Most life-sustaining resources are supplied by Earth’s critical zone—the thin outer terrestrial layer of the planet that extends from the surface of unaltered bedrock to the top of vegetation and the atmospheric boundary layer. The critical zone addresses the critical dependence of humans on the terrestrial surface layer, which is under critical pressure from human activity and is a critical interface of Earth planetary systems. The critical zone exhibits dominant material, information and energy fluxes governed by land-atmosphere, soil-vegetation, shallow geosphere-lithosphere, and land-ocean interactions. The role of soil in the processing of energy, material, and biodiversity is essential to the critical zone provisioning of life-sustaining resources. Soil is alive and probably contains more unraveled biodiversity than any other genetic reservoir of the terrestrial biosphere. This soil life is organized into a highly complex web.
Within the critical zone, soil plays a central role as a dynamic, porous interface that connects the atmosphere, vegetation, shallow geosphere, groundwater and surface waters. Through the development of soil structure, soil horizons form a reactive, porous interface that transforms and transmits flows and transformation of material, energy, and genetic information between atmosphere, biosphere, hydrosphere, and lithosphere. The geographical distribution of soil microbial biodiversity is influenced by environmental habitat factors, including plant cover, soil physical-chemical properties associated with soil type soil pH and land use. Mechanistic linkages across the soil surface between soil microbial and vegetation biodiversity include organism transfer and plant-rhizosphere signaling or— indirectly—the selective pressures exerted by changes in the vegetation and soil communities or in environmental conditions.
Bacteria and fungi feed on organic residues and plant root exudates; protozoa and nematodes feed on the bacteria and fungi; mites and insects feed on all of the above and on each other; and earthworms ingest soil and decomposing organic matter, absorbing nutrients released by microorganisms thereon. Some soil organisms also feed directly on plant roots, but in a healthy soil with good biodiversity, such pests are in the minority and pose little threat to vigorous plants. Plants release a significant portion of their annual photosynthetic product into the soil, supporting a vibrant microbial community in the root zone and forming a vital link between plant and soil. One of the most important groups of soil fungi, the mycorrhizae, grow within plant root tissues and extend hyphae (filaments) some distance into the surrounding soil. The fungus forms mycorrhiza with plant roots, and through those connections pass substances that both organisms need to grow. Mycorrhizal symbioses expand several-fold the volume of soil from which plant roots can absorb moisture and nutrients, and strongly enhance uptake of phosphorus and trace minerals. During evolution, mycorrhizal fungi have refined their biotrophic capabilities to take advantage of their hosts as food sources and protective niches, while plants have developed multiple strategies to accommodate diverse fungal symbionts. Intimate associations with pervasive ecological success have originated at the crossroads between these two evolutionary pathways and fungi act as biofertilizers and bioprotectors. This important symbiosis also plays a key role in both global carbon and phosphate cycles. here is another level of interaction; an exchange not only back and forth between the fungus and the plant, but also between neighbouring plants, using fungi as a thoroughfare.
As the fungal threads spread, they can link up to multiple trees, plants, creating webs known as ‘common mycorrhizal networks’. These networks provide nutrients pulled from the soil in exchange for a supply of photosynthetic sugars and exchange, nutrients, water and more. In some cases, the fungal filaments have been shown to connect the roots of multiple trees — providing a pathway for nutrients to shuttle between organisms. It is also mentioned that trees and plants can receive early warning signals about threats via the network if a neighbor is under attack — for example, from aphids or caterpillars — and preemptively produce defensive chemicals to protect themselves from an assault and that mature trees send messages and sugars to their offspring via mycorrhizal connections (not -yet?- supported by any published, peer-reviewed research).
Fungi can cover a large surface area by developing white fungal threads known as mycelium. Mycelium spreads out on top of tree roots by up-taking sugars from the tree and by providing, as aforementioned vital minerals back to the tree, such as nitrogen and phosphorus. This symbiotic relationship between tree roots and fungi is known as the mycorrhizal network. With all their inhabitants linked up, forests look less like collections of individuals, and more like giant superorganisms.
Trees and plants can provide mutual support and help shape the ecosystems they inhabit. The networks they form are complex, often encompassing not just multiple plants but multiple species, and depending on the type of fungi involved, different materials can be exchanged. In some cases, the fungal filaments have been shown to connect the roots of multiple trees — providing a pathway for nutrients to shuttle between organisms. In the past few years it has been demonstrated that trees and plants can exchange more than just nutrients.
Human activity poses threats to global soil resources through land use conversion and intensification. These changes are resulting in desertification from soil organic carbon decline, physical erosion of soil from productive land to depositional environments, and loss of biodiversity. A further threat is the negative impact on vegetation from loss of soil fertility, which is due to (a) compaction by overgrazing or agricultural machinery; (b) chemical contamination; (c) acidification from anthropogenic atmospheric deposition and intensive biomass cultivation, which mobilizes metals and disrupts the formation of organic-mineral complexes that chemically bind organic matter and protect it from degradation; (d) sealing by urbanization; and (e) salinization from groundwater irrigation.
Trees and plant rely on a healthy ecosystem to thrive and protect themselves from danger. Humans rely on a healthy ecosystem to be able to inhale clean oxygen. Last year, millions of people around the world experienced the devastating effects of climate change, biodiversity loss, soil degradation. It is not only impacting human health and wellbeing, but it is also affecting the ecosystem of our oceans and forests. Human-initiated deforestation contributes to climate change by reducing the number of trees that are available to soak up carbon dioxide. Deforestation not only removes the trees that are being cut down, but also impacts trees that are still alive by disrupting the mycorrhizal network that is important for intra-tree communication.
Changes in climate, as seen through increased droughts and extreme temperatures, may further disrupt the biodiversity of the microbes in the forest. This decline in biodiversity is known as human assisted evolution, or unnatural selection. The altered microbiota of the forest may then change the nutrients that trees are able to receive and we may start seeing changes in tree morphology, particularly in the shape of leaves. This would change the photosynthetic capacity of trees and plants ; for example, smaller leaves have less surface area for light absorption, which will negatively impact their ability to absorb the sun’s rays and produce sugars through photosynthesis. This could potentially inhibit tree and plant growth and the amount of carbon that trees can share with fungi. Furthermore, without a biodiverse mycorrhizal network, trees are becoming more susceptible to destruction from invasive, harmful insect species. It is clear that the impact we are making on the environment is self-perpetuating and heading in a dire direction for the health of our forests, but there is still hope. Some scientists are trying to combat climate change by using gene-editing techniques to restore ecosystems that have become extinct and by engineering synthetic microbes that are important for a thriving ecosystem.
Trees and plants are considered to be the oldest living organisms on the planet. Over centuries, they have been resilient to changes in their environment due to their symbiotic relationship to fungi and other microbes. There are so many more discoveries to be made to understand the ancient wisdom of our forests and the invisible microbes that keep our ecosystems in harmony.
Living soil and mycorrhizal fungi have been underpinning life on Earth for millions of years. By disrupting the complex webs they form beneath our feet, we are also endangering the organisms we depend on to survive.