The team at sustainably FED think soil carbon sequestration is critical. We like it a lot.
It might solve many problems from those we have now, like finding ways to pull carbon dioxide from the atmosphere to those we will see from the growing demand for food. The biggest one is declining soil health, already here and about to become acute. And whilst we are not advocates of the silver bullet approach to problem-solving, there are benefits to soil carbon sequestration that are hard to resist.
Previously we wrote about hedging on the future, the choice made by the Australian government to finally go with CSIRO predictions of how beneficial soil carbon sequestration would be to hedge future GHG emissions.
Here we expand on why we like soil carbon so much.
Soil carbon sequestration is crucial to soil health.
That’s it, soil health.
The climate benefits are good because sequestration draws carbon from the atmosphere and puts it into storage. It’s not an avoided emission option but a genuine greenhouse gas removal, which is crucial to the success of any climate action.
Only here is the thing.
Climate change is an existential threat to humanity because it will disrupt the food supply, make some places uninhabitable, and put millions of people at risk of weather-based catastrophe. But it is not the only existential threat and not even the most acute.
What happens if there is not enough food? All the climate bets are off as people scramble to feed themselves. Secure food supplies are essential to human survival.
Soil scientist Dr Rattan Lal won the 2020 World Food Prize for developing and mainstreaming a soil-centric approach to increasing food production that restores and conserves natural resources and mitigates climate change.
Like many other ecologists, Rattan Lal suggests that soil organic carbon level is a crucial determinant of ecosystem services. Attention to soil carbon benefits food production in the millions of tons for many crop types.
Increasing the soil organic carbon pool within the root zone by 1 tonne of carbon per hectare per year or by 0.07 g/100g/yr in the surface 10 cm, has been estimated to increase food production in developing countries by 30 to 50 Mt per year; this includes 24 to 40 Mt per year of cereals and legumes, and 6 to 10 Mt per year of roots and tubersLal, R., Follett, R. F., Stewart, B. A., & Kimble, J. M. (2007). Soil carbon sequestration to mitigate climate change and advance food security. Soil science, 172(12), 943-956.
So why does increasing soil carbon by 1 t/ha in production lands produce millions of tons of extra food?
Because biology is the engine for nutrient transfers to plants in soil, and carbon is the fuel and lubricant for the engine in its myriad forms.
Soil carbon helps plants grow.
What does carbon do in soil?
Carbon is the raw material for life on Earth and the food source for all soil organisms, from microbes to moles.
As Mathew Evans in his fascinating and accessible book ‘Soil’ says: ‘There are more living things in a teaspoon of healthy soil than there are humans on Earth’. Fule these organisms with carbon and biological activity increases from a more significant number and diversity of soil organisms. This biological ‘more making’ becomes self-perpetuating as the organisms mobilise nutrients into plants.
As organic matter, carbon also holds onto moisture, the other essential for life. So organic-rich soil tends to dry out more slowly and maintain biological activity for longer.
How this all comes together depends on the soil type, especially the texture and amount of clay minerals, land use, and local land management history.
Most of the time, soil carbon is present as SOM or soil organic matter—the organic matter component of soil, that consists of plant and animal detritus at various stages of decomposition, cells and tissues of soil microbes, and substances that soil microbes synthesise.
SOM is critical to what soil contributes to humanity because of several key features like these…
- Nutrient source
soil organic matter is a significant source of nutrients in soils and a vital part of the cycling of nutrients in soils.
- Cation exchange capacity
organic matter provides charged sites that hold exchangeable cations. This becomes especially important for soils with lower clay or more highly weathered clays like kaolinites.
- Buffering capacity
soil organic matter can help buffer soils against acidification from agricultural land use.
soil organic matter can bind heavy metals and pollutants, preventing them from being leached into the groundwater and into streams. The effects are complex, but the capacity of organic matter to bind at least some pollutants is known.
- Improved structure and aggregate stability
organic matter favours the formation and stabilization of soil aggregates, which is important for aeration, infiltration, the friability of soils for tillage and resistance to compaction. Well-aggregated soils are less susceptible to crusting, surface sealing and compaction.
- Water holding and retention
extra soil organic matter can increase soil water holding capacity.
- Soil conservation
increased aggregation promoted by organic matter can reduce soil susceptibility to wind and water erosion.
Exposed soil on poorly managed grazing land in NSW, Australia. Photo by Alloporus.
Soil carbon can be managed
In degraded soil or when soil organic carbon levels are low (< 1.0 to 1.5 g/100g); appropriate land management changes—cover crops, reduced tillage, fertliser use, crop sequences, fallow, deep-rooted perennial plants—can increase soil organic matter levels and deliver the kinds of crop yield gains Rattan Lal describes.
maximum yield of biomass (not grain) increased from 8t/ha for soils with organic carbon concentrations of 0.6 g/100g to 20 t/ha for soils with organic carbon concentrations of 2.1 g/100g.Tittonell, P., & Giller, K. E. (2013). When yield gaps are poverty traps: The paradigm of ecological intensification in African smallholder agriculture. Field Crops Research, 143, 76-90.
Rainfall, temperature, soil type and history of management determine when soil organic carbon could be considered “low”, but when SOM is below the norm, there is potential to increase the levels with prudent agricultural practices.
We will look into these soil restoration practices in other posts. Broadly, they are actions that maintain
- vegetation on the soil (no bare patches or periods)
- mixture of plant species growing and decaying
- mixture of plant species with diverse root architecture
- water on the landscape by reducing runoff after rainfall
- retention of dead leaves, roots and twigs
- fire regimes
In other words, soil carbon as SOM is manageable.
Most agricultural practices have known impacts on soil carbon sequestration through SOM levels, and those impacts can be avoided, mediated, and even reversed in most cases.
This is important because not all costs and externalities of agricultural production can be avoided without halting production altogether.
There will be more soil carbon in the soil inside this grazing exclosure on a property near Canberra, Australia. Photo by Alloporus.
Soil carbon as a climate change fix
When degraded soil is regenerated through soil carbon sequestration, it is likely to return crop yields or vegetation closer to those enjoyed by the farmers before the soil quality declined. This is a virtuous cycle because more carbon tends to generate more carbon up to a biological activity threshold limited by the soil type and climate.
Exactly how much carbon is returned to the soil (sequestered) depends on the locality, but typically
0.5 MgC/ha/yr resulting in longer-term changes in soil carbon stocks of 2 to 2.5 Mg C/ha.Govers, G., Merckx, R., Van Oost, K., & van Wesemael, B. (2013). Managing soil organic carbon for global benefits: a STAP technical report. Global Environmental Facility, Washington, DC, 1-72.
Over 20 years, these rates translate to 7 to 9 tCO2e per hectare. This is not a huge amount, but if you are an Australian cattle farmer with an average size farm of 13,000 ha and the carbon credit price is reasonable, it might be worth waiting 20 years for a seven-figure payday.
The problem with the modest accumulation rate of 0.5 MgC/ha/yr is not the cumulative volume but that this amount is at the detection limit of available measurement methods. Consequently, the level of uncertainty is high.
If the soil restoration practices listed above are already adopted, there is limited potential to increase soil carbon sequestration through management changes. The good guys cannot get the carbon benefit of sound management practices.
As with all solutions to complex problems, there are issues to resolve.
Overall, restoration of degraded soil through soil carbon sequestration as a long-term objective is good for production and carbon offsets as the accumulation rates are sufficient in most cases to reward the patient farmer and investor.
In fact, smart farmers would do it anyway.
What sustainably FED suggests.
Soil carbon sequestration is excellent.
Keeping SOM and enhancing the levels in soil have huge impacts on soil health and plant productivity. In agriculture, this means better yields, more efficient use of fertilisers and less need for irrigation. All significant benefits to the farmer.
Where soil is degraded or below its carbon potential, there is an opportunity for soil carbon sequestration as part of the climate change fix. Even one ton of carbon per hectare can make a difference if the land area is large enough.
Australia emits roughly 500 million tCO2e annually. Offsetting a year of emissions from soil carbon sequestration at one tC per hectare would require 136 million hectares or 40% of the total grazing land. Not a silver bullet for government targets but a significant contributor to a broader emission reduction and offset agenda.
The big bonus of soil carbon sequestration is resilience for agricultural production from the additional biological activity and attributes that enhance soil health.
Climate change will bring drought, fire and flood to Australia in frequencies and intensity beyond historical records. Farmers are devastated by these events and must recover quickly or their business fails and food production drops.
Soil carbon levels are critical to how vegetation responds to routine and extreme weather events, droughts, wildfires and the farmer’s management actions. At or near the optimum, soil organic matter is the most effective way to gain resilience.
Our post on why we love soil carbon has more on this topic as does our deeper dive into the soil carbon sequestration case for climate mitigation.
Go check them out.
More Science Sources
Bot, A., & Benites, J. (2005). The importance of soil organic matter: Key to drought-resistant soil and sustained food production (No. 80). Food & Agriculture Org.
Henry, B., Murphy, B., & Cowie, A. (2018). Sustainable land management for environmental benefits and food security. A Synthesis Report for the GEF (Global Environmental Facility). GEF, Washington, DC, USA.
Krull, E. S., Skjemstad, J. O., & Baldock, J. A. (2004). Functions of soil organic matter and the effect on soil properties (p. 129). Canberra: Cooperative Research Centre for Greenhouse Accounting.
Murphy, B. W. (2015). Impact of soil organic matter on soil properties—a review with emphasis on Australian soils. Soil Research, 53(6), 605-635.
Sarker, J. R., Singh, B. P., Dougherty, W. J., Fang, Y., Badgery, W., Hoyle, F. C., … & Cowie, A. L. (2018). Impact of agricultural management practices on the nutrient supply potential of soil organic matter under long-term farming systems. Soil and Tillage Research, 175, 71-81.