Can Phytoliths Save the World?

Can Phytoliths Save the World?

Or: Plants do Carbon Capture and Storage Quite Naturally

Can Phytoliths….. Can what?? Many people have never heard of phytoliths, but I have worked on them for nearly 40 years, so let me tell you a little about them, and then I will describe some of my recent research. Yes, these unheard of phytoliths might help in the fight against climate change. But read on.

Pampas grass

Have you ever cut your hand pulling up grass? Have you noticed how sharp the edges of Pampas grass are? 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. If you know a little Greek you will know 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. So if you had a microscope you would be able to see that the edges of Pampas grass had 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). But the phytoliths are much more resistant to breakdown in the soil and can persist for hundreds or thousands of years. Because they last a long time, and have shapes and sizes that are characteristic for the plants they come from, phytoliths are used by archaeologists and palaeoecologists to work out what people grew and ate, and past environments and climates.

There is increasing interest in carbon sequestration in soils. It is recognised that the soil is a huge carbon store and that if we could find ways of increasing that storage then it could really help to suck carbon dioxide out of the atmosphere. But one of the big problems with this idea is that carbon sequestration in the soil is reversible. So once plant materials enter the soil and form humus it is susceptible to breakdown, releasing the carbon dioxide back to the atmosphere.

Back in 2005 some Australian phytolith experts, Parr and Sullivan, had a brilliant idea. They realised that phytoliths encapsulate carbon within their structures. According to their calculations, phytoliths store a lot of carbon in the soil and potentially sequester it within the silica for a very long time. Their paper and their idea created a whole new area of phytolith research. The idea has not been without controversy, particularly over how much carbon can be stored in phytoliths. Nonetheless, the area remains a major focus for phytolith research.

In the 1980s I spent a long time looking at how phytoliths developed within the plant, and I have kept up this interest, publishing a major review on the topic in 2016. Plants have two main types of phytolith: those developing in the cell lumen; and those that form in the cell wall on a carbohydrate (largely cellulose) matrix. If you did biology at school you might remember the cell wall as a kind of box around the lumen. The lumen contains the cytoplasm and all of the organelles, including the chloroplasts, nucleus etc.

I was invited by the Frontiers journal organisation in 2017 to be a guest associate editor for a special collection of papers, "Frontiers in Phytolith Research". I assembled an editorial team of experts from around the world and started to invite potential authors. But what would I write my own paper on? I decided that I wanted to look at one aspect of the carbon sequestration in phytoliths story that I felt had been neglected. Which types of phytolith are most important in storing carbon in the soil? Is it the cell lumen or the cell wall types?

Wheat inflorescence phytolith

To answer this question I needed to consult a wide range of literature. I began by outlining the history of carbon sequestration in phytoliths and discussing the major methodological controversy over how much carbon they contain. Next, I wanted to determine exactly which phytoliths were cell wall phytoliths. In most cases, it is pretty obvious, but there is one important type where we lack clarity. Then I looked at what is known about carbon concentrations in the two types of phytolith, and not surprisingly the cell wall types have much more carbon than the lumen types.

I then needed to find out what happened to the two types of phytolith in the soil. The received wisdom is that cell wall phytoliths break down faster in soil than lumen types. But I carried out a major literature survey looking for evidence to support this contention, and I couldn't find any! Moreover, when I investigated the archaeological and palaeoecological literature I found that cell wall phytoliths were present in a wide range of contexts and could be found in samples that were thousands of years old. Having done all this, I then constructed two hypotheses: one to consider what happens to phytoliths when they are prepared in the laboratory (this also addressed the question of how much carbon is stored in phytoliths); and the second concerned what happens in the soil.

Can phytoliths save the world? Probably not! But I think we need to look far more carefully at the rather neglected cell wall phytoliths. As I say in my paper, phytoliths are unlikely to be a "silver bullet" for climate change, but they may have a role to play. 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? Can we find ways to increase carbon sequestration in phytoliths and in soils? In my paper, I have outlined a whole lot of work that we need to do over the next few years. Let's get on and do it!

Martin J. Hodson
(July 2019)

Images:
1) Pampas grass image- Shirley Hirst on Pixabay:
https://pixabay.com/photos/grass-pampas-grass-pampas-56993/
2) Wheat inflorescence phytolith- MJH

The Fourth Edition

The Four Editions!

I first arrived in Oxford Polytechnic (now Oxford Brookes University) in September 1989, and for the first few years I shared an office with ecologist, Peter Morris. I was hired to teach plant physiology, but I soon found myself teaching soils as well. Not long after I arrived, Peter started working on a new Master's course in Environmental Assessment and Management. This was to be a joint course with the Department of Planning, and I remember Peter had lots of meetings with Riki Therivel who was his joint course leader from Planning. In September 1990 the first cohort were recruited, and Peter, Riki and I all taught on the first day (28^th September). I was involved in quite a bit of the course in the early days, but specialised in soils. Basically, what students needed to know were the likely impacts of developments on soils and how to mitigate them. The course was a great success. Fairly soon the idea for a textbook arose with chapters from many of those teaching on the course. Naturally, Peter and Riki were the co-editors, and they asked me to write the chapter on “Soils and Geology”. The first edition of “Methods of Environmental Impact Assessment” came out in 1995 (white cover) and my chapter was 18 pages long. The book seemed to fill a niche in the market, and soon there was talk of a second edition. Peter and Riki were keen to give the second edition a more applied feel and asked the authors to recruit some environmental impact assessment practitioners as co-authors. Peter recruited a friend of his, Roy Emberton, to help me with contaminated land issues. I recruited Chris Stapleton, a soils consultant who specialised in the assessment of agricultural land. Chris had taken a Planning course at Brookes including an option with my soils classes, and I asked him to come and teach with me (he still is!). So with such expertise available, I mainly concentrated on the more introductory soils material and on getting the whole chapter into shape, a role that I have continued to play in subsequent editions. The second edition was published in 2001 (blue cover), and the Hodson, Stapleton and Emberton chapter was now quite a lot longer (26 pages) and was entitled “Soils, Geology and Geomorphology.” Time moved on and I left the full-time staff at Oxford Brookes, but I continued to teach an annual soils session in tandem with Chris Stapleton for the Master's course. Chris and I were asked to take part in a third edition of the book. Roy Emberton moved to help Peter Morris with his Ecology chapter, so we needed someone to look after contaminated land issues. Chris found Kevin Hawkins of WSP Environmental, and we went to meet him in Reading. I suggested that Chris should be first author on the third edition, and I settled into the “senior author” position (last), which I felt was appropriate as my main role was one of coordination. In 2009 the third edition (brown cover) came out, including the Stapleton, Hawkins and Hodson chapter on “Soils, Geology and Geomorphology” (34 pages). Chris and I kept going with our annual session and whenever we met we wondered whether there would be a fourth edition. Then in 2015 we had the sad news that Peter Morris had died. I felt that would be the end of any hope for another edition. But Riki Therivel had other ideas and teamed up with Graham Wood in Built Environment to tackle quite an ambitious project. The publisher, Routledge, wanted the fourth edition to be rather different to the previous three. We had always concentrated on the UK and Europe, but now the book was to be international in scope. But no greater length than the third edition! This time Chris and I were joined by Hugh Masters-Williams of Jacobs UK who took on the contaminated land brief. Revising the chapter proved quite a challenge. I set it now in the context of the new Sustainable Development Goals (SDGs), and managed to include coverage of all the soil types in the world. And so the fourth edition (green cover) now sits on my coffee table. It has a somewhat expanded title: “Methods of Environmental and Social Impact Assessment”. The book is dated 2018 and is dedicated to Peter Morris. Without him there would have been no course and no book. The chapter on “Soils, Land and Geology” by Stapleton, Masters-Williams and Hodson is now 39 pages long, over double the length of my original 1995 effort. I would like to thank my co-authors, Chris, Roy, Kevin and Hugh for their contributions to the project over the years, and our editors Peter, Riki and Graham for their endless patience.

Now let's see. The average time between editions is about seven years. That makes 2024 for the fifth edition. Should just about be functional by then……

Martin Hodson