Homegrown soil carbon – soil carbon basics
Industry Best PracticeUnderstanding soil carbon is fundamental to improving orchard soil health.
Key definitions
- Soil organic matter (SOM) is any material in soil produced originally by living organisms, plants, animals and microbes. SOM is approximately 58 per cent carbon, the rest being minerals such as nitrogen, phosphorus, potassium, sulphur, calcium, magnesium and trace elements.
- Soil organic carbon (SOC) is the carbon contained in soil organic matter.
- Soil carbon sequestration is the process of transferring carbon dioxide (CO2) from the atmosphere and storing it in the soil as SOC.
- Soil microbes are mainly bacteria and fungi.
Growing your own cover crops to increase soil carbon sounds wholesome, but what does it really mean, do I really need to do this, what are the best sources of carbon, and how much gain is practically achievable?
Why care about soil carbon?
On a global scale, soil contains the largest terrestrial reservoir of carbon on the planet, holding three times the amount stored in plants and twice as much as the atmosphere. This is why it has become central to carbon sequestration policies throughout the world.
On a farm scale, soil’s role as a carbon reservoir is no different. But we also value carbon in an orchard scenario because it supports a sustainable high-functioning soil that supports a high-performing orchard.
Soil carbon or soil organic carbon (SOC) contributes to the key biological, physical and chemical functions in soil.
- Biological functions: provides the energy for biological processes, provides food for microbes, stores nutrients, improves soil resilience.
- Physical functions: improves structural stability and water retention, buffers temperature, resists erosion and compaction.
- Chemical functions: improves cation exchange capacity, stores nutrients, buffers soil acidity, binds pollutants and heavy metals.
The first step to growing your own soil carbon levels, known as soil carbon sequestration, is to understand the processes at play below and above ground that lead to carbon remaining (captured) in the soil. This becomes important when asking the question ‘Is it feasible to store enough soil carbon to make carbon markets economically attractive?’.
Storing carbon – soil carbon sequestration
In selling the soil carbon sequestration story, the soil carbon cycle has been simplified to basic carbon capture cycles (Figure 1). The soil carbon sequestration process is now viewed as a more complex model driven by the microbial ecosystem. Nonetheless, the two fundamentals of all carbon capture models remain the same:
- The sun is the primary energy source.
- Plants photosynthesise and store carbon in roots, stems, shoots and leaves.
Figure 1: A ‘simple’ carbon cycle. Source: Climate Central
Keeping more carbon in the soil for longer
For soil carbon sequestration, we are chasing soil carbon persistence.
In older soil carbon models, soil carbon sequestration was thought of as a process where microbes break down large compounds and leaf litter to smaller compounds that are labile (short lived) or recalcitrant (resilient to breakdown over time). It was also believed that the size and structure of these compounds controlled their decomposition rate, with large complex compounds persisting longer than smaller compounds.
This understanding is changing because modern technology allows us to track plant organic compounds more accurately. Large compounds such as plant lignin and lipids (fats) were previously thought to be highly resistant to decomposition, or recalcitrant. There is now evidence that they can be labile, turning over rapidly in the right environment, while sugars and other small molecules can be recalcitrant, persisting for decades rather than weeks.
Current models now emphasise the role of microorganisms and microbial-derived compounds in the soil carbon cycle. A useful way to visualise carbon cycling, and how we can increase carbon in our soils, is a constant spinning wheel of soil carbon (Figure 2a) rather than pools of carbon breaking down to ‘end products’ over time.
Figure 2a: Carbon cycle.
Source: Bot A and Benites J (2005) The Importance of Soil Organic Matter, Food and Agriculture Organization of the United Nations
Figure 2b: New spinning wheel soil carbon cycle.
Source: Miltner A, Bombach P, Schmidt-Brücken B et al (2012) “SOM [soil organic matter] genesis: microbial biomass as a significant source”, Biogeochemistry, 111:41–55
This understanding helps us manage soil ecosystems to retain more carbon by considering the living microbes and their habitat. In orcharding we consider the specific regional growing conditions (climate, weather events, humidity) and the individual orchard system (irrigation schedule, tree line and inter-row management) in deciding how we can improve the microbial habitat to incorporate and retain more carbon.
Key point: Soil carbon sequestration is the outcome of a microbial ecosystem rather than the result of a decay process of different sized carbon compounds.
Feeding the beast, speeding up sequestration
It is often overlooked that carbon dioxide (CO2) is continually being lost from the soil. Think of the soil as home to a hungry living organism of mainly fungi and bacteria: ‘the beast’. If no new residues are added to soil, microbes continue to consume stored carbon and release CO2, depleting carbon stores. You need to continually ‘feed the beast’ to make this ecosystem wheel spin. This delivers more soil function benefits and allows more carbon to be stored (captured). A fast-spinning wheel releases more CO2 from the soil, but produces a net gain of soil carbon, speeding the soil carbon sequestration process.
Key point: As we increase carbon inputs, we feed the beast and the wheel spins faster. The faster it spins the more soil functional benefits flow to sustain your orchard and the more carbon is stored.
How to increase soil carbon storage – ‘crash mow’
Most soil carbon storage studies focus on pasture and grazing systems. We can adapt these principles to the ‘pasture’ growing within the orchard. Luxuriant, crash grazed perennial pastures are big contributors of carbon. Obviously the more you grow, the more carbon is produced. Nitrogen and other nutrients are essential to this, with carbon and nitrogen being intimately linked. Typically, one tonne of soil carbon locks up 80kg of nitrogen, 20kg of phosphorus and 14kg of sulphur for future access by the orchard trees. The value of these nutrients to the farm budget and the long-term production and sustainability benefits to the orchard system most likely outweigh potential income from carbon market schemes. The PIPS 4 Profit Building sustainable soils (AP22003) project will investigate this.
Key point: The cost:benefit of different orchard management systems will be a key analysis of the PIPS 4 Profit trials in Tasmania.
Crash grazing, or ‘crash mowing’ in orcharding, may be more effective than keeping a neat and trim orchard inter row. Allowing the pasture or inter row to grow tall and lush stimulates perennial root systems to grow deeper. Crash mowing then causes many of the roots to die off, then regrow. This provides food and carbon to the microbial ecosystem and, importantly, is a source of carbon that is retained more efficiently in soil than above-ground inputs.
Key point: The Building sustainable soils project will establish a range of mowing treatments to mimic crash grazing and quantify the benefits to soil carbon levels and orchard production. Potentially less time and money spent on mowing!
Converting plant carbon to soil carbon efficiently
The process of converting plant carbon to soil carbon is horribly inefficient. Often, less than 10 per cent of residue becomes soil carbon, with the rest being lost to the atmosphere as CO2. Residues that are more composted and have a lower carbon to nitrogen ratio (< 20:1) are converted to soil carbon more efficiently. Figure 3 shows the different C:N ratio of a range of carbon inputs.
Indicative carbon to nitrogen (C:N) ratio of various organic residues
| Poultry manure | 5:1 |
| Humus | 10:1 |
| Cow manure | 17:1 |
| Legume hay | 17:1 |
| Green compost | 17:1 |
| Lucerne | 18:1 |
| Field pea | 19:1 |
| Lupins | 22:1 |
| Grass clippings | 15–25:1 |
| Medic | 30:1 |
| Oat hay | 30:1 |
| Faba bean | 40:1 |
| Canola | 51:1 |
| Wheat stubble | 80–120:1 |
| Newspaper | 170–800:1 |
| Sawdust | 200–700:1 |
Figure 3: Carbon to nitrogen ratio of common products used as mulch and in compost. Source: Grains Research and Development Corporation (2013) Managing Soil Organic Matter, A Practical Guide.
Key point: In the PIPS 4 Profit trial at Rookwood, locally available composts and mulches with different carbon to nitrogen ratios have been applied to tree lines. We will assess how these impact soil carbon levels over time and, more importantly, how they impact soil function as a flow-on effect of ‘feeding the beast’.
Preventing carbon loss
Soil carbon sequestration is about achieving a net gain in soil carbon. Preventing carbon loss is just as important as carbon inputs for soil carbon sequestration. In an orchard system, carbon is removed as fruit and prunings, and in orchard renovation or replacement. Although cultivation is rare in orchards, the potential for soil and carbon loss through erosion is possible. Bare soil is the enemy of carbon storage.
How far can we go? Constraints to carbon sequestration
There is a finite limit to how much carbon can be stored in soil. Soil type, texture and environment (climate or weather events such as drought and flooding) dictate the amount of carbon that can be stored in the soil. Clay soils in cold, wet environments have the greatest capacity to retain carbon, while plants grow and store carbon more luxuriantly in warm, tropical environments. While carbon concentrations in deeper subsoils are low, they contribute more than half of the global soil carbon stocks.
Most models tend to overestimate soil carbon sequestration potential because they simplify the influencing processes, including underestimation of carbon loss pathways. Unfortunately, most models also only account for soil carbon in the top 30cm of the soil profile. This means that carbon stored by apple and pear trees in deep roots is often unaccounted for.
The greatest potential to improve soil carbon levels and benefit from currently available carbon market schemes is to start from a very low base with a poor or degraded soil. Most apple orchards do not fit this picture!
The next article in this series will discuss the pros and cons of carbon markets for apple and pear production and where to access current resources on carbon accounting.
Acknowledgement
The PIPS 4 Profit Program’s Building sustainable soils (AP22003) project has been funded by Hort Innovation, using the apple and pear research and development levies, contributions from the Australian Government and co-investment from the Tasmanian Institute of Agriculture. It is supported regionally by Pomewest, Lenswood Apples and the NSW Department of Primary Industries. Hort Innovation is the grower-owned, not-for-profit research and development corporation for Australian horticulture.
PIPS 4 Profit Program
This article was first published in the Autumn 2024 edition of AFG.
