A cluster of independent researchers, agronomists and farmers think doubts about the ability of Australia’s agricultural soils to sequester and hold carbon in meaningful quantities are unfounded.
Mainstream science, largely represented by soil scientists within CSIRO and the State departments of Primary Industry, have harboured such doubts, especially about the amounts of carbon sequestered, being enough to earn farmers an income from emissions trading schemes.
But independent scientist Dr Christine Jones has argued that giving soil carbon its due will require a re-evaluation of our agricultural methods.
Since 2005, she has helped put soil carbon on the national agenda, by hosting the first conference on the subject.
Most studies of soil carbon have been based on standard agricultural practices.
They rely for measurement on the “Roth C” model, which focuses on the breakdown of organic matter into forms of soil carbon.
Dr Jones, however, contends that conventional agriculture and biomass breakdown are not alone going to deliver the soil carbon gains necessary to make agriculture part of the climate change solution.
She is instead, promoting “regenerative agriculture” to develop “liquid carbon pathways”—plants pumping stable carbon-rich compounds into the soil as part of their interaction with soil microbes.
The best understood of the soil microbe families—and potentially the most useful to agriculture—are the mycorrhizal fungi.
They're found on the roots of around 80 per cent of flowering plants, including the grasses.
Mycorrhizae attach themselves to plant roots and grow thread-like hyphae out into the surrounding soil.
The fungi feed on sugars exuded by the plant. In return, the hyphae act like a greatly extended root system, siphoning nutrient molecules back to the plant.
For the soil carbon story, the most significant part of this interaction is the protective carbon-rich sheath that is created around the hyphae of arbuscular mycorrhizae - a family of fungi associated with grasses like wheat and rice.
The sheath is made of a substance called glomalin, which was only identified in 1996.
Glomalin is 30-40 per cent carbon, and according to some research can account for a quarter of the carbon held in fertile soils.
A greenhouse trial by Mike Amaranthus of Mycorrhizal Applications, Inc (MAI) in the United States found that inoculation of tall fescue with mycorrhizial fungi almost doubled carbon production in a year. Professor Amaranthus believes that glomalin was responsible for the increase.
Glomalin can resist breakdown for between seven and 42 years, making it a long-term carbon store.
As an organic glue, it contributes to the production of humus.
As well as being a highly stable form of carbon storage, humus underpins natural fertility.
According to humus researcher Glenn Morris, in its pure form humus can hold at least four times its weight in water.
“Humus is critically under-studied and thoroughly underestimated as a component of land and water management systems,” wrote Mr Morris, a northern NSW farm manager, in his 2004 Masters dissertation.
Dr Jones believes the “humification process” that glomalin assists is behind some outlier results she has collected showing that soil carbon accumulation in Australian agricultural soils can be much higher than has been thought possible.
However, farmers wanting to encourage mycorrhizal fungi may have to reconsider how they farm.
“Many conventionally grown crops have little or no dependency on mycorrhizal fungi because they receive lots of inorganic fertilisers that don’t warrant the carbon ‘cost’ of forming the relationship with the fungi, for want of a better expression,” said Dr David Johnson, a mycorrhizae specialist at the University of Aberdeen, Scotland.
“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.”
Long fallows, heavy tillage and many agricultural chemicals are known to also kill off mycorrhizae and other soil life.
On the upside, former CSIRO forestry researcher Dr Nick Malajczuk, who manages MAI’s Australian operations, said that in MAI’s estimation, mycorrhzia-inoculated cereal crops needed about a third less fertiliser than crops without good populations of the fungi.
Dr Jones believes agricultural systems need to move toward low-input “year-long green farming” methods that maintain green, growing plants for as much of the year as possible.
* She will make a case for these methods on May 22, when the Australian Soil Carbon Accreditation Scheme (ASCAS), the voluntary soil carbon program Dr Jones set up with the backing of philanthropist Rhonda Wilson, presents test results and carbon sequestration payments to ASCAS farmers at its “Managing the Carbon Cycle” forum in Canberra.
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