THE figures have changed, but Sydney University’s message that highly fertile soils may be capable of sequestering more methane than cattle produce hasn’t.
The University has acknowledged a “transcription error” in a report published by Rural Press last year, which suggested that certain Snowy Mountains soils were capable of oxidising (and effectively rendering climate-neutral) 8750 kilograms of methane per hectare per year.
At the time, Professor Mark Adams, Dean of Sydney University’s Faculty of Agriculture, Food and Natural Resources, told Rural Press that grazing of cattle on these soils was “easily methane-neutral and may even offset cow-methane from other parts of the landscape”.
The sums altered radically when it was discovered that micrograms of methane had been wrongly transcribed as milligrams, a much larger unit of measure.
Dr Robert Simpson, a post-doctoral research fellow who supplied the corrected values, said the methane oxidation rate measured by University researchers is actually 8.75 kilograms per hectare per year.
“We still feel this is a very high methane (CH4) sink in a global context, and can offset significant percentages of enterprise-scale CH4 emissions where grazing intensity is low or seasonal and soils have a high intrinisic ability to oxidise atmospheric CH4,” Dr Simpson said.
Even with the corrected figures, the methane oxidation rate in high country soils is only known to be exceeded by pristine New Zealand forest.
Prof. Adams affirmed that despite the significant rescaling of the figures, certain high country soils oxidised methane at sufficient rates to fully offset enteric methane production in low intensity grazing systems.
He also restated his belief that any accounting approach to greenhouse gas emissions needs to properly account for “sinks” as well as “sources”.
In a paper using the correct figures, Dr Simpson, Prof. Adams and Dr Tina Bell of the University of Melbourne did some sums on a hypothetical 1000 hectare high country grazing area made up of 49 per cent woodlands, 49 per cent grassland and two per cent bog.
Grazing by 200 head of cattle for six months of the year across this area would produce about 5.4 tonnes of methane per year.
Methane oxidation rates in soil, according to University measurements, would total about 7.6 tonnes across the 1000 hectares, ensuring that the grazing emissions would be clearly offset.
“Clearly not all systems will have a net sequestration of CH4, however these data underscore that any quantification of CH4 production from ruminants for the purposes of accounting cannot be calculated as a gross number,” the authors said.