Plant-soil interactions: the cycle of life

Last spring, I gave a TED talk about plant-soil interactions and their importance in the global carbon cycle at a TEDx event organised by Amsterdam University College. You can watch the video below, but for those of you who rather read (actually, I am one of those people, as I never have the patience to watch a video from beginning to end!) you can also read the full text below.

Do you ever think about soils? Do you ever think about soils, other than, when your boots are muddy, or your vegetables dirty? Well, I’m going to talk about soils.

Soils! Without soils, we would not be here. Soils sustain all life on land. And that is because all energy flows through soils, via photosynthesis and respiration.

Have soils always been here?

No!

Have you ever thought about how soils are formed? Where plants came from? And the tiny invisible microbes that live in the soil?

More than 4.5 billion years ago, there was no soil. There wasn’t even life. There were only oceans. But somewhere between 4.5 and 3.5 billion years ago, the first microorganisms appeared in the oceans. There wasn’t even free oxygen at that time! But then, photosynthesis evolved in bacteria, and cyanobacteria started producing oxygen around 2.7 billion years ago. About 1.5 billion years ago, the first fungi appeared, and much later, around 500 million years ago, the first land plants arose. Probably, photosynthesis in these plants was derived from photosynthetic bacteria inside plant cells (the endosymbiosis theory). Those first land plants – like this little liverwort – had no, or very rudimentary roots (remember, there was no soil that they could grow their root in, only rock!), and were likely helped on land by symbiotic fungi. 

And this is where soil started to form. 

An extremely dry summer in Manchester

Right at the end of the extremely dry period we had this summer I decided to do a little experiment: I started taking a photo of the patch of grass in my street in Manchester, in the North West of England, every couple of days. I study the effects of drought on ecosystems (see my previous posts about the effects of drought belowground here and here) and I thought it would be nice to show how the grass in my street would bounce back after the rain had started.

Only… it didn’t. The rain did not come as intensely as I expected, and the grass did not bounce back as quickly as I expected. The first (top left) photo was taken on the 12^thof July, the last (bottom right) on the 20^thof August. And still you can see bare soil and brown patches! This patch of grass would look a lot lusher and greener during a normal Manchester summer.

The grass in my street in Manchester this summer. 

But, more importantly, while aboveground plant growth seems mostly recovered, the composition of the community has changed (which you can’t see in these photos), and as I’ve shown in my research, this might continue to affect belowground communities and the processes they perform.

Of course, this little patch of grass in Manchester is not that important for the functioning of our ecosystems. But it is a nice illustration of the impacts of an extremely dry summer on grassland and how long it takes for these fast-growing plants to regain their biomass!

What is the current weather doing to our soils?

There has been plenty of media coverage of the current extremely hot and dry weather. The drought is revealing archaeological features, (see also here), and we can even see the browning of our landscape from space. But this drought is not good news for our ecosystems at all, and one example of that are the recent wildfires in the Peak District. These fires are not just bad news for the plants and animals that live there, but also they make large amounts of carbon that have been sequestered over many years go up in the air as CO₂, and this can amplify climate change. Drought also affects our ecosystems more subtly than that, but the long-term consequences might be as severe.

We can all see the effect the drought is having on plants: lawns are turning yellow, corn leaves are rolling, and in extreme cases, trees even lose their leaves. They become inactive, and can even die. This is not just bad news for the plants, it is also bad news for the soil. When plants stop growing, they are not photosynthesing, and when they are not photosynthesising, they are not removing CO₂from the atmosphere. A large part of this photosynthesised CO₂ goes straight into the soil to fuel the activities of microbes, which carry out important functions such as the decomposition of plant litter and the release of nutrients for plant growth.