Environmental scientist spends more than four decades researching a potential climate game-changer

In the ongoing fight against climate change, one remarkable discovery has gained significant attention over the past years: phytoliths.

These microscopic particles of silica form in plants and are deposited in the soil when the plant dies, where they can survive for hundreds or even thousands of years. 

Dr Martin Hodson, a Visiting Researcher and Associate Lecturer at Oxford Brookes University, began working with phytoliths 43 years ago. Most recently he has worked with a global team of researchers looking at the potential for carbon storage in phytoliths -  a process known as carbon sequestration. Here Dr Hodson talks about his research past and present and outlines the steps needed for future progress.

Tell us more about phytoliths
We don’t think of plants as hard but have you ever cut your hand pulling up grass? Have you ever been stung by a stinging nettle? This is all due to phytoliths. Soluble silica is taken up by the plants and is deposited in some of the cells as hard, solid phytoliths. In Greek ‘phyton’ is plant and ‘lithos’ is stone or rock, so phytoliths are plant stones. They take the shape of the cells they are deposited in. If you had a microscope you would be able to see that the edges of pampas grass have razor sharp prickles made of silica. If you looked at the lower surface of nettle leaves you would see silica hairs which act like minute hypodermic syringes to inject poison into your skin.

When a plant dies, the leaves, stems and flowers fall into the soil and are incorporated into the soil organic matter (humus). The phytoliths are much more resistant to breakdown in the soil and can persist for hundreds or thousands of years. Phytoliths are used by archaeologists and palaeoecologists (scientists who study fossil animals and plants) to work out what people grew and ate, and their climates and environments, thousands of years ago.

How does carbon sequestration in phytoliths work? 
In 2005 Australian phytolith experts, Parr and Sullivan, realised that phytoliths store carbon within their structures. According to their calculations, phytoliths store a lot of carbon in the soil and potentially store it within the silica for a very long time. 

Their idea created a whole new area of phytolith research. This could be a method for removing some of the carbon from the atmosphere that is causing the climate change crisis. We are spending large amounts of time, money and energy on trying to get carbon capture and storage to work on power stations. Why not see if plants can do it naturally?

However, we needed to work out how much carbon is stored in phytoliths and for how long and these aren’t easy problems to solve.

Describe your latest research into phytoliths
It’s a joint study with Dr Félix de Tombeur, a scientist from the University of  Montpellier in France, and Professor Martin Saunders and Professor Peta Clode, from the University of Western Australia. The resulting paper,  How important is carbon sequestration in phytoliths within the soil? was recently published in the journal Plant and Soil in May 2024.

For the research we used state-of-the-art microscopic equipment in a laboratory in Australia. Carbon has such a low atomic weight that previously, without this equipment, we weren’t able to look at carbon composition in phytoliths. The method involves firing electrons at the material which give off X-rays of particular wavelengths that are specific for the elements present (including carbon and silicon). The more X-rays the more carbon and silicon in this case.

Our latest research, using these sophisticated electron microscopes showed that there is 15-20% carbon stored in cell wall phytoliths. This may mean that phytoliths could provide an effective way of removing carbon from the atmosphere. 

So what needs to happen next? 
We know that phytoliths last hundreds or thousands of years in the soil, but we don’t know how many of them last that long. That will be the next part of the puzzle we need to crack to determine how effective phytoliths are in carbon sequestration. 

This will involve multidisciplinary teams of researchers all bringing their particular expertise to bear on the problem. It will need laboratory investigations and some in fields or forests. We will almost certainly require computer modelling to determine the percentage of phytoliths that survive and are useful in carbon sequestration.

If phytoliths are proven to be effective for storing carbon, what will that look like in practice?
There are many possibilities. Parr and Sullivan's work suggested that some cereal varieties sequester more carbon than others. We might even be able to breed plants that are better at carbon sequestration. 
There has been much interest in recent years in conservation agriculture, leaving the soil covered with plant material at the end of the season. If that plant material contained a lot of phytoliths, they could be naturally incorporated into the soil.

Very recently, Graphyte, a start-up company in the United States, suggested burying bricks of plant material to sequester carbon very cheaply in the soil. The story was covered by the Washington Post. Maybe if the plant bricks contained lots of phytoliths they would sequester carbon for longer? Who knows!

You were working on phytoliths long before carbon sequestration was discovered. What else have they been used for?
The first project I worked on at Bangor University was a collaboration with the Imperial Cancer Research Fund, looking at whether plant silica could act like asbestos and cause oesophageal cancer. This was never proven, but my career has involved work in a range of other areas. These include agriculture, archaeology, palaeoecology, Belgian beer, and now climate change. Although phytoliths seem like a single concept, they have many aspects to them. They are so tough and hard that plants use them as a defence against predators. 

There’s a real interest from an agricultural perspective. I’ve been to China and Japan where there are big industries in silica fertilisers as silicon is essential for rice production. Rice and cereals are particularly effective at producing phytoliths. The cereals include the barley used in Belgian beers. I worked with Belgian scientists on a paper about the silica in beers as it’s really good for human bones. The biogeochemical cycle of silica, how silica is transported through the Earth’s systems, is also getting a lot of attention. 

What other environmental work are you involved in?
I’ve been heavily involved in the Christian environmental movement for many years with my wife, Rev Margot Hodson, an environmentalist and Anglican priest. Margot holds a first-class theology degree from Oxford Brookes.

We have written many books on environmental issues together. We both work for the John Ray Initiative - an educational charity with a vision to bring together scientific and Christian understandings of the environment. I am also the Principal Tutor of Christian Rural and Environmental Studies, a distance-learning course looking at these issues.