In the first study of its kind, research undertaken at the University of Southampton has cast doubt on the potential of biochar to alleviate climate change.

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Advocates suggest it can be used to sequester carbon when buried in soil, potentially cutting emissions from agriculture and reducing nutrient leaching. Biochar is thought to have a significant potential to reduce air pollution from agriculture; experts have suggested that up to 12 per cent of greenhouse gas emissions could be reduced by biochar soil application. It also increases soil fertility and research has tended to suggest it can provide protection against certain foliar diseases.

However, key researchers into its effects have suggested that agricultural uptake of biochar has been limited as, "Its agronomic value in terms of crop response and soil health benefits have yet to be quantified, and because the mechanisms by which it improves soil fertility are poorly understood."

Although the potential of biochar as a soil treatment has been touted relatively widely, the mechanisms by which it is said to benefit plants remain largely unknown. Researchers from Southampton, working with partners from the National Research Council (CNR) of Italy and James Hutton Institute in Scotland, may have found an explanation, though they also discovered that the application had certain negative effects.

Professor Gail Taylor, From Southampton's Centre for Biological Sciences, found that when increasing amounts of biochar were mixed into soil it stimulated the growth of thale cress and lettuce plants by over 100 per cent. The scientists also studied the response of over 10,000 genes to the biochar application, to investigate its effects.

In contrast to previous studies, which have suggested biochar can boost plants' defences to certain fungal diseases, Taylor's international team found that a suite of genes known to affect plants' ability to withstand attacks from pests and pathogens were inhibited; these defence genes were consistently reduced following biochar application.

The international team used the equivalent of up to 50 tonnes per hectare per year, if biochar were being applied in the field. The researchers warned that the observed effects on immunity "could have wide implications for the use of biochar on commercial crops… if reproduced in the field at larger scales."

Professor Taylor, who co-ordinated the research, explained, "Our findings provide the very first insight into how biochar stimulates plant growth – we now know that cell expansion is stimulated in roots and leaves alike and this appears to be the consequence of a complex signalling network that is focussed around two plant growth hormones. However, the finding for plant defence genes was entirely unpredicted and could have serious consequences for the commercial development and deployment of biochar in future."

Though the European researchers' findings are concerning, the full effects of biochar usage remain relatively poorly understood. Research published last year by scientists at Rice University, Texas suggested that bochar's effect on soil-dwelling bacteria can provide some defence benefits. The researchers revealed that biochar application tended to interfere with the chemical signals that some microbes use to communicate, including those used by plant pathogens to coordinate their attacks.

Taylor suggested that more research would be necessary to work out whether the phenomena observed by Southampton researchers would present a risk to plants in the field. The professor added, "Any risk to agriculture is likely to prevent wide scale use of biochar and we now need to see which pest and pathogens are sensitive to the gene expression changes."