CCS in agricultural soil is impractical, says experts

Both the Australian Government’s Carbon Farming Initiative and the Opposition’s Direct Action Plan recommend offsetting greenhouse gas emissions by increasing carbon storage in agricultural soils. But in a paper published in Scientific Reports, experts from the University of Melbourne have claimed that this method is both technically and economically very limited.

The research team at the Melbourne School of Land and Environment analysed 56 papers to understand the effects of agricultural management practices on soil carbon sequestration in Australia. Paper co-author Professor Rick Roush said the studies covered “172 direct comparisons for the effects of conservation tillage, 116 for residue retention, 83 for the use of pasture and 64 for increased application of nitrogen fertiliser”.

Using the findings of these studies, the team then ran calculations based on the 100 million ha of managed cropland and modified pasture in Australia, to see what changes the new practices could bring and how economically feasible they would be to carry out. They did this under generous conditions, assuming 100% take-up of the practices and ignoring their potential for increased emissions of methane from grazing animals and nitrous oxide from additional nitrogen fertiliser.

The study’s results indicate that the four measures increased soil carbon concentration in the surface layer only (0-10 cm) by the following amounts:

• Nitrogen fertiliser application - 7%
• Residue retention - 8%
• Conservation tillage - 10%
• Pasture usage - 13%

But the researchers believe these small increases might not be technically credible due to possible errors and uncertainties, and even if they were, Professor Roush noted this would result in “only 53.3 million tonnes of CO₂ equivalent sequestered in soil and would therefore not meet the 85 million tonnes targeted in the Coalition’s Direct Action Plan”. Furthermore, the researchers said Australia’s surface layer can and does continually lose carbon, “and the management-induced increases of soil carbon in this surface layer are very vulnerable to environmental and management pressures”.

As for the cost analysis, at the current carbon price of $23/tonne, all practical soil management practices lost at least $3/ha per year. The minimum carbon price for a net profit was estimated to be $36/tonne.

“Even when nitrogen fertiliser was used to increase crop growth and subsequent soil carbon, the costs of the extra fertiliser use generated a financial loss across all systems even assuming that you can keep the carbon in the ground despite drought, and changing land ownership and practices,” said co-author Professor Deli Chen.

The researchers thus conclude that carbon sequestration in Australian agricultural soils, for the purpose of offsetting emissions, “is technically limited and economically non-viable at the present time”.