closeup of dry soil with withered plants

Fixing climate with soil carbon has a big bonus

Like a bolt from the blue, soil carbon is the sequestration option to help fix the climate problem. It is no surprise to us scientists and isn’t even the biggest bonus of extra soil carbon.

The team at sustainably FED believe in soil carbon. 

We think 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.

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 the real reason for liking soil carbon so much—it is crucial for soil health.

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.

Along with many other ecologists, 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 tubers 

Lal, 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 result in millions of tons of extra food production?

Because biology is the engine for nutrient transfers to plants in soil and carbon in its myriad of forms is the fuel and the lubricant for the engine.

Carbon fuels and lubricates the soil engine.


What does carbon do in soil?

Carbon is the raw material for life. It is the food source for all soil organisms, from microbes to moles. Increase carbon in the soil, and generally, there is more biological activity from a more significant number and diversity of soil organisms. 

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’. This biological more making becomes self-perpetuating as these organisms mobilise nutrients into plants.

Carbon as organic matter also holds on to moisture, the other essential for life. So organic-rich soil tends to dry out more slowly and maintain biological activity for longer.

Exactly how this all comes together depends on the soil type, especially the texture and amount of clay minerals, plus the land use and local land management history.

Most of the time, soil carbon is present as soil organic matterthe 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

Nutrient source

soil organic matter is a major source of nutrients in soils and a key 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 contents or soils with more highly weathered clays such as the kaolinites. 

Buffering capacity

soil organic matter can help buffer soils against acidification from agricultural land use. 

Detoxification 

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

additional 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.

Science sources for the effects of soil organic matter: 

  • 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.

In degraded soil or when soil organic carbon levels are low (< 1.0 to 1.5 g/100g), appropriate land management changes can increase soil organic matter levels and deliver 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, SOM is manageable

Most agricultural practices have known impacts on 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.


Soil carbon as a climate change fix

When degraded soil is regenerated by improving soil organic carbon, 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 $47 per tonne, it might be worth waiting 20 years for $4.2 million.

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. 

Plus, if the soil restoration practices listed above are already adopted, there is limited potential to increase soil carbon through management changes. The good guys cannot get the carbon benefit of sound management practices.

However, restoration of degraded soil as a long-term objective is good for production and for carbon offsets as the accumulation rates are sufficient in most cases to reward the patient farmer and investor.


Soil organic carbon is excellent. 

Keeping it 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 to sequester carbon 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. Clearly, not a silver bullet for government targets but a significant contributor to a broader emission reduction and offset agenda. 

The big bonus of increasing soil carbon is resilience.

Climate change will bring drought, fire and flood to Australia in frequencies and intensity beyond historical records. Farmers are devastated by these events and need to recover quickly or go out of business.

Soil carbon levels are critical to how vegetation responds to routine and extreme weather events and wildfires.

At or close to the optimum, soil organic matter is the most effective way to gain resilience.


Hero image from photo by Çağlar Oskay on Unsplash

Mark

Mark is an ecology nerd who was cursed with an entrepreneurial gene and a big picture view making him a rare beast, uncomfortable in the ivory towers and the disconnected silos of the public service. Despite this he has made it through a 40+ year career as a scientist and for some unknown reason still likes to read scientific papers.

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