Biochar, the once and future agricultural mainstay

It is one of humanity’s oldest and simplest forms of agricultural technology – using little more than wood, soil and fire. It is also heralded as potentially the single most important farming technique of the future, one that will help feed the world and prevent runaway climate change 

Biochar – a charcoal-like product made by subjecting wood or other biomass to heat in conditions where oxygen is limited – has been used by farmers for thousands of years to enrich soils. Traditionally, this has been done by heating biomass in soil-covered pits. Depriving the process of oxygen means the material doesn’t burn away to ash, releasing all its carbon into the air, and can be used to keep carbon in the ground.

Its production and use is thought responsible for creating the rich dark soils (known as terra preta or black earth) that sustained the large-scale agricultural societies of pre-Columbian South America. Amazonian terra preta was first identified by soil scientist Wim Sombroek in 1950s. Similar sites have since been found in West Africa.

In recent decades, interest in biochar has grown into a global movement. Australian palaeontologist and environmental activist Tim Flannery is one of its adherents. Biochar, he has said, is a uniquely powerful solution to address food security, the fuel crisis and the climate problem.

So what makes this simple substance so marvellous? The latest contribution to understanding its underlying properties sheds light on its precise chemistry by using spectromicroscopy and mass spectrometry to analyse its elemental composition at the nanoscale.

The results, published in the journal Nature Communications, suggest biochar’s capacity to store nutrients and promote plant growth comes down to a super-thin but complex organic coating created on the outer and inner surfaces of its particles as they compost in soil.

The research, led by Nikolas Hagemann and Andreas Kappler of the University of Tubingen, in Germany, calls this coating, “rather than biochar surface oxidation, as previously suggested”, the the dominant molecular mechanism by which it achieves its “extraordinary” effectiveness in retaining nutrients and working as a slow-release fertiliser.

The team’s findings add weight to the prospect that biochar can be commercialised as an inexpensive and eco-efficient technique to both reduce excessive use of nitrogen-based fertilisers and increase carbon sequestration in soil.

Combining biochar with organic matter such as manure is, the researchers suggest, “the key strategy” to create fertiliser effective enough to work at low applications.

“Annual application of such biochar-based fertilisers could sequester carbon and mitigate global warming – a global mitigation strategy that could be implemented due to its economic value for farmers, without the necessity for further subsidies,” the paper concludes.