Published on March 16th, 2021 | by Elsa Panciroli


Episode 123: Soil

Terrestrial life as we know it couldn’t exist without soil. Soil is a layer of minerals, organic matter, liquids, gasses and organisms that not only provides a medium for plant growth, but also modifies the atmosphere, provides a habitat for animals and retains and purifies water.

However, this kind of soil hasn’t always existed, so in order to understand early conditions on land, we first need to understand what can be constituted as a soil and when these first appeared. Is there soil on the Moon? Can soil fossilise?

Since most terrestrial ecosystems are rooted in soil, if we want to understand how life established itself on land, we need to know how soils form, how they have changed over geological time and which kinds of plants and fungi can live without it.

Joining us in this episode is Dr Ria Mitchell, Experimental Officer in X-ray Computed Tomography at the University of Sheffield, UK.

Regolith is a layer of loose material covering solid rock. It is essentially composed of rock debris. If a regolith contains organic matter it can be considered a soil, although this is not always the case and is largely dependent on how a soil is defined. Regolith can be found on Earth, the Moon, Mars and even asteroids!
Image: Buzz Aldrin’s footprint in the lunar regolith/soil.
Back on Earth, we can find the first evidence for fossil soils as early as the Paleoarchean Era, around 3.46 billion years ago. These fossilised soils are termed ‘palaeosols’ and the earliest of these were more comparable to regolith, with little organic content compared to modern soils.
Image: Ria on fieldwork in the Upper Peninsula in Michigan, USA, in the Sturgeon Falls river, hunting for 1.1 billion year old palaeosols.
It was only later in the Paleozoic when plant life became established on land that large quantities of organic content was added to the soil.
Image: On fieldwork in Iceland looking at modern analogues of the 407 million year old Rhynie chert – a silicified and exceptionally preserved geothermal wetland, the oldest preserved plant-based terrestrial biosphere.
Cryptogamic organisms are those that reproduce using spores. Such organisms are capable of living on solid rock, or without a well-developed soil. As such, they are ‘pioneer species’, helping to form the first soils and keeping them consolidated (held together).
Image: Example of a moss and lichen dominated cryptogamic ground cover from Iceland.
Lunularia cruciata, a thalloid liverwort, is another example of a cryptogamic plant. Instead of roots, as we see in vascular plants, liverworts have single celled ‘rhizoids’.
Example of a moss and lichen dominated cryptogamic ground cover from Iceland. From images such as this one, it is easy to see how cryptogams can start to produce a soil on top of barren rock.
Example of an X-ray computed tomography (X-ray CT) scan through a moss cryptogamic ground cover. Green shows the surface plants, red the high density volcanic soil grains. Each of these can be digitally segmented based on their different densities. Here, you can see how the moss is able to hold together the soil on top of the regolith.
Ria’s research focusses on using modern plants and their role in the soil formation process as proxies to similar organisms and processes in the geological past.
Image: Structural differences observed from X-ray CT scans of cryptogamic ground covers in volcaniclastic (a‐c) and geothermal (d‐f) soils. Each soil is dominated by a different type of plant: liverworts (a), mosses (b, e), hornworts (d) and mixed (c, f). From Mitchell et al., (2021), Geobiology.
Potential extent of various cryptogamic ground covers through the geologic past. From Mitchell et al., (2021), Geobiology.

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