Soils and carbon for reduced emissions

Speaker 1:

Many people ask, "So, what's the story with soil carbon?" We know it's important, and for a farmer, it can boost soil health, fertility, water holding capacity, and soil structure. In fact, you can think of it as a foundation block for productive agriculture.

While oceans are the world's largest carbon sink, our soils contain more stored carbon than is found in all the vegetation, and atmosphere combined. Every additional ton of soil carbon that we create, can remove the equivalent of 3.67 tonnes of CO2 from the atmosphere. Since CO2 is the most significant greenhouse gas, this is important, but, in reverse, every tonne of soil carbon lost from our soils will emit the equivalent of 3.67 tonnes of CO2 back into our atmosphere. It's important that we look after our soil carbon, and better understand it.

Let's look at it more closely. Soil carbon comes in both organic, and inorganic forms. Here, our focus will be on soil organic carbon, which typically makes up 58% of the total soil organic matter content. It's this component that we can most readily influence. Stored soil carbon is a bit like inheriting a bank account. With the size of your soil carbon bank balance being mainly driven by natural, primary productivity. With the two most influential factors being climate, and soil type and depth.

Climate is a key driver. The largest soil carbon stores occur where there is high rainfall, and cooler temperatures. Think about peat bogs. Similarly, the lowest soil carbons stores occur in low rainfall, hot areas, more like a [inaudible 00:02:02] desert. Soil type and depth is also important, where clay based soils hold more soil carbon, than sand. Soil carbon levels can vary from rich peat soils, with greater than 10% soil carbon, right down to highly cultivated and sandy soils, which can have as little as half a percent.

Although land management tends to be the minor player, it can influence carbon content over time. Our soil carbon wealth account is made up of three fractions. The labile, or particulate fraction. The humus fraction, and the resistant carbon fraction. The labile, or particulate fraction operates like an access account. It's readily available for use by soil microbes, which makes it the least stable, shortest lived, and the easiest to lose.

Humus is a bit slower, and more like a long term deposit, or investment in real estate that adds to your wealth, being stable over years, and decades. Resistant carbon is the equivalent of being locked away in a vault, for your great grandkids. It's very stable, and can last for hundreds of years.

A balanced soil carbon account generally requires a regular supply of plant residues, and organic matter, providing regular deposits into the account. Meanwhile, soil microbes eat away at the organic matter, using some nutrients for themselves, and releasing remaining nutrients for plants to access. It's a bit like having regular expenses taken from your account. That's why we have to keep making regular deposits, so that we can maintain a soil carbon account balance.

If your deposits match your withdrawals, then you'll have a stable soil carbon account. Just like a bank, if your deposits are greater than your withdrawals, your account will grow. Unfortunately, since the introduction of agriculture in Australia, it's been more common to be losing carbon from soils, rather than increasing them, with soil carbon levels often declining from natural levels. Across the Australian wheat belt, it has been estimated that over 60% of soil carbon has been lost from the top ten centimeters of soil. This is largely because little carbon is produced during the fallow period. Compared to what could be achieved by either permanent pastures, or native vegetation, which can accumulate some carbon input most of the year, after each rainfall event.

Land management practices, such as cultivation, stubble burning, annual cropping, overgrazing, and erosion, are all activities which tend to cause soil carbon loss. It's a bit like a banking situation, where our deposits are less than our withdrawals. Our balance shows a loss over time. Thankfully, many farmers are keen to try and turn this around. Using perennial pastures, cover crops, and other ways of increasing additions of plant biomass, and organic matter, which can help to maintain, or sometimes even increase soil carbon levels.

However, farmers need to be aware that this will not always increase the soil carbon level. When we add more inputs to our account, the soil microbes sometimes just increase their activity. Resulting in more carbon turnover, but, not necessarily adding more stored carbon in our bank. It's a bit like expenses, where we can earn more money, but, just end up spending more. That's why it's important that we measure your bank balance over the longer term, to see if your soil carbon wealth account is growing or reducing.

Earlier, we talked about the fact that soil microbes use nutrients, and release some to the soil as they eat away at soil carbon. This can result in crop and plant benefits, via mineralization. When nutrients in the soil organic matter, and carbon bank are released. Overall, a balanced farming system ensures we put back what we take out. This means that while it's a great idea to try and grow your soil carbon, it's important to remember, that when you store carbon, it also locks away other nutrients with it. This might require the addition of extra nutrients.

Also remember that any land use or management changes, which increase soil carbon, will need to be maintained indefinitely, if you want to keep that higher carbon bank balance over the longer term. There you have it. There's been a lot of great research on soil carbon across Australia, and how to look after it. To find out more, just head to these sites. This cash, can I keep this?