Soil carbon sequestration

Soil carbon sequestration, the process of capturing and storing carbon in the soil, has gained attention as a potential solution to mitigate climate change. However, achieving significant and consistent increases in soil carbon levels is not without its challenges. There are a number of strategies farmers can use to build soil organic carbon within their farming systems.

Understanding natural fluctuations in soil carbon

One of the main difficulties in soil carbon sequestration lies in the complexity of soil systems. Numerous factors influence the accumulation and loss of soil carbon, including climate, land management practices, vegetation cover, and soil properties. Some practices such as liming can be detrimental in the short term and helpful in the long term. While some practices, such as reducing tillage, planting cover or perennial crops, adding organic amendments, can enhance soil carbon sequestration, the effectiveness of these techniques may vary depending on soil type, seasonal conditions, climate, and other site-specific conditions. Finding the right combination of practices that maximize carbon sequestration potential can be a daunting task.

It is important to understand that soil carbon levels naturally fluctuate over time. Factors such as changes in land use, vegetation growth cycles, and climatic variations can cause shifts in soil carbon stocks. For instance, when land is converted from natural ecosystems to agricultural or urban areas, there is often a decline in soil carbon levels due to disturbance and loss of organic matter. Conversely, reforestation or restoration efforts can lead to carbon accumulation in soils.

While soil carbon sequestration holds promise for climate change mitigation, its implementation faces challenges. The complexities of soil systems and the need for site-specific approaches make it difficult to achieve consistent increases in soil carbon levels. Additionally, natural fluctuations in soil carbon over the years remind us of the dynamic nature of these processes. To effectively harness soil's carbon sequestration potential, it is crucial to address these challenges and develop adaptive strategies for sustainable land management.

The carbon cycle

The carbon cycle put simply is this: plants photosynthesise energy from the sun to take CO2 from the air, split it into carbon and oxygen, release the oxygen back into the air and use carbon to grow.

Humans and animals eat the plant (or animal in the food chain that originally ate the plant), metabolise the carbon, use it for their own growth and repair, and breathe out the CO2 or excrete the rest.

Carbon is continually cycled (and recycled) in many ways. It can be present as a gas, liquid or solid - and it moves easily and constantly between each of these forms as it is cycled by plants and animals through respiration, rumination, reproduction and all other processes of life.

Carbon sequestration

Carbon sequestration is defined as the process of the removal and storage of carbon from the atmosphere (as CO2) in sinks such as vegetation (including grasses, forbs and trees) or soils through physical or biological processes such as photosynthesis.

Forests and trees are often considered as key contributors to carbon sequestration however, grasslands and soils are also important carbon sinks

Our soil has a large capacity to absorb (or sequester) carbon. Sequestration of carbon in agricultural soils, through appropriate management actions, has been recognised as an important tool to mitigate climate change. Our soil is full of microorganisms and when plants take carbon from the air they use it in a symbiotic relationship to feed soil microbes in exchange for nutrients and other ecological functions.

Plants do this by releasing exudates via their roots in the form of many different carbon compounds - exuding liquid carbon.

These exudates can drive numerous interactions within the soil, including reproduction and life-cycles of many different soil microbes. Organic matter from roots, plant biomass and grazing animals all add to the carbon (as well as other nutrients). The microbes then convert this into complex and stable carbon compounds called humus.

By increasing biological activity, soil health and plant production is enhanced, increasing the capacity to store a greater amount of carbon in plant biomass and more importantly the soil.

Carbon farming is one activity that has developed rapidly in recent years and, along with other environmental markets, is expected to continue evolving into the future.

Sources of soil carbon

Organic matter is a diverse group of organic materials of differing composition and at different stages of decomposition. It comprises of partially decomposed organic residues, microscopic organisms, well - decomposed humus, and burnt residues such as charcoal. The transformation of organic residues into humus by soil organisms requires nitrogen, phosphorus and sulphur (and other elements in smaller quantities). These elements are constituents of organic matter and must be present in organic residues or added to the soil for humus to form. Note that by definition, all organic compounds contain carbon.

Root exudates are the most important source of carbon. These are substances released from the plant root directly into the soil system. Roots exude a vast range of organic compounds primarily sugars, amino acids and organic acids in addition to proteins hormones and enzymes. Carbon released from plant roots in this form is essential to stimulate biological activity and nutrient cycling. Roots also secrete mucilage, polysaccharide compounds which enhance soil structure.
Plant material as both roots and shoots contribute the largest amount of carbon entering the soil. Root cells which are sloughed off as well as senesced (dead) roots provide a source of energy for soil biota. Above ground plant parts, depending on environmental and management factors, generally contribute a relatively small proportion of the total soil carbon pool. On average leaf material contains 42% carbon and roots around 58%. It’s not until it’s at least partly decomposed that plant material is considered at contributing to the soil carbon pool.
Microbial biomass is potentially a significant contributor to soil carbon depending on the biological health and activity in soil. It has been reported to contribute up to 5% of total soil organic carbon under conventional management. Microbial biomass carbon will be influenced by environmental factors, land use, management and a range of soil characteristics.
Humus is the stable end product of decomposition of organic matter (plant and animal) by soil microbes. Humus is the building block of soil and building new soil, by storing carbon in this stable form, is called carbon sequestration. Under healthy aerobic conditions, up to 40% of the carbon captured from the air by plants can be sequestered in the soil in this way.

image showing dirt and grass growing with words to show what happens above and below the surface

Figure 1: Sources of soil carbon

Increasing soil carbon

Soil carbon increases through increased biomass production and retention and application of carbon rich amendments. The main losses of carbon from the soil are through organic matter decomposition by microorganisms, soil erosion, biomass burning, and product removal in food and fibre.

The process of building soil carbon starts with the process of photosynthesis. Essentially the conversion of sunlight energy, water and CO2 into a range of plant materials which are ultimately delivered to the soil. The resulting carbon compounds contained in plant shoot material which are not harvested mechanically or by livestock are returned to the soil as leaf litter.

The greatest source of soil carbon though is from the products of photosynthesis that are exported to roots and released into the soil. Around half of the carbon fixed by photosynthesis in shoots is exported to roots.

Ten ways to build soil carbon

	Adding carbon to the system ↓	Reducing carbon losses from the system ↓	Protecting carbon in the system ↓
1. Pasture management Optimise pasture growth through species selection and input management	More biomass production	Good ground cover / less soil erosion	Good ground cover / less soil erosion Deep roots adding carbon at depth Improved soil structure Nitrogen available for microbial population (from legumes)
2. Grazing management Optimise the intensity and timing of grazing (and rest)	More biomass production	Good ground cover / less soil erosion	Deep roots adding carbon at depth Improved soil structure Diverse microbial population
3. Cover crops Grow crops (incl. green manure and inter-row crops) to keep the soil covered in between main crops	More biomass production	Less soil disturbance	Nitrogen available for microbial population (from legumes)
4. Pasture cropping Sowing winter cereals into perennial pastures	More biomass production	Less soil disturbance	Improved soil structure
5. Changing crop-pasture sequence Increase the frequency or duration of pastures in a cropping rotation	More biomass production, especially in roots		Improved soil structure Nitrogen available for microbial population (from legumes)
6. Adding lime, gypsum, nutrients Optimise plant growth by managing chemical and physical soil constraints	More biomass production		Nutrients available for microbial population
7. Adding carbon-rich materials Compost, manure, biosolids	Addition of organic matter More biomass production		Addition of microbes
8. Minimising or strategic tillage Eliminate or reduce mechanical cultivation of the soil		Less soil disturbance	Improved soil structure
9. Stubble retention Retain crop residues on the soil surface	Remove less biomass from system	Surface protected by residues from erosion
10. Restoring degraded sites Changing land use to repair land degradation	More biomass production	Good ground cover / less soil erosion Less soil disturbance	Improved soil structure

For more information, download the factsheet Ten Ways to Build Soil Carbon PDF, 3167.95 KB

Increasing soil carbon in grasslands

A feature of perennial grasses is that the dry weight of above ground leaf material is approximately equal to the biomass of the dry weight of roots. On average leaf material contains 42% carbon and plant roots contain 58% carbon so the total plant biomass will be 50% carbon.

Since plant roots and below ground processes are the largest contributors to the soil organic carbon pool management to enhance soil conditions will result in increases to soil carbon and organic matter.

Increasing soil carbon in grasslands starts with photosynthesis. The conversion of sunlight energy into plant material. The rate of photosynthesis can be increased in the following ways:

Increased plant density. The area of photosynthetic material i.e. green leaf, may be increased by increasing the density of plants per unit area of the soil surface
Increasing plant species diversity. A higher diversity of plant species with different growth cycles will ensure an increased presence of green, actively growing plants for a greater period throughout the year
Maintain optimal residual herbage mass. A minimum of 5 cm of green leaf should be retained on individual plants to increase potential growth post grazing and minimise any potential reduction of the root system
Improved plant growth and vigour. Healthy plants will have larger root systems and produce more growth and leaf area for a longer period throughout the year, capturing more sunlight energy
Increase water use efficiency. Rainfall is critical for optimal plant growth and more open porous soil will hold more water for plant uptake enhancing plant growth.

For more information download the factsheet Soil Carbon and Organic Matter PDF, 786.28 KB