What is soil organic carbon?
Soil organic carbon refers to carbon within soil that originated from plants and animals.
It is one of the best indicators of soil health, and building it can enhance pasture productivity.
Our soil organic carbon levels are determined by the combined effect of climate, soil properties, plant inputs and land management.
Decades of current agricultural management have depleted our soils of organic carbon.
Restoring it can offset operational greenhouse gas emissions as well as secure the productive value of our land for years to come.
What controls soil organic carbon?
Areas with high rainfall produce more biomass and generally have higher soil organic carbon.
This is because there is more food and water available for soil microbes.
Live and dead microbe biomass makes up most of the carbon in the soil.
Soils with high natural fertility magnify the impact of high rainfall on plant production, and soils high in fine particles (clay and silt) can store more carbon within their matrix.
This is because these fine particles are more reactive, aggregate better and have a higher surface area than sand particles.
Plants draw carbon dioxide out of the atmosphere via photosynthesis.
This carbon is then stored in many forms throughout the plant.
Around 4-6% of the net drawdown of plants then becomes stored as soil carbon.
By increasing plant biomass we can increase our feed on offer and make more carbon available to the soil. Increasing soil carbon can be a win-win for productivity and sustainability.
We can’t control our climate or soil type.
But on the Northern Rivers, we have high rainfall and fertile soils throughout the region.
We are well placed to potentially store more soil carbon through good land management which promotes ground cover, increases biomass and protects from erosion.
This helps to maintain the soil ecosystem and provide the best chance for our soils to realise their full carbon potential.
Building carbon through land management
We can improve soil organic carbon by making land management decisions that increase plant inputs to the soil and protect the soil ecosystem.
- Rotational grazing: Adequate pasture rest keeps pasture species in Stage II of their growth curve and provides good quality feed and promotes below ground biomass and carbon inputs.
- Addressing soil constraints such as acidity and nutrient deficiencies (macro and micro): This increases plant growth and carbon, and pairing this with permanent improved pasture can lead to productivity gains and soil improvements.
- Using diverse species mixes, such as legumes and brassicas: This can help fill winter feed gaps and provide co-benefits to permanent pastures, such as nitrogen fixing and improved soil structure. To be successful, these need to be paired with grazing management and pasture rest.
Overall, improvements we make to our pasture productiveness and soil fertility will build our soil ecosystem.
Soil carbon across our region
Data for the region shows soil organic carbon levels ranging from 4% to greater than 10%.
Set stocked, naturalised or unimproved hillslope paddocks occupy the lower end of the scale (as pictured above left).
High input, rotational systems on vertosol creek flats occupy the higher end (above right).
Soils high in carbon are typified by a dark colour and fluffiness due to the colour and the low bulk density of organic matter.
Hillslopes that have been set stocked and overgrazed can be depleted due to the compound effects of low plant inputs and erosion.
How much carbon can we store, and can it offset on-farm emissions?
Soil carbon equilibria take about 20 years to achieve following a significant land management change.
Based on research currently being undertaken by Southern Cross University, increasing carbon from these low to high scenarios has potential to offset on-farm emissions over these transition periods.
There are uncertainties. Southern Cross University and our project partners are working to develop farm specific information to help farmers understand their ability to build soil carbon.
For more information, or to be involved in Southern Cross University research, email Dr Abe Gibson, Research Fellow in Sustainable Grazing Systems: abe.gibson@scu.edu.au