Green cover

400-360mya

Green Covering

At the end of this time - around 360-350mya, the land was covered, at least in parts, by green plants. This was mainly mosses and ferns that provided both cover and two new moist environments - one 'above' ground in the moss and fern plants and now also 'underground - their rhizosphere..

These new environments encouraged small arthropods like springtails and hence loads of their predators, as well as small segmented worms, smaller than earthworms but larger than nematodes.

You can see what is happening today to get an idea of what could have gone on then, by looking at the first dozen photogrpahs. (Zanella et al 2018) showing how soil could have started to grow on top of existing rocks.

Devonian plant hypothesis

The evolution and geographical spread of trees with deep, complex rooting systems and their role in the development of soils is widely regarded as the ‘Devonian engine’ that drove major changes in global biogeochemical cycles as the planet became increasingly forested. This is known as the ‘Devonian plant hypothesis (Berner 1997; Retallack 1997; Algeo and Scheckler 1998; Beerling and Berner 2005; Beerling 2007) "It has recently been hypothesized that the rise of forests enhanced the biotic processes that promote pedogenic clay mineral production, as vegetation reduced soil erosion and the rapid loss of clays, while organic matter debris retained cations and increased fluid residence time. In effect, forests act as ‘clay mineral factories’.The ‘clay mineral factory’ (CMF) hypothesis requires that the kinds of clay minerals formed in well-drained soil profiles become more complex in nature, origin and organization as the ‘factory’ processes become more advanced and efficient over time“ (Morris et al 2015)

“The Devonian Plant Hypothesis (DPH) was the first concept to offer a full and logical explanation of the many environmental changes associated with the evolution of trees/forests that took place during this time period"(Pawlick et al 2020)

The DPH tests the “graphical spread of trees with deep, complex rooting systems and their role in the development of soils and major changes in global biogeochemical cycles due to the forests. Molecular clock evidence indicates that the rooting systems of land plants co‐evolved and maintained symbiotic arbuscular mycorrhizal (AMF) associations, (Morris et al 2015)

Plants & soil evolution in this period

“It has recently been hypothesized that the rise of forests enhanced the biotic processes that promote pedogenic clay mineral production, as vegetation reduced soil erosion and the rapid loss of clays, while organic matter debris retained cations and increased fluid residence time. In effect, forests act as ‘clay mineral factories’. The ‘clay mineral factory’ (CMF) hypothesis requires that the kinds of clay minerals formed in well-drained soil profiles become more complex in nature, origin and organization as the ‘factory’ processes become more advanced and efficient over time “ (Pawlick et al 2020)

“The success of the archaeopterid pro-gymnosperms may have been due to their ability to create and maintain favourable microenvironments. Mass-shedding of their leafy lateral branch systems would have produced deep litter mats that affected soil moisture, pH, and humic content… The first large root systems increased to 80-100 cm as archaeopterids spread during the Frasnian & Famennian…Arborescence was accompanied by development of larger root systems that left distinctive `signatures' in contemporaneous palaeosols" (Algeo & Sheckler 1998)

This landscape was to spread in the next period

Vital process

The vital 3-way symbiosis between roots, fungi and springtails is the most important building process of the soil had started to develop. All else has been built upon it, and it is still today one of the most vital life processes on earth. Roots of plants produce exudates which make their surrounding attractive to Arbuscular Mycorrhizal Fungi (AMF). Root exudates may act as signals for microbial recognition and can also indirectly suppress pathogens by attracting antagonists of plant pathogens. AMF are now found on 80% of plants roots. Carotenoid-derived molecules signal after release from roots that enable mycorrhiza to detect their host plants; they have been identified in AM fungi and can induce AM spore germination and hyphal branching.

The fungal filaments provide the roots with much more water and nutrients particularly phosphates. In return the roots provide the energy for fungi which produce glomalin. This leads to one of the most significant soil processes, and I have called it glomalisation, Glomalin was discovered about 30 year ago, which makes up a quarter of all soil carbon, and is the main food for springtails, often around 100,000 per sqm. This vast number chew the glomalin and poo it out as glomalin related soil proteins (GRSPs), thus spreading the carbon compounds, captured in the leaves conducted into the roots, throughout the soil. The springtails also help distribute the fungal spores and keep the roots clean of dead fungal matter. GRSPs are gluey substances which bind together various mineral and vegetable particles to make the aggregates.

Aerobic

This is the essential building block for soil and is made aerobically - it needs air to breathe in the soil. Most of these early processes would have been aerobic – needing oxygen. Any decomposition, by saprophytic fungi is also aerobic. Also, each of the three key characters, springtails, mites and worms, seems to be ‘generalistic’ in that it can make do with a wide variety of hosts, rather than specific to a particular group. This has enabled adaptations changes over the years. These three key characters are all over the world, and can be considered living fossils This tri symbiotic process is much the same today, while roots and environmental conditions have changed. It is probably the most important process ever of life, having evolved all this time ago, adapted throughout hundreds of millions of years, and today hold the key to improve soils.

Relations

The new found relations between roots, bacteria, fungi and springtails helped disperse life across the surface of the planet, greening it as it went along. The creatures had lots to live on, including themselves. The substrate would have been lacking some key ingredients for plants – like potassium, making them dependent on fungi, and possibly cyanobacteria.

When you read about the evolution of organisms on land, many records say they ‘go back' to Devonian times. Yet when you check out the evidence there is often very little, nor any explanation as to how those organisms lived at this time. We have seen that despite much work on morphological or molecular evidence, there is still much debate about what evolved at this time. Here we try and clarify that be looking more at the environment the creatures live in - both then and now.

Green covering

It seems that the ground in various places was covered with moss and fern growth, but will little breakdown of their debris. This would have led to some places thick with plant debris, following from the green covering. This would have looked like peat does to this day. You could imagine mosses on peat – much like sphagnum moss on peatbogs today. Basically the ‘soil’ then is the equivalent of what we now call Horizon O. The first paleosol found at this time was a histosol - again indicating peaty conditions - not humified matter. (humus).It also means the later developments of soil must have happened below the green cover and litter layer – without affecting either.

“The first terrestrial ecosystems are not thought to have been based on plants of a moss-like grade of organisation, mats of algae, crusts of lichens and cyanobacteria, or even on plants unlike any alive today. These mats were probably colonised by air breathing arthropods long before the arrival of vascular plants" (Shear 1991)

“About 367 million years ago, during the Late Devonian period, a powerful force caused a mass extinction on Earth. It is estimated that some three-quarters of all living species did not survive. What caused this massive loss of species diversity, one of only five such crises in geologic history? Was it global cooling? A catastrophic impact, or several impacts? Palaeontologists cannot agree.

Based on two decades of research this book reviews the many theories that have been presented to explain the event, considering in particular the possibility that the extinction was indeed triggered by the multiple impacts of extra-terrestrial objects”.

Which of our new found soil friends survived? What changes occurred to others?

We shall see..

Palaeosols

Alfisols are moderately leached soils that have relatively high native fertility. These soils have mainly formed under forest and have a subsurface horizon in which clays have accumulated.

"Alfisols and spodosols, which develop primarily (though not exclusively) under modern forests, appeared in the Late Devonian to early Carboniferous, respectively" (Retallack 1986, 2003).