Friday, March 27, 2009

Glomalin - Long Term Carbon Glue.

The name Glomalin derives from Glomalis, an order of common root dwelling fungi such as Mycorrhizae that colonise the root systems of plants, and was discovered only as recently as 1996. Glomalin itself is a glue-like protein which builds a carbon-rich sheath around the hyphae (thread-like tendrils) that grow out from the fungus to form a secondary root system. Glomalin contains 30 - 40% of its weight of carbon, and it is thought might account for up to one quarter of all the carbon that is contained in fertile soils. Glomalin is also a highly resistant material, and can survive being decomposed in soils for anywhere between 7 and 42 years, thus making it potentially significant in carbon storage by soils. Glomalin also helps to glue-together soil aggregates of other organic (humus) and mineral components, and it is believed to help in the formation of humus - a complex process called humification.

Glomalin gives the soil "tilth", which is a discrete texture that allows experienced farmers and gardeners to "know" good soil just by feeling its smooth granules as they run past their fingers. It is thought that glomalin may also make the hyphae sufficiently rigid they can span the air-spaces between particles of soil. It is believed that hyphae have a lifespan of days to weeks, but the much greater longevity of glomalin suggests that the current technique of weighing hyphae samples to estimate fungal carbon storage may undervalue grossly the amount of carbon stored in the soil. Sara Wright, the discoverer of glomalin, and her colleagues discovered that glomalin makes a far greater contribution of nitrogen and carbon to the soil than is made by hyphae or other soil microbes.

Dr Christine Jones, who is an independent scientist based in Australia, proposes that changes in farming methods to those of "regenerative agriculture" are necessary for the full carbon-capture potential of soil to be realised, particularly for Australian soils. She is promoting "liquid carbon pathways", in which plants pump stable carbon-rich compounds into the soil, as part of a symbiosis with root-fungi, which in return syphon nutrients and water from the soil back to the plant via their extensive hyphae systems.

The relationship between the glomalin and the humus is also symbiotic, since the glomalin contributes to the humification and the humus increases the overall fertility of the soil. Humus is an important material in the retention of water in soil. Dr Jones thinks that the assistance of the humification process by glomalin is a reason for a found much higher accumulation of carbon in some Australian soil than had been thought possible. However, she stresses, farmers may need to rethink how they farm to derive full benefits from the process. She is of the opinion that the answer lies in establishing low-input "year-long green farming" methods which maintain green, growing plants throughout much of the year.

At the University of Aberdeen, Dr David Johnson who is a specialist on mycorrhizal fungi, said:
"Many conventionally grown crops have little or no dependency on mycorrhizal fungi because they receive lots of inorganic fertilizers that don't warrant the carbon 'cost' of forming the relationship with the fungi, for want of a better expression. So, moving to low-input farming systems is likely to encourage plants to form mycorrhizas and therefore increase carbon allocation to this group of organisms." It is also known that long fallow periods, heavy tilling of soil, and a number of agricultural chemicals (including nitrogen fertilizers) can damage the fungi and other forms of soil life.

Now, there is corollary line of thinking from the United States, which proposes that it is soil-depth that is critical to whether or not no-till methods actually result in carbon storage. In essence, no-till involves leaving crop residue on the surface of the soil rather than ploughing it underneath. This saves on labour, wear and tear on machinery, soil-erosion, fossil fuels and artificial (oil and gas derived) fertilizers and pesticides, makes the soil more productive (brings it "back to life"), improves habitats for wildlife and overall biodiversity and conserves water in the soil. If the carbon input (storage) exceeds the carbon output (lost), then the method can be considered successful, or the converse if more is lost than gained.

Results from no-till studies are found to vary from region to region, and for example 40% of Ohio's cropland is good for carbon-storage. Where no-till (practised on a mere 6% of the world's cropland overall, and most of that in the U.S and Canada, Australia and South America - Brazil, Argentina and Chile) does not prove effective, other carbon-capture methods can be applied instead; e.g. mulching, cover crops, complex crop rotations, mixed farming systems, agroforestry and biochar . A survey has been carried out of no-till land in Ohio, Michigan, Indiana, Pennsylvania, Kentucky, West Virginia and Maryland by Rattan Lal and his colleagues at the Ohio State's Ohio Agricultural Research and Development Centre, where he is director of the Carbon Capture Management and Sequestration Centre. Lal says:

"Basically, those soils that are well-drained, are silt/silt-loam in texture, warm quickly and have some sloping characteristics prone to erosion are excellent candidates for no-till. Clay soils or other heavy soils that drain poorly are prone to compaction and are in areas where the ground stays cooler may not always encourage carbon storage through no-till."Lal concludes that soil depth is the crucial factor in carbon storage. He says that if you go down just 8 inches, in general, no-till fields will store carbon better than ploughed fields. However, at depths of 12 inches and more, the situation may be reversed.

"You have to go deeper," he said. "We recommend going down to as much as one metre below the soil surface... [to establish a soil ratings guide for applying different conservation tillage systems at regional and national scales].

Put another way, you have to know your soil, as farmers traditionally do. "Soil" is part of a complex interactive system, and there is not a simple "one size fits all" solution. The means must be tailored to get the best results wherever we are. The real solution is likely to be found in the sum of many smaller "solutions".

Related Reading.


Unknown said...

Well written! This complex issue is not easily summed up. My hat is off to you for helping to decimate and make understandable the data on Glomalin and its role in the carbon cycle.

Professor Chris Rhodes said...

This is an important issue, as are all of the aspects of regenerative agriculture, in terms of feeding the world when oil is both expensive and scarce!