Carbon Capture in the soil

There is a wide spread view that simply capturing carbon in the soil will both improve soil quality and mitigate climate change.  There are truths in these views but the situation is more complex than simply increasing the level of soil carbon in the soil, total soil carbon content is only part of the story.

What really matters is the form of the carbon in the soil.  If the carbon is in an unstable form, e.g. readily digested by bacteria, any benefits for climate change and food production is limited.  If it is a stable form, what we know as humus, it will have long lasting benefits for both climate change and food production.

 The critical factor is not the total amount of carbon in the soil, but the amount of stable carbon or humus in the soil.

Background

 In the mid seventies Australia suffered severe dust storms loosing millions of tonnes of top soil.

I started a research project on how to regenerate top soil, using a block in which al the top soil had been lost by excessive grazing by goats leaving the underlying clay base. See www.waterright.com.au/soil_regeneration. The characteristics of the clay were only to clear.  When dry it was like concrete almost impossible to work, but when wet it would become little more than an unworkable gooey mess.  Plant growth was extremely poor.

 The carbon or organic content was very low however simply adding organic material or even the so called clay breakers had little change.  It was simply a mix of organic material and clay with essentially the same characteristics.  However using the appropriate process the characteristics could be completely transformed producing a soil which is easy to work, wet or dry, exhibiting reduced density and significant elasticity (springiness) but above all it was highly productive.  Plant growth was used as a measure of the effectiveness of soil regeneration.

 The process to achieve this transformation is simple, the soil must be maintained moist and plants continuously grown.  The mechanics behind this transformation is not obvious and only partially understood.

 We know that organic material on the surface will be decomposed by the combination of UV light and oxygen, that in aerobic conditions that bacteria will rapidly attack the softer organic material releasing carbon dioxide, that under anaerobic conditions bacteria will release methane and that fungi are more adapt at attacking the harder components particularly the lignin.

 Of course bacteria and fungi attack organic material because it is a source of energy. Exactly the same reason that every living creature depends on organic material.  This organics material which is devoured for energy provides very little benefit for the soil structure or for averting climate change, simply releasing carbon dioxide and methane back to the atmosphere. Measuring soil carbon may indicate a high level of carbon but much of this will be temporary.

 However a certain amount of the organic material will be converted into a substance which is generally referred to by its common name of humus.    The critical issue is the amount of carbon which is locked into the soil as stable humus. Despite the apparent lack of a scientific name, or in fact having been the subject of intensive scientific research humus is vital for life on earth.

 Humus however has two properties which are of immense importance.  Firstly it is stable and will resist decomposition for long periods of time; - some people claim for hundreds of years.  This makes it valuable as a means of averting climate change.  Secondly is its remarkable effect of soil quality transforming virtually unworkable clay into top grade soil.  This is achieved by a process of aggregation in which fine particles are held together in small clumps.

 Some people say that this is physical, the long chain molecules simply entwine around individual soil particles while other say it is the result of Van de Waal forces, those secondary forces which occur when molecules are in close contact.

 The process of forming humus is of great importance. At the macro level it is clearly a result of microbiological action. Creating beneficial conditions for the appropriate microbiological action has a major influence on the proportion of humus that is formed.

 At a micro level it has been suggested that enzymes which fungi release from their hyphae to dissolve mineral particles is involved in the formation of humus.

 If I can quote from James Amonette, (Pacific Northwest National Laboratory, Richland, WA.) as an indication of our scientific understanding.

 ‘While incompletely understood, the humification process by which soil C is stabilized (Stevenson, 1994) is believed to involve several parallel pathways. Of these, the polyphenol formation pathway generally dominates. The rate limiting step for this pathway is believed to be the oxidation of polyphenols to polyquinones, which then polymerize with amino acids to form humic material. This oxidative polymerization reaction is catalyzed by polyphenol oxidase (PPhO) enzymes such as tyrosinase (Martin and Haider, 1969, 1971; Nelson et al., 1979), but soil minerals such as allophane (Kyuma and Kawaguchi, 1964), Fe and Mn oxides (Shindo and Huang, 1984; Stone and Morgan, 1984; McBride 1987), and smectites (Kumada and Kato, 1970; Thompson and Moll, 1973; Filip et al., 1977; Wang et al., 1978) also promote the reaction.’ www.flyash.info/2003/47amon.pdf

 While science may not fully understand the process of humification this does not prevent us taking advantage of the process to resolve the great challenges facing humanity. This is relatively simple and easily backed by simple experiments.

 The wicking bed process with the moist conditions and recycling of air and water and water can be observed to be highly effective in creating humus.  This can be accelerated by the use of fungal initiators or inoculants into the water stream.

 One of the major hurdles to adoption of any method of carbon capture is measurement.  It can take time for organic material to form stable humus so a common approach has been to measure carbon content over time resulting in farmers having to wait sometimes years to receive payment from carbon trading.  This removes a major incentive for farmers to adopt carbon capture.

 The wicking beds use organic waste from external source so it is easy to measure the quantity added.  Only a small proportion of this organic waste is converted to the stable humus. The ratio of organic waste to humus can be measured in a separate controlled trial by measuring the amount of humus generated from a given quantity of organic waste.  This ratio can then be used to predict the amount of humus which will be created, so providing a simple basis for carbon trading.