4 per 1000: Soil Carbon to mitigate Climate Change

13 November 2015

Required soil C sequestration rate (in tonne C per hectare per year)
Required soil C sequestration rate (in tonne C per hectare per year)

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One of the major aims of COP21 meeting in Paris is, "…for the first time in 20 years of UN negotiations, to achieve a legally binding and universal agreement on climate, with the aim of keeping global warming below 2°C". In response the French government is focussing on the soil's ability to sequester carbon and mitigate the greenhouse effect. To achieve this, the French Minister of Agriculture Stéphane Le Foll has set an ambitious international research program, the "4 pour mille" or "4 per 1000", meaning an increase global soil organic matter stocks by 0.4 percent on an annual basis. Backed by the FAO and the US, we can already meet this challenge and determine the amount of carbon that needs to be sequestered locally across the globe using the recently published maps of global soil C stocks from research developed by the Faculty of Agriculture & Environment.

Soils store lots of carbon! At about 2400 billion tonnes (2400 × 1015 g) of C, the soil is the largest terrestrial carbon pool. The top 2 metres of soil in fact hold 4 times the amount of carbon that is stored in plant biomass. Soils also have the potential to store carbon for a very long time by various protective mechanisms.

The potential of soil to sequester C to offset greenhouse emissions has been widely debated. So the million dollar question here is how do we realistically increase the soil organic matter stock by 0.4% on a year-to-year basis? Some would interpret this figure as an increase of 0.4% soil C per year, which is virtually impossible.

The 4 per 1000 soil carbon sequestration initiative
The 4 per 1000 soil carbon sequestration initiative

First we need to understand where "4 pour mille" or 4 parts per thousand comes from. The annual greenhouse gas emissions from fossil carbon are estimated at 8.9 billion tonnes (8.9 x 1015 g), and a global estimate of soil C stock to 2 metres of soil depth of 2400 billion tonnes (2400 x 1015 g). Taking the ratio of global anthropogenic C emissions and the total soil organic C stock (8.9/2400), results in the value of 0.4%. But to come up with a meaningful number for farmers to use, we need to work out how much C the soil is storing on a per hectare basis. 0.4% of this total estimate will then give us the required annual sequestration rate that needs to be achieved to reach the "4 pour mille" goal.

The land area of the world has 149 million km2, and it would be estimated that on average there are 161 tonnes of C per hectare. So 0.4% of this equates to an average sequestration rate to offset emissions at 0.6 tonnes of C per hectare per year. We know that soil varies widely in terms of C storage, for example peat soils in the tropics hold about 4000 tonnes of C per hectare, while sandy soils in arid regions may only hold 80 tonnes of C. The type of aboveground vegetation and how quickly the soil biota uses the carbon also can affect this rate. Taking this into account, we would need to add about 4 times the amount of organic matter to meet this sequestration rate.

Studies across the globe have measured soil C sequestration rates and they suggest that a C sequestration rate of 0.5 tonnes of C per hectare per year is possible, after the adoption of best management practices such as reduced tillage in combination with legume cover crops. These estimates do change however with soil type and climatic regions. Our work mapped out the global C stock in the world and showed that some cropland areas in the world have C contents that are below critical limits. Restoring the soil's C content in these areas is a win-win situation, as it offsets greenhouse gases emissions and provides benefits of enhanced soil quality.

Now let us look at some numbers for Australia, where we are considered to be one of the worst carbon emitters per capita. Our current annual CO2 emissions are 309,000 kilo tonnes (or 106 x 1012 g C). If we use the C stock estimates for Australian soil of the top 30 cm as 25,000 million tonnes (or 25,000 x 1012 g), then the ratio of C emissions over soil C stock is remarkably at 0.4%. If we consider the proportion of agricultural land at 470 million hectares, then annually we need to sequester an average of 0.22 tonnes of C per hectare. This value falls between the estimation of sequestration rates, 0.19-0.3 tonnes of C per hectare per year, for best management practices in Australia where water is not severely limiting.

The "4 per 1000" is an ambitious aspiration, however it is for the first time setting a global goal to promote good soil management that can help mitigate climate change. To achieve this we need disruptive technologies that can help agricultural practices to soak up more carbon in the soil, create soil security to achieve food security and mitigate climate change. Australia should be part of it!

Contact: Budiman Minasny

Phone: +61 2 8627 1131

Email: 1a231e081f0e345e1f1b5f11305e2f2f20164b29534f662336471e2407