A research paper has highlighted the limitations of grasslands to offset emissions generated by the livestock grazing upon them, with huge gaps between amounts emitted and soils’ ability to offset the carbon dioxide.
Published in the journal Nature Communications, the research attempted to address the capability of soils to offset emissions released by livestock in pastoral systems.
Typically, soil in pastoral systems is capable of storing more carbon than that in disrupted, cultivated arable systems. Research by scientists in 2018 in the midwestern United States concluded that emissions from a grasslands grazing system could be offset completely by soil carbon sequestration and may help in mitigating climate change.
Project Drawdown, a climate change mitigation project, has cited farming’s potential to support the role of natural carbon “sinks” as one of its top 10 sector solutions.
But the latest work highlights that this “business as usual” approach will not suffice and the findings were based on short-term data only.
Soils in fact have a finite ability to store carbon, the new research shows, with incremental gains in stored carbon ultimately plateauing – while the livestock grazing above them continue to emit greenhouse gases (GHG).
The researchers improved their measurements by accounting for the short-term nature of methane, the main gas released by ruminants, balanced against the long-lived finite levels of soil carbon that could offset those emissions.
Scientists calculated the amount of soil carbon sequestered that would be required to cancel the methane and nitrous oxide emissions from livestock across the world.
It was found to be nearly twice the current soil organic carbon held in the world’s managed grasslands, with various world regions requiring up to 20 times more grassland area to offset their livestock’s emissions.
Accounting for soil carbon sequestration of livestock emissions makes the appeal of a “closed loop” system particularly strong among advocates for leaving food-producing livestock out of GHG emission equations.
But the scientists found that even in the most optimistic scenario, with the maximum possible carbon sequestered at the lowest methane emission rate, compensating for cattle producing about 40kg of methane a year would require an additional 40t/ha of soil carbon to be stored in the grassland.
This equalled 0.8ha of grassland per one head of cattle per hectare, meaning only relatively extensive systems could wholly deal to their methane emissions.
In New Zealand a single cow produces about 80kg of methane a year, double the amount of methane the researchers used, and they are stocked significantly more intensely at about three cows per hectare.
The researchers also noted that actually achieving the 40-50t/ha of stored carbon was “rather challenging” and may be only rarely achieved.
Additionally, this size of accumulation has not been observed in NZ.
Globally, the increases in grasslands needed to offset emissions at current livestock densities are eye-watering.
The researchers found the increase in global soil carbon storage in grasslands needs to be the equivalent to all the soil carbon losses caused by agriculture for the entire period of human farming history, about 12,000 years.
The regional variations in amount of grassland increases were also huge, ranging from 25% more in Eastern Europe, to 2000 times more in southern Asia.
They proposed some reduction in livestock numbers where possible, improving feed availability, better handling of urine and manure and also putting more effort to restore degraded grasslands, preserving the carbon held under grasslands as much as possible.
Professor Louis Schipper of Waikato University said this modelling work aligns with the approach NZ has been taking.
That was why this country has not become overly obsessed with using soil carbon sequestration as an offset to methane emissions from farms.
“We already have high soil carbon stocks and no evidence of gains on flat land. A possibly more important consideration is avoiding losses of existing soil carbon stocks, such as from drained peat soil, excessive periodic cropping with bare ground and large offtakes.”
The full paper can be read here
