Soil organic carbon – what is it and how do we measure it?

Summary

This video script delves into the concept of total carbon, distinguishing between organic and inorganic components in soil. It emphasizes the significance of soil organic carbon (SOC), which nourishes soil biology and supports nutrient cycles, crucial for plant growth. SOC also contributes to soil structure, aeration, and erosion mitigation. The script outlines methods for measuring SOC, such as the Walkley-Black wet chemistry method and loss on ignition, and discusses the relevance of SOC in carbon trading and farming systems, suggesting it as a potential revenue stream and environmental asset.

Takeaways

• 🌿 Total carbon in soil includes both organic and inorganic carbon, with inorganic carbon primarily being carbonates.
• 🔍 To isolate organic carbon, soil samples are treated with acids to remove inorganic carbonates.
• 🌱 Soil organic matter consists of decomposed plant and animal material and living organisms in the soil.
• 🧪 Measuring soil organic matter is challenging, so soil organic carbon, which is about 40% of soil organic matter, is often measured instead.
• 🌱 Soil organic carbon is crucial as it feeds soil biology, supports nutrient cycles, and contributes to soil structure.
• 💧 Organic carbon acts as a binding agent, helping to hold soil particles together, which improves aeration and mitigates erosion.
• 🌍 Soil organic carbon is significant in the context of climate change, as increasing it in soils can aid in carbon sequestration.
• 📈 For carbon farming or credit purposes, knowing the soil organic carbon levels is essential.
• 🔬 Soil organic carbon is measured using methods like the Walkley-Black wet chemistry test or loss on ignition using a LECO machine.
• 📊 The lab results provide the concentration of carbon in the soil, which can be converted into soil organic carbon stock by considering soil bulk density.
• 💰 Monitoring soil organic carbon can benefit farming systems by improving soil health and potentially generating additional income through carbon trading.

Q & A

What is meant by 'total carbon' in the context of soil?

Total carbon refers to the sum of organic carbon and inorganic carbon found in soils. Inorganic carbon is primarily composed of carbonates, while organic carbon is derived from plant and animal residues and living organisms within the soil.

Why is it necessary to remove carbonates when measuring soil organic carbon?

Carbonates are removed because they are inorganic and not representative of the organic carbon content, which is the focus when assessing soil health and fertility.

What is soil organic matter and how is it related to soil organic carbon?

Soil organic matter consists of decomposed plant and animal material, as well as living organisms in the soil. Soil organic carbon is a component of this matter, making up approximately 40% of it, and is often used as an estimate for the total soil organic matter.

Why is soil organic carbon important for soil health?

Soil organic carbon is crucial as it feeds the soil biology that drives nutrient cycles, contributes to soil structure by acting as a binding agent for mineral particles, and helps in mitigating erosion.

How does soil organic carbon relate to climate change mitigation efforts?

Soil organic carbon is significant in climate change mitigation because it represents a carbon sink where carbon can be stored, thus reducing the amount of carbon dioxide in the atmosphere.

What are the two forms of soil organic carbon mentioned in the script?

The two forms of soil organic carbon are labile carbon, which is readily available to feed soil biology and drive nutrient cycles, and recalcitrant carbon, which is more stable and persists longer in the environment.

What is the purpose of measuring soil organic carbon in farming systems?

Measuring soil organic carbon helps farmers understand and improve soil health, structure, and fertility, which can lead to better crop yields and potentially open up opportunities for carbon trading or credits.

How is soil organic carbon typically measured in a laboratory?

Soil organic carbon is commonly measured using either the Walkley-Black wet chemistry method or through loss on ignition using a machine like a LECO, where the carbon is burned off and measured.

What is the significance of bulk density in calculating soil organic carbon stock?

Bulk density is important because it allows for the conversion of soil organic carbon concentration per unit volume to an estimate of the total carbon stock in a field, which is necessary for carbon trading and assessing soil carbon sequestration potential.

How can soil organic carbon measurements benefit a farmer?

Soil organic carbon measurements can help farmers improve soil health, structure, and fertility, which can enhance crop productivity. Additionally, it can provide opportunities for carbon credits and trading, potentially generating additional income.

Outlines

00:00

🌱 Understanding Total Carbon in Soils

This paragraph introduces the concept of total carbon in soils, which includes both organic and inorganic carbon. It explains that inorganic carbon, primarily carbonates, is often removed using acids to focus on the organic carbon, which is crucial for soil health. Organic carbon is derived from plant and animal matter and is a key component of soil organic matter. The paragraph emphasizes the importance of soil organic carbon in feeding soil biology, driving nutrient cycles, improving soil structure, and aiding in climate change mitigation. It also touches on the existence of soil organic carbon in labile and recalcitrant forms, with the latter being more stable and persistent in the environment.

Keywords

💡Total Carbon

Total carbon refers to the combined presence of organic and inorganic carbon in soils. Organic carbon is derived from the remains of plants and animals, while inorganic carbon is primarily composed of carbonates. In the context of the video, it's emphasized that while inorganic carbon can be significant, the focus is on organic carbon due to its role in soil health and fertility. The script mentions the removal of carbonates using acids to isolate organic carbon for measurement.

💡Soil Organic Matter (SOM)

Soil Organic Matter (SOM) is the living and non-living organic material in the soil, including plant and animal residues, which are in the process of decomposition, as well as living organisms such as microbes. It plays a crucial role in soil fertility and structure. The video points out that measuring SOM directly is challenging, so soil organic carbon (SOC) is often used as a proxy.

💡Soil Organic Carbon (SOC)

Soil Organic Carbon (SOC) is a component of SOM and represents the carbon content in the organic matter of the soil. It is approximately 40% of SOM, as mentioned in the script. SOC is vital for soil health because it feeds the soil biology that drives nutrient cycles, which are essential for plant growth. The video also highlights the importance of SOC in climate change mitigation strategies, as increasing SOC can sequester carbon from the atmosphere.

💡Labile Carbon

Labile carbon is a form of organic carbon that is readily available to soil organisms and can be quickly decomposed and utilized. It is an important component of SOC because it directly feeds the soil biology, contributing to nutrient cycling and soil fertility. The script discusses labile carbon as the form of carbon that actively supports the biological processes in the soil.

💡Recalcitrant Carbon

Recalcitrant carbon is a more stable form of organic carbon that is resistant to decomposition. It is often associated with humic substances or minerally occluded carbon. This form of carbon persists in the environment for longer periods, contributing to the long-term storage of carbon in soils. The video describes recalcitrant carbon as a stable form that is less likely to be quickly utilized by soil organisms.

💡Soil Structure

Soil structure refers to the arrangement of soil particles, including the spaces between them, which influences the soil's physical properties such as aeration, water infiltration, and resistance to erosion. In the video, it is mentioned that SOC acts as a 'glue', helping to bind soil particles together and providing structure, which is beneficial for aeration and mitigating erosion.

💡Carbon Sequestration

Carbon sequestration is the process of capturing and storing carbon dioxide from the atmosphere in carbon sinks, such as forests, oceans, and soils. The video discusses the importance of increasing SOC as a means of sequestering carbon, which can help in the fight against climate change by reducing the amount of CO2 in the atmosphere.

💡Walkley-Black Method

The Walkley-Black method is a wet chemistry technique used to measure SOC in soil samples. It involves the use of acidic potassium dichromate to oxidize the organic matter, and the amount of carbon is determined by titration. The video mentions this method as one of the two primary ways to measure SOC in a laboratory setting.

💡Loss on Ignition (LOI)

Loss on Ignition (LOI) is another method for measuring SOC, where soil samples are heated to high temperatures, causing the organic matter to combust and the inorganic residue to remain. The weight difference before and after heating indicates the amount of carbon present. The video describes LOI as a method often performed using a machine like the LECO, which measures the carbon content by burning it out of the soil.

💡Bulk Density

Bulk density is the mass of dry soil per unit volume, including both the solid particles and the pores between them. It is an important factor in converting the concentration of SOC in a soil sample to an estimate of the total SOC in a field setting. The video explains that by multiplying the SOC concentration by the bulk density, one can estimate the soil organic carbon stock, which is crucial for carbon trading and management.

💡Carbon Credits

Carbon credits are a form of tradeable certificate that represent the right to emit a certain amount of carbon dioxide or the equivalent amount of another greenhouse gas. They are part of a broader carbon trading market aimed at reducing greenhouse gas emissions. The video suggests that knowing the level of SOC in one's soil is essential for those interested in farming carbon or seeking carbon credits.

Highlights

Total carbon includes both organic and inorganic carbon found in soils.

Inorganic carbon primarily consists of carbonates.

Organic carbon is derived from plant and animal residues and current soil life.

Soil organic matter is challenging to measure directly.

Soil organic carbon is used as an estimate for soil organic matter, representing about 40% of it.

Soil organic carbon is crucial for feeding soil biology and driving nutrient cycles.

It acts as a glue, providing soil structure and aiding in aeration and erosion mitigation.

Soil organic carbon is significant in the context of climate change and carbon sequestration.

Soil organic carbon exists in labile and recalcitrant forms, with different stability and persistence.

Measuring soil organic carbon is essential for farming systems and potential carbon credits.

Soil organic carbon is measured using wet chemistry methods like Walkley-Black or loss on ignition.

LECO machines are used to measure carbon through combustion in soil samples.

Lab analysis provides the concentration of carbon in soil samples.

Bulk density of soil is necessary to calculate the soil organic carbon stock.

Soil organic carbon stock is vital for carbon trading and assessing carbon sequestration potential.

Monitoring soil organic carbon can improve soil health and potentially generate additional income.

Transcripts

Hi there! Some of you have been asking questions  about carbon, so we wanted to give you a  

brief video to basically talk through  some of these things about carbon.  

One of your first questions is ‘What is  total carbon?’ Well total carbon is the  

organic carbon and the inorganic carbon that  we find in our soils. The inorganic carbon  

is basically the carbonates. And when we look  at a soil, and it maybe has high carbonates,  

we try to remove those using acids because  what we’re interested in is the organic stuff.  

Now this exists as our soil organic matter  and what that is, is basically former bits  

of plants animals, all that dead stuff that’s  turning over in our soils and also the life  

that’s in our soils at the moment. But it’s really  hard to measure soil organic matter, so what we  

usually do is measure soil organic carbon. Now  soil organic carbon is only about 40% of the  

soil organic matter, so we use an estimation  or approximation of our soil organic matter  

based on the soil organic carbon measurements. Soil organic carbon is important because it’s the  

food that feeds the biology that drives the  nutrient cycles which we rely on to grow our  

plants. It also acts as a type of glue, helping  to stick the mineral components of our soils  

together, and in doing so it gives us structure  and that structure allows better aeration and also  

can help mitigate some of the effects of erosion  events. And I guess finally, soil organic carbon  

becomes important because it’s something we hear  a lot about in relation to our fight or struggle  

against climate change. Our ability to put more  carbon into our soils is often something we’re  

encouraged to do. And this is why we might want to  look at our soil organic carbon, but in our soils,  

that soil organic carbon can exist in two other  forms: the labile form, which is what’s feeding  

that biology and driving those nutrient cycles  and a recalcitrant form (which is nearer to the  

inorganic often, but is more like humic substances  or what we call minerally occluded carbon) and  

that’s the type of carbon that’s more stable and  likely to persist in our environment for longer.  

So our soil organic carbon is really important  to help us with a number of things we rely on  

in our farming system. That said, suppose  you did want to try and farm carbon or  

look for carbon credits – yes you need to  know what your soil organic carbon is.  

So when we send our soils off to the lab to  measure soil organic carbon, it’s generally  

measured in one of two main ways. The first way  is using Walkley-Black, which is a wet chemistry  

measure. The second is using loss on ignition, and  that’s often done on a machine like this behind  

me – a LECO where the carbon is basically burned  out of the soil and it’s measured. So this is how  

it’s done and these are the results you get back  from the lab analysis. Now what tells you is how  

much carbon you have in a gram or in a kilogram  of soil – if you like it’s giving you a percentage  

of carbon in your sample. And that’s fine, but as  many of us will know, when we actually think about  

a field, the density of our soils in those fields  is much higher than a gram per centimetre cubed  

or a tonne per metre cubed -it’s bigger than that.  So what we need to know is the bulk density of our  

soils. We can then multiply that concentration  of carbon in our soils by the bulk density in  

our soils and that gives us our soil organic  carbon stock. And that’s what we’re interested  

in when we’re looking at carbon trading. So there you go – if you’re interested in carbon  

trading or not, soil organic carbon is definitely  something you want to have in your system; it’s  

something you maybe want to measure and monitor in  your farming systems, because just having it will  

help with your soil biology, it’ll help with your  soil structure, and you never know – it just might  

make you a few extra pennies on the side. That’s soil organic carbon!