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Enhanced rock weathering might lock away tons of CO2 even on inhospitable arid farmlands

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Enhanced rock weathering might lock away tons of CO2 even on inhospitable arid farmlands

Researchers pushed the limits of spreading rock dust on agricultural soil to fringe landscapes and found—to their surprise—that it still soaks up significant amount of carbon.
November 3, 2023

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We may be overlooking the potential of crushed rock to trap tons of carbon across swathes of arid farmland, a new study shows. 

Rock weathering has such significant carbon-fixing potential that research finds it can supercharge us on our path towards our climate targets. Studies from the UK show that this technique could capture 45% of the carbon needed to reach the country’s net zero goals. But, these studies have so far focused on wetter climates, where rainfall is abundant. That’s because moisture facilitates the weathering process that locks carbon into rock, and so is seen as essential to this process

But this new research is the first to show the that carbon-trapping rock dust has unrealized value on farmland in dry weather climates, too.

The California-based scientists didn’t have to design ideal lab conditions to make this discovery: during their study period, California experienced a period of extreme drought that created perfect field conditions for the research. 

On a five acre stretch of fallow farmland where corn had previously grown, the researchers spread a mixture of crushed basalt and olivine, at a rate of about 70 tons per hectare, which they applied over two winter seasons. Crushing rock into dust accelerates a natural rock weathering process that traps CO2, and basalt and olivine are two rock types that are especially known for their fast-weathering rates and therefore, their potential to capture more CO2. 

 

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Enhanced Rock Weathering

 

Then, the researchers used instruments called lysimeters to detect the chemistry of the water that percolated into the soil, when very limited amounts of rain did fall during the study period. These lysimeters were used to measure levels of bicarbonate, a mineral that forms when silicate minerals in rock react with CO2, as weathering occurs. Bicarbonate levels were therefore taken as an indicator of how much CO2 was being fixed into the rock dust-enriched soil. 

When the researchers compared these lysimeter measures to those from a field where no rock dust had been mixed into the soil, the differences were surprising. Samples from the farmland that had been amended with rock dust contained almost three times more bicarbonate than in samples from the control field. In carbon terms, that translated to a not insignificant 0.15 tons of carbon dioxide per each hectare. 

“Even the infrequent heavy rains we get in the [US] West might be enough to drive enhanced rock weathering and remove carbon dioxide,” the researchers say.

While the captured amount might not seem huge, the study period covered just a few months, which suggests a fairly rapid rate of carbon-fixing on the arid land. And, if extrapolated to all Californian farmland, carbon sequestration at that rate would suck up emissions equivalent to removing 350,000 cars from the road each year. 

That additional potential is the point the researchers seem keen to emphasize with their discovery: if rock weathering can achieve decent levels of carbon sequestration in even the most unsuitable conditions, then why not expand it to these fringe landscapes? Focusing only on ideal weather conditions for rock weathering risks overlooking a significant carbon sink in large swathes of arid farmland worldwide, the study indicates.

For now, these dryland findings are preliminary, and the researchers say their experiment needs to be trialed across larger swathes of land, with more lysimeters, and at deeper depths, to find out how much carbon is being sequestered and whether the rate of sequestration declines. 

But for now, it builds up the evidence on the benefits of this simple technology to fix carbon into the soil. If rock dust works, why not spread it far and wide? 

Holzer et. al. “Direct evidence for atmospheric carbon dioxide removal via enhanced weathering in cropland soil.” Environmental Research Letters. 2023. 

Image: Dominicus Johannes Bergsma via Wikipedia Commons

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