In the first study of its kind, research undertaken at the University of Southampton shows that biochar in soil has acts as a powerful tonic to stimulate plant growth.

The findings are published in the journal Global Change Biology Bioenergy.

The scientists, led by Professor Gail Taylor, found that when thale cress and lettuce plants were subjected to increasing amounts of biochar mixed with soil, up to 50 tonnes per hectare per year, plant growth more than doubled.

Plant growth hormones activated

They also tracked for the first time the changes in genetic expression that followed from applying biochar.

The response of more than 10,000 genes was followed simultaneously, and two growth promoting plant hormones - brassinosteroids and auxins, together with their signalling molecules - were stimulated by the biochar.

Professor Taylor said: "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.

A huge potential for biochar - to store carbon, and fertilise

Many previous reports have shown that biochar can also stimulate crop growth and yield, providing a valuable co-benefit when the soil is treated with biochar, but the mechanism enabling this to happen was unknown - until now.

Biochar is produced when wood is combusted at high temperatures to make bio-oil and has been proposed as a method of geoengineering. When buried in the soil, this carbon rich substance could lock-up carbon and reduce greenhouse gas emissions, while also acting as a potent fertiliser.

The global potential of biochar is considerable. In principle up to 12% of human carbon emissions could be sequestrated by biochar soil application.

The greatest limitation on the use of biochar is the availability of organic material to convert into charcoal, without leading to the destruction of natural forests and other ecosystems.

Integrated agroforestry systems have the best potential to conserve soils and ecosystems while producing food and organic material for conversion to biochar - as conducted by Carbon Gold in Belize.

But increased vulnerability to pests?

The positive impacts of biochar were coupled with negative findings for a suite of genes that are known to determine the ability of a plant to withstand attack from pests and pathogens.

These defence genes were consistently reduced following biochar application to the soil - for example those leading to jasmonic and salcyclic acid and ethylene production - suggesting that crops grown on biochar may be more susceptible to attack by pests and pathogens.

However the research did not investigate the question of whether crops grown on biochar enriched soils really were more susceptible to attack by pests and pathogens.

"The finding for plant defence genes was entirely unpredicted and could have serious consequences for the commercial development and deployment of biochar in future", commented Taylor.

"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."

However it maybe that the biochar itself deters pests, or that it activates other plant defence mechanisms. In the absence of field evidence that biochar-fertilised plants are more vulnerable to pests and pathogens, Professor Taylor's fears may be misplaced.

Oliver Tickell edits The Ecologist.

The research was carried out by Professor Gail Taylor, Director of Research at the University's Centre for Biological Sciences and research colleagues, in collaboration with National Research Council (CNR) scientists in Italy and The James Hutton Institute in Scotland.