Andrew Crane-Droesch, from the University of California, Berkeley, US, and his colleagues conducted a meta-analysis to assess the potential of biochar. They gathered data from 84 biochar studies and analysed the data statistically to tease out the impact of different soil types, different agricultural techniques and different types of biochar. From this they estimated the potential yield increase that biochar could bring about in different parts of the world.

The team found that biochar produces the biggest yield increase on highly weathered soils, often found in the humid tropics. "Our model implies positive yield response over much of Sub-Saharan Africa, parts of South America, Southeast Asia, and southeastern North America," said Crane-Droesch. And better still, the scientists found that yield responses continued to increase further in subsequent seasons. However, richer organic soils, including those found in much of the world’s most important agricultural areas – northern Eurasia, North America and Indonesia – appear to be less likely to benefit from biochar addition, with negligible or even negative yield increases. The findings are published in Environmental Research Letters (ERL).

Previously, Dominic Woolf from Cornell University and his co-workers estimated that biochar could mitigate a maximum of 12% of annual greenhouse gas emissions (based on 2010 emissions) if the technique was used to its maximum extent and applied globally (Nature Communications). Although Crane-Droesch and his colleagues did not investigate the potential of biochar to sequester carbon, their findings suggest that this figure is likely to be a significant overestimate, given that it will be uneconomic to apply biochar across the large swaths of the globe where it doesn’t increase yields.

Although it is perhaps disappointing that biochar is unlikely to be able to have much impact agriculturally on the ‘breadbaskets’ of the world, there is still much excitement about its potential in some of the Earth’s poorest regions. "I think this may be the major story with biochar – enabling intensification on some low-potential soils," Crane-Droesch told environmentalresearchweb.

Exactly what biochar is doing to the soil is still not clear. "There are many possible processes that may explain the increase in yield after biochar addition, including an increase in the cation exchange capacity (reserve of nutrients), an increase in water retention and an increase in soil organic matter," said Samuel Abiven, co-author on the paper. In this study the scientists couldn’t identify a specific mechanism; they suspect that biochar improves soil in different ways in different places, depending on the soil and the crop being grown.

There are still many unknowns surrounding biochar and many questions that need to be answered. How and where should it be made, how should it be transported and distributed, and where is it economically feasible to use? "It holds a lot of promise, but before biochar production takes off in a big way, we need to understand better what some of the risks may be with biochar production and application," said Crane-Droesch.

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