Soil Survival

66-0 mya Cenozoic Era

Plants Grass-Herbivore Soil animals Ground-dwelling beetles

The role of soil in the delayed diversification of flowering plants between the mid-Cretaceous and early Cenozoic eras is multifaceted. Soil conditions and changes influenced nutrient availability, the evolution of new soil types, plant-soil feedbacks, microbial interactions, and biotic relationships, all of which played crucial roles in shaping the patterns and timing of flowering plant diversification. This interplay between the evolving soil environments and the adaptability of flowering plants to these conditions drove the complex, uneven diversification that we observe in the fossil record. There was a massive increase in grasses, forming prairies, which impacted on the soil.

Extinction

What do we know about soils and the changes in and on them? What other plants and animals went extinct, and what effect did that have on soil ? Lots of animal and plants overground were destroyed but some survived. Which ones survived below and how were they protected?

Fire

We can get some clues as to the impacts on soil of fires then, as we have a lot of recent work looking at the impact of fires on soil biota. That’s because there are more fires caused by global warming. It is probably that there were different responses between the area around the asteroid and soil conditions on the other side of the world.

Elsewhere

So what survived ‘in’ land? It must be quite a different story, but not one that has been investigated very much.

Reduced microorganism biomass

“Fire reduced microorganism biomass and abundance by up to 96%. Bacteria were more resistant to fire than fungi. Fire reduced nematode abundance by 88% but had no significant effect on soil arthropods. Fire reduced richness, evenness and diversity of soil microorganisms and mesofauna by up to 99%. We found little evidence of temporal trends towards recovery within 10 years post-disturbance suggesting little resilience of the soil community to fire. Interactions between biome, fire type, and depth explained few of these negative trends" (Pressier et al 2018)

Soil communities can be impacted by fire both directly through mortality during the disturbance and indirectly through physical, chemical, and biological changes to the soil environment. There will be organic matter loss through combustion, modification of organic matter quality through the addition of pyrogenic organic matter, alterations in plant community composition and subsequent inputs loss of soil structure changes in soil moisture dynamics and increases in pH

Soil web survived

It seems clear it did not kill most of the existing soil web of life. But which ones were decimated and which survived? We could imagine the ones living deeper – the oribatids would have survived n the main whereas the more superficial – Sminthurid springtails for example would have evaporated in the vicinity of the collision. But what about elsewhere?

“Even though soil biota play a key role in maintaining the functioning of ecosystems, the vast majority of existing studies focus on aboveground organisms. Many questions about the fate of belowground organisms remain open, so the combined effort of theorists and applied ecologists is needed in the ongoing development of soil extinction ecology" (Veresoglou et al 2015) So let’s get on with it.

Susceptibility to Extinction

For any form of conservation we need a basic understanding of the susceptibility to extinction. For soil organisms this issue so far has been dealt with only cursorily compared with the aboveground world. The state of soil extinction ecology reveals a worrying paucity of existing information. However, the challenge of understanding and modelling belowground extinction also opens up a rich frontier of opportunities for future research.

Microbes

More sensitive than bacteria

It is a widely held view that fungi are more sensitive to fire than bacteria, and this may be due to both the lower thermal tolerance of fungi and, for mycorrhizal fungi, the mortality of plant hosts during fire. Unlike fungi, bacterial richness is not significantly different between burned and unburned sites. The meta study indicates that the effect of fire on soil mesofauna, like nematodes and arthropods, is weaker than for microorganisms, bacteria, fungi, and microbes. Microbes possess an arsenal of ‘ silent’ genes that allow them to cope with environmentally adverse conditions for extensive periods. Soil microbial taxa can also pursue very divergent lifestyles, like being a root-parasite and a decomposer of soil fungi or of a nitrogen fixer. Microbes came through the extinction 250 mya, and could survive this one.

Arthropods

Highly resilient

Soil arthropods appear to be highly resilient to fire as their abundance did not significantly increase or decrease with fire. Soil arthropod richness, evenness, and diversity did decrease significantly with fire. “Soil arthropods are more agile than other invertebrates and have the ability to move about soil pore spaces in search of more favourable conditions…Some taxa (e.g. oribatid mites) are more heavily armoured than others (e.g. Collembola), which results in greater protection against increased temperatures during fire. Soil arthropod communities may be resistant to fire because they are able to either withstand or escape high soil temperatures by migrating to unburned patches or deeper into the soil…may serve as colonizing populations for recovering arthropod communities…the resistance of soil arthropods to fire may result in an increase in top–down pressure on their microbial prey that may exacerbate microbial sensitivity to fire and contribute to their limited recovery…Our analysis found that fire reduces arthropod richness to a greater degree in grasslands than in forests" (Veresoglou et al 2015)

Microhabitat

The nature and structure of soil also gives rise to unparalleled microhabitat complexity with numerous physiochemical gradients, like aerobic and anaerobic microhabitats. They can occur within a few dozen micrometres of each other, due to diffusion limitation in pore networks. Natural CO2 levels in the soil atmosphere are much higher than in the air above, because of soil biological activity (including root and microbial respiration). Also, soil offers temperature insulation, and at the microscale it may be partially insulated against drought events through capillary water reserves. Thus, the features of the soil micro-habitat poses a range of considerations for the ecologist delving into belowground extinction susceptibility.

Extinction Ecology

“Considering the fabric of belowground habitats in extinction ecology is an exceptionally important point. The nature and structure of soil also gives rise to unparalleled microhabitat complexity with numerous physiochemical gradients, for example, aerobic and anaerobic microhabitats can occur directly adjacent to each other (that is, within a few dozen micrometres) due to diffusion limitation in pore networks. The resulting high complexity of the soil landscape could render soil biota considerably more resistant to change than aboveground organisms, as suggested in thelandscape-moderated insurance hypothesis. In addition, soils may be insulated against many drivers of climate change, including drought, warming and extreme events. For example, natural CO2 levels in the soil atmosphere are much higher than in the air above, because of soil biological activity (including root and microbial respiration) coupled with gas diffusion limitation. Also, soil offers temperature insulation, and at the microscale it may be partially insulated against drought events through capillary water reserves". (Tscharntke et al 2012)

Specialist v Generalist

Have we yet seen any soil-dwelling organisms go extinct? Surface ones yes, but certainly not the main groups of springtails and mites, many existing ones looking like 'living fossils'. Many organisms that existed hundreds of millions of years ago are related to living species. Soil biota persistence may be tightly linked to hosts plants through symbiosis. Others such as soil decomposers may not be so host specific, but have evolved a high affinity to specific compounds, like white-rot fungi and lignin. They could be susceptible to high-impact events that can modify the availability of their substrate.

Asexual answer

An additional difference is the fact that most groups of belowground taxa can propagate asexually. Bacteria, fungal pores and oribatids too fit this category. Asexual organisms need only a single individual to make a viable population, and require less energy to procreate and perform sexual behaviours. Furthermore, specifically for microbial taxa, the fact that they can be extremely physiologically and functionally versatile makes for difference from ng aboveground extinction ecology. Differential extinction susceptibility is influenced by three particularities of belowground food webs, their extreme population sizes, physiological versatility; and high adaptation potential.

Specialised species more prone to extinction?

While many soil fauna are omnivores with little apparent specialisation, some are more specialised when it comes to food. A ‘pervasive idea in extinction ecology’ is that highly specialised species may be more prone to extinction than generalists because their persistence additionally depends on the persistence of their prey. "In general, fungal feeding decomposer animals appear to be food generalists rather than specialists." (Maraun et al 2003), pretty important for soil ecosystems. Similarly 'endosymbiotic' soil microbes could also be more susceptible than free-living microbes, but they may work mutualistically to survive. (Lombardo 2007).

After the extinction

The earth underwent significant climatic changes, which influenced soil development and distribution. These changes, such as increased rainfall or shifts in temperature, led to the formation of new soil types and the redistribution of existing ones. The development of new soil types, like podzols and laterites, formed under certain climatic conditions (such as high rainfall and warmer temperatures), which could favour certain flowering plant families adapted to those soils.

Grasslands/Prairies

Soils that form under prairies (mollisols) differ from those that develop under forests (alfisols). The major difference between forest and prairie soils is the thickness of the zone of organic accumulation - carbon from living organisms.

“This zone starts at the soil surface. In forests the zone is very thin, consisting of leaf litter and the thin layer of developing soil beneath it. The leaf litter is acidic, and nutrients are leached through the organic zone. Clay is also leached into the deeper layers in forest soils.

In prairies the organic layer is thick (deeper) because there is more litter. The clay content is higher in the surface layers and the nitrogen and carbon content is also higher than that of forest soils. Forest soils contain less nitrogen and carbon than prairie soils; and are therefore less fertile than prairie soils.”

The spread of grasslands following the fires would have affected soil development...

Early (50-20mya)

Formation of Grasslands and Early Soil Development (50–20 Million Years Ago)

Climate Changes During the Cenozoic Era: Around 50 mya, during the Eocene epoch, the Earth’s climate was warmer and more humid, with extensive forests. As global temperatures gradually cooled, and arid conditions began to develop in certain regions, grasslands started to emerge. This process accelerated during the Oligocene (around 30 mya) when further cooling led to the spread of open habitats.

Vegetation Shift and Organic Matter Accumulation: The transition from forested to open grassland ecosystems introduced new types of vegetation, primarily grasses with extensive, fibrous root systems. Unlike trees, grasses shed leaves annually, decomposing into organic material that enriched the soil. This organic matter accumulation provided the initial nutrient base for grassland soils, promoting the development of soil structure and fertility.

Changes in Soil Structure: The shift in vegetation also led to significant changes in soil structure. Grasses' dense and fibrous root systems created a thick mat close to the soil surface, stabilizing it and reducing erosion. The decomposition of roots and other plant materials contributed to forming a nutrient-rich upper soil layer (A horizon), which is vital in grasslands today.

Later 20mya - now

Development of Grassland Soil Types (20 Million Years Ago to Present)

Formation of Mollisols: One of the most significant developments in grassland soils was the formation of Mollisols. Mollisols are dark, fertile soils rich in organic matter, characteristic of temperate grasslands like the prairies of North America, the steppes of Eurasia, and the pampas of South America. These soils are typically formed under prairie vegetation with a deep root system that continually adds organic material to the soil as plants grow, die, and decompose.
Key Characteristics of Mollisols:
- Thick A Horizon: Mollisols are known for their thick, dark-colored topsoil layer (A horizon), rich in organic carbon. This is due to the continuous decomposition of plant roots and surface litter.
- High Cation Exchange Capacity (CEC): Mollisols have a high capacity to retain essential nutrients like calcium, magnesium, and potassium, which makes them highly fertile.
- Good Structure and Drainage: The combination of organic matter and clay particles gives Mollisols a granular or crumb structure, which allows for good water retention and drainage.
Other Grassland Soil Types: Besides Mollisols, other soil types also developed in different grassland regions:
- Vertisols: Found in tropical and subtropical grasslands (savannas), these soils have a high clay content that swells and shrinks with moisture, leading to deep cracks in dry conditions.
- Alfisols: Present in grasslands with slightly more precipitation, Alfisols have a thinner organic layer than Mollisols but still support grassland vegetation.
- Aridisols: In arid and semi-arid grasslands (like deserts transitioning to grasslands), these soils are typically less fertile, with limited organic material and more mineral content due to low decomposition rates.

Factors

3. Factors Influencing Grassland Soil Evolution

Climate and Precipitation: Precipitation is one of the most critical factors influencing grassland soil evolution. Grasslands typically form in areas with moderate rainfall — enough to support grasses but not dense forests. The balance of moisture affects soil organic content, mineral leaching, and overall fertility.

Vegetation and Root Dynamics: The development and persistence of grassland soils are closely tied to the characteristics of grassland vegetation. Grasses have extensive root systems that promote organic matter accumulation in the soil. This root mass also helps stabilize the soil and prevent erosion, allowing for the gradual development of fertile, well-structured soil horizons.

Biological Activity: Soil evolution in grasslands is significantly impacted by biological activity. Earthworms, fungi, bacteria, and other decomposers break down plant material and contribute to the organic content of the soil. This activity also enhances soil structure by creating pore spaces for air and water.

Parent Material: The geological substrate, or parent material, from which the soil forms, affects the soil's mineral content and texture. In grasslands, parent materials may include loess (wind-blown silt), volcanic ash, glacial till, or weathered rock. These materials contribute to the soil's overall properties, including nutrient availability and drainage capacity.

Burrowing beaver

A burrowing beaver may tell us something about the soil. Volcanic soil helped the preservation of Palaeocastor a bit like a burrowing beaver. Palaeocastor burrow walls were densely infiltrated by grassland plant roots throughout their entirety, as the burrow provided an optimal micro-habitat. The root mats helped stabilize the burrows while beaver activity kept the roots in check. If a burrow was abandoned, it was quickly infilled with roots, seen fossil burrows.
The volcanic soil provided a source of amorphous silica which, dissolved ground water, was readily absorbed by the grassland root systems for deposition throughout the burrow system “probably resulted in a very rapid partial lithification of beaver burrow walls", perhaps within a few years of the death of the plants occupying this microhabitat.

This burrowing beaver may well have gone for a wide range of ground-dwelling beetles, insect larvae and worms, all now more prevalent in the upper layers of richer soil.What the soil animals are getting up to.

“Consistent with the relative importance of belowground biomass, burrowing activity by animals appears to be greater in grasslands than in other ecosystems, although the difference is difficult to quantify. There are many observations of remarkably extensive burrowing by earthworms, insects (especially ants and cicadas), and rodents in North American and Eurasian grasslands. Termite-burrowing and mound construction is extensive in tropical and subtropical savannas and grasslands." (Mason & Zanner 2005)

Let's look at the grasslands and herbivores

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