequestration and reduce emissions in the agricultural sector in Australia, which recognise the critical role of carbon farming in mitigating climate change.

Government incentives

  • Carbon Farming Initiative
  • Emissions Reduction Fund
  • Climate-Smart Agriculture Program
  • Net Zero Plan
  • National Climate Policy Review

These policies and incentives aim to:

  • Support farmers and the agricultural sector to adopt climate-smart practices.
  • Encourage the reduction of emissions.
  • Building resilience against climate change.
  • Increase productivity, competitiveness, and sustainability in agricultural operations.
  • Promote sustainable natural resource management practices.

Section 5K

Recommended Practices for Soil Carbon Capture and Sequestration

Recommended Practices for Soil Carbon Capture and Sequestration

Land Clearning

1. Avoid land clearing

  • Maintain ground cover
  • Avoid burning stubble
  • Cover crops

2. Reduced soil disturbance (+ image)

  • Low-till or no-till practices
  • Avoid intensive conventional tillage
  • Avoid tillage operations with disc ploughs
  • Prevent soil erosion

3. Grazing management

  • Rotational grazing
  • Avoid over-grazing
  • Reducing grazing pressure

4. Smart cropping

  • Plant perennial crops with deep roots
  • Plant cover crops during fallow periods
  • Double cropping
  • Crop rotation
  • Mixed cropping
  • Agroforestry

5. Organic soil amendments

  • Composting
  • Manuring
  • Retain and incorporate crop residues

6. Soil health management

  • Balanced soil moisture
  • Balanced soil nutrients i.e. C:N ratio
  • Improve soil biodiversity
  • Apply microbial stimulants
  • Avoid soil compaction

7. Monitoring soil carbon levels

  • Measure baseline SOC
  • Apply recommended practices to increase SOC
  • Measure for SOC
Figure: Increasing soil carbon inputs and reducing losses on farm.
Source: The Global Soil Partnership

Case Studies

Here are some examples for successful Soil carbon capturing activities conducted in NSW (Courtesy: NSW DPI).

Water Ponding

Water ponding on scalded land in the semi-arid areas of NSW was a potentially successful project to sequester carbon as SOC. Scalded soils have very low SOC levels and revegetation by ponding rainwater significantly increased these levels. The land management system for water ponding and its specific operations have been clearly defined and well documented.

CAMBI Project

A pilot scheme was developed in Central West NSW to trial the use of a market-based instrument to encourage farmers to change farm management practices to increase SOC. The changes in SOC stocks on the farms were calculated based on the initial measured levels of SOC and the predicted levels of SOC after 5 years.

The following four land use practices were recommended for adoption.

  1. Reduced tillage cropping
  2. Reduced tillage cropping with organic amendments (e.g. biosolids or compost)
  3. Conversion from cropping land to permanent pasture
  4. Conversion from cropping land to permanent pasture with organic amendments.

At each site, a minimum of 10 locations were sampled and soil was analysed for total carbon using dry combustion method (LECO elemental analyser), and soil bulk density was measured. The SOC stocks at 0 – 30cm soil depth were assessed before and after the adoption of practice and calculated on the initial equivalent soil mass. A significant increase in SOC stocks were observed in 60% of sites. Pasture had a higher rate of SOC sequestration than reduced tillage cropping, and sites with organic amendments had higher rates of SOC sequestration than without organic amendments.

The results demonstrated increases in SOC, using quantification methods consistent with the current measurement method of the Australian Government’s Emissions Reduction Fund Policy used to generate Australian Carbon Credit Units.

What effect does high rainfall have on carbon sequestration?

In NSW agricultural lands, pastures in the higher rainfall regions (>450 mm), either as permanent pastures or as ley pasture is estimated to have a considerable increase in soil organic carbon sequestration by pasture improvement and improved management practices.

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