Sustainable agriculture research: The value of soil carbon
Healthy soils are the foundation of productive agriculture and are vital for sustainable food production and many natural processes.
Soils are one of the world’s largest carbon reservoirs and have enormous potential for reducing the impact of GHG emissions. The world’s soil contains more than 2 trillion tons of stored carbon. This is at least three times more than there is in the Earth’s atmosphere.
However, soils have come under pressure. Over half of the soil organic matter in the world’s agricultural ground has already been lost, with over 2 billion hectares of farmland affected. This leads to declining crop yields and increased pollution of water systems from fertiliser run-off.
Soil organic matter is mainly composed of carbon, leading to the release of enormous amounts of carbon into the atmosphere. Over the last 10,000 years, agriculture and land conversion led to 840 billion metric tons of carbon dioxide (GtCO2) being released into the atmosphere. Many cultivated soils have lost 50–70% of their original organic carbon.
Other issues impacting soil health include salination and compaction. For example, in 2010, soil degradation in England and Wales was estimated to cost £1.2 billion annually.
So how can soil health be restored and how can farmers make money out of the valuable resource beneath their feet? How can improving the health of soils also support sustainable food production?
What is soil health?
Soil health is the capacity of soil to function, within ecosystem boundaries, sustain crop and animal productivity, maintain, or enhance environmental sustainability, and improve human health. In agroecosystems, soil health can change due to human activities, such as intensive cropping practices, which can impact soil functions.
Soil organic matter (SOM) is vital to soil health. SOM consists of all of the carbon-based compounds that were once in living organisms. It is a mixture of materials ranging from fresh plant residues to highly decomposed material known as humus.
Soil organic carbon is the net difference between carbon inputs (photosynthesis, organic amendments such as manure) and outputs (decomposition, soil erosion) in agroecosystems. Enhancing soil carbon storage by even a few percentage points makes a significant difference.
A recent study estimates that soil carbon sequestration could be scaled up to sequester 2–5 Gt CO2 per year by 2050, with a cumulative potential of 104–130 GtCO2 by the end of the century at a cost of between $0 and $100 per ton of CO2.
How to build soil carbon and enhance soil health?
Soil health and soil carbon levels can be enhanced by a range of farming practices. This includes reduced tillage systems, permanent soil cover and effective use of crop rotations including intercrops and covers crops and reducing the fallow period.
Reducing or eliminating tillage can reduce the mixing of the soil’s crop residues, resulting in lower decomposition rates and CO2 emissions. Research in Brazil has shown that a no-tillage system leads to higher soil organic matter levels and enhanced soil fertility. The study used legumes as a cover crop which also built-up soil nitrogen levels.
Expanding the growing season can be achieved through cultivating perennial crops or incorporating winter crops or cover crops into the rotation. This results in greater capture of CO2 via photosynthesis and thereby increases carbon inputs to the soil. Minimizing residue removal or burning can also bolster soil carbon.
Soil organic Carbon can be maximized by growing crops with high biomass production (e.g., corn vs. soybean) or by using deep-rooted crops e.g., alfalfa and by reducing the fallow period. Improved irrigation techniques (e.g., deficit irrigation) can also reduce CO2 emissions in arid and semiarid environments.
How can soil carbon be monetised?
There are growing opportunities for farmers to derive income from soil carbon through carbon trading schemes. Farm-based carbon credits are becoming more widely available. For example across the world in Australia, Alberta, Kenya, and California, there are active programs to reward on-farm carbon sequestration.
A recent case study in western Victoria showed the benefits of high soil carbon. The increased nutrients and water holding capacity were worth as much as $150 per hectare compared to $15 to $20 per hectare for soil carbon trades. Soils with high levels of carbon are also likely to be more productive, leading to higher yields and savings on fertiliser and other inputs.
Richard Eckard, Professor of Sustainable Agriculture at the University of Melbourne, and Director of the Primary Industries Climate Challenges Centre think farmers would be better off saving any carbon credits for their commodities as consumers increasingly demand carbon neutrality. According to Professor Eckard:
Once it goes to another entity like the coal industry who use it as their offset, what happens to that farm when the supply chain says we will only buy your product if it’s carbon neutral?
Healthy soil is vital for sustainable food production and is an increasingly valuable resource. Farmers and landowners are starting to appreciate the need to build soil carbon and maintain soil health, but it is not clear yet how to best turn the soil into more tangible wealth.
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