Compost can turn agricultural soils into a carbon sink - but no match to biochar

Applying organic fertilizers, such as those resulting from composting, to agricultural land could increase the amount of carbon stored in these soils and contribute significantly to the reduction of greenhouse gas emissions, according to new research published in a special issue of Waste Management & Research. The findings are interesting, but the bioenergy community has meanwhile gone way beyond this idea, and is instead looking at a similar but much more promising concept, namely creating soil carbon sinks with biochar, coupled to biofuel and bioenergy production.

The addition of biochar - inert carbon obtained from the pyrolysis of biomass - to soils makes for a much more stable and permanent soil carbon sink than storing carbon via compost; it also reduces fertilizer needs because of increased cation exchange capacity and the prevention of leaching and runoff of nutrients; it reduces nitrous oxide emissions substantially, boosts soil fertility and organic matter build up, and holds a huge overall potential for soil carbon storage (in fact, the system can effectively halt and reverse climate change).

When added to soils, biochar (also known as agrichar) remains unaltered for hundreds, possibly thousands of years (see the ancient Terra Preta soils). Compost derived carbon biodegrades in a matter of years, to release greenhouse gas emissions. Moreover, biochar systems can be based on the production of carbon-negative bioenergy, because the soil amendment is obtained from pyrolysis - itself a promising bioconversion technology that yields both biofuels (bio-oil) and combustible gas.

The new data on organic fertilizers and how they can play a role in creating soil carbon sinks, is fascinating though, in that they precisely highlight biochar's far greater potential. Carbon sequestration in soil has been recognized by the Intergovernmental Panel on Climate Change (IPCC) and the European Commission as one of the possible measures through which greenhouse gas emissions can be mitigated.

One estimate of the potential value of the compost approach - which assumed that 20% of the surface of agricultural land in the EU could be used as a sink for carbon - suggested it could constitute about 8.6% of the total EU emission-reduction objective.

An increase of just 0.15% in organic carbon in arable soils in a country like Italy would effectively imply the sequestration of the same amount of carbon within soil that is currently released into the atmosphere in a period of one year through the use of fossil fuels. - Enzo Favoino and Dominic Hogg, authors

Like biochar, increasing organic matter in soils via compost may cause other greenhouse gas-saving effects, such as improved workability of soils, better water retention, less production and use of mineral fertilizers and pesticides, and reduced release of nitrous oxide.

However, capitalizing on this potential climate-change mitigation measure is not a simple task. The issue is complicated by the fact that industrial farming techniques mean agriculture is actually depleting carbon from soil, thus reducing its capacity to act as a carbon sink:
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According to Hogg and Favoino, this loss of carbon sink capacity is not permanent. Composting can contribute in a positive way to the twin objectives of restoring soil quality and sequestering carbon in soils. Applications of organic matter (in the form of organic fertilizers) can lead either to a build-up of soil organic carbon over time, or a reduction in the rate at which organic matter is depleted from soils (graph, click to enlarge). In either case, the overall quantity of organic matter in soils will be higher than using no organic fertilizer:

What organic fertilizers can do is reverse the decline in soil organic matter that has occurred in relatively recent decades by contributing to the build-up in the stable organic fraction in soils, and having the effect, in any given year, of ensuring that more carbon is held within the soil. - Enzo Favoino and Dominic Hogg, authors

But calculating the value of this technique to climate change policies is complicated. To refine previous calculations and to take account of the positive and negative dynamics of carbon storage in soil, Favoino and Hogg modelled the dynamics of compost application and build-up balancing this with mineralization and loss through tillage.

Their results suggest that soils where manure was added have soil organic carbon levels 1.34% higher than un-amended soils, and 1.13% higher than soils amended with chemical fertilizers, over a 50-year period. This is clearly significant given the evaluations reported above regarding carbon being lost from soils, and the increasing amount of carbon dioxide in the atmosphere, they say.

The article about the potential role of compost in reducing greenhouse gases is published on today in a special issue of Waste Management & Research, entitled Green House Gases and Solid Waste Management. The article will be free online for two months.

References:
Enzo Favoino and Dominic Hogg. "The potential role of compost in reducing greenhouse gases", Waste Management & Research, Vol. 26, No. 1, 61-69 (2008), DOI: 10.1177/0734242X08088584

Amonette, J.; Lehmann, J.; Joseph, S., "Terrestrial Carbon Sequestration with Biochar: A Preliminary Assessment of its Global Potential", American Geophysical Union, Fall Meeting 2007, abstract, December 2007.