Climate Change and Carbon Sequestration in Soils: Can Phytoliths Help?

In October 2018 the Intergovernmental Panel on Climate Change (IPCC) released its report on the feasibility of us keeping under the 1.5°C target suggested by the United Nations at the Paris climate change meeting in 2015. The IPCC gave the world around 12 years to radically cut carbon emissions or we will face very serious climate disruption. However, they were not only interested in cutting the burning of fossil fuels, but also in “negative emissions”, how we can take carbon dioxide out of the atmosphere. Plants and soils are huge carbon stores and are getting a lot of attention as a result.

On 5th July 2019 Bastin et al. published a paper in Science advocating a massive global tree planting exercise to help solve the climate change problem. The paper caused some controversy. Everyone agrees that planting trees is a good thing, but the statement made in the abstract, “This highlights global tree restoration as our most effective climate change solution to date”, worried many climatologists. They felt that the best solution to climate change is to cut emissions. Top climate scientist, Stefan Rahmstorf, wrote a detailed response to the paper. Planting trees is a good idea, “But we must not fall for illusions about how many billions of tons of CO₂ this will take out of the atmosphere. And certainly not for the illusion that this will buy us time before abandoning fossil fuel use”, wrote Rahmstorf.

Globally the amount of carbon storage in soils is greater than in plants, about 75% of the global terrestrial carbon pool, and there is considerable interest in how we might increase sequestration and thereby take carbon dioxide out of the atmosphere. A key problem with carbon sequestration in soils is its reversibility. So soil organic matter is vulnerable to breakdown by microorganisms with consequent release of carbon dioxide back into the atmosphere.

In the same week as the Bastin et al. paper appeared in Science, with rather less fanfare, I published my paper on carbon sequestration in phytoliths in Frontiers in Earth Science. I have worked on phytoliths for nearly 40 years and have seen the field grow remarkably in that time, but still many scientists, even plant scientists, have no idea what a phytolith is!! Soluble silica is taken up by plant roots, and phytoliths are solid silica bodies that form in plant cells. More silica is taken up by grasses and cereals than most other groups of plants and they contain more phytoliths as a result. There are two main types of phytolith, those that form in the cell lumen and those that form in the cell wall on a carbohydrate matrix. The silica strengthens plants and acts as a defence against grazers. The phytoliths take the shape of the cells they are formed in and much work by the International Committee for Phytolith Taxonomy (ICPT) has focussed on categorising them. Phytoliths are resistant to breakdown in the soil. So they are routinely used by archaeologists and palaeoecologists to determine past diets, agriculture and environments.

In 2005 Parr and Sullivan, the Australians, suggested that phytoliths could sequester substantial amounts of carbon in the soil. If true, then sequestration would decrease the reversible nature of carbon storage in soils. But this idea has been controversial, mainly because different methods for preparing phytoliths give different values for the carbon concentrations in phytoliths. A high value would suggest that sequestration in phytoliths was important globally, but a low value would imply that it was insignificant.

Rather strangely, there has been very little consideration of which types of phytolith were most significant in sequestration. Was it the lumen types or those in the cell wall? In my paper I tried to find out. I first outlined the history of carbon in phytoliths, and the current debates surrounding this subject. I then investigated the literature to discover which phytoliths were cell wall types. It is often not easy to be sure using light microscopy, so I looked for studies using transmission or scanning electron microscopy and x-ray microanalysis. Many of these were published in the 1980s, often in the Annals of Botany, and rather frequently by me and my collaborators! The epidermis of leaves and stems is the major location, particularly in grasses and cereals, and primary cell walls, macrohairs, prickle hairs, and the wall protrusion of papillae are often silicified.

Next, I tried to work out the carbon concentrations in the lumen and cell wall phytoliths. There is very little data on this, but what there is suggests that cell wall types have a lot more carbon in them than lumen types. So are the cell wall types more important in carbon sequestration? The problem is that they are regarded as delicate and fragile and more susceptible to breakdown in the soil, or at least this seems to be the received wisdom. But when I investigated the literature I was unable to find any direct evidence for this assertion. Smaller phytoliths with a large surface area to volume ratio break down faster, but nobody seems to have looked at cell wall types and compared them with those from the lumen. I next turned to the archaeological and palaeoecological literature. What I discovered was that cell wall phytoliths can survive intact in soils and sediments for hundreds or even thousands of years. In two cases they had been found associated with dinosaur remains.

So at least some cell wall phytoliths can survive a long time in soil, and it seems quite possible that they contain substantial amounts of carbon, protected within their siliceous structure. So can phytoliths help in the fight against climate change? We do not know for sure, but it is at least a possibility that is worth investigating. In my paper, I outlined a number of research areas which urgently need tackling so that we can find out.