In brief
A living skin on open dry ground, made of cyanobacteria, algae, lichens and mosses. It binds the surface and protects it against wind and water.
What is a biological soil crust?
On open dry ground, between the shrubs of a semi-desert or on a bare patch of hillside, the soil surface is often not bare at all. Look closely and you find a thin, dark, slightly lumpy skin, here and there with tiny moss cushions or flat lichens in grey, black, orange or green. This is a biological soil crust, often shortened to biocrust: a community of cyanobacteria, green algae, lichens, mosses, liverworts, fungi and bacteria living in and on the top few millimetres of soil.
The cyanobacteria are the founders. Their filaments move through the moist soil after rain and leave behind sticky sheaths that glue mineral grains together. Lichens and mosses settle on this foundation as the crust matures. Biocrusts are among the oldest forms of life on land, and according to a global estimate they still cover around twelve percent of the earth's land surface, mostly in dry regions where vascular plants leave gaps.
It is important not to confuse them with the crust that forms on bare, tilled soil after heavy rain. That physical crust arises when raindrops break the soil crumbs and fine particles clog the pores. It hinders water and seedlings and is a sign of damage. The biological crust is the opposite: a living protective layer.
What biocrusts do
Their most obvious service is protection. Where there are no plants to hold the ground, the crust does it: its filaments and threads bind the surface so firmly that wind and rain carry far less soil away. On land threatened by desertification, that alone is worth a great deal.
Biocrusts are also small factories. They photosynthesise whenever they are moist and add carbon to soils that have little organic matter. Many cyanobacteria and lichens fix nitrogen from the air and release part of it into the soil. They trap wind-blown dust, which brings in fine particles and nutrients. Their influence on water is more complex than often claimed: depending on the type of crust, the roughness of the surface and the climate, they can increase or reduce infiltration. Generalisations do not hold here.
In the language of ecological succession, biocrusts are pioneers of the very first hour. They settle where nothing else can, stabilise the surface and prepare the ground in which the first seeds can germinate.
Fragile and slow to return
Their weakness is mechanical. A dry crust is brittle, and a footstep, a hoof or a tyre crushes it. Once broken, the surface is exposed to wind and water, and the soil beneath it begins to move. Recovery is slow: depending on climate and composition, cyanobacteria can recolonise within years to decades, while lichens and mosses in very dry deserts may need decades or even centuries. A global study published in 2018 expects that climate change and more intensive land use could reduce biocrust cover by a quarter to two fifths by around 2070.
Part of the danger lies in how inconspicuous biocrusts are. In the dry season they look like ordinary dark soil, and only after rain do mosses swell and turn green and lichens show their colours. Many crusts are destroyed simply because nobody noticed them.
How Terra Humana understands biological soil crusts
For Terra Humana, biocrusts are a reminder that bare-looking ground is not always empty. Restoration often begins with the urge to cover every open patch. But where a living crust has formed, the surface is already protected, by organisms that have done this work far longer than any gardener.
This calls for a careful eye. Open dry patches are worth examining before anything is placed on them or anyone walks across them. Where a crust exists, the simplest help is not to disturb it: paths that keep to a few lines, no grazing animals on fragile surfaces, and patience with the slowest pioneers of all.
Frequently asked
What is a biological soil crust?
A biological soil crust, or biocrust, is a thin living layer on the surface of open ground in dry regions. It consists of cyanobacteria, green algae, lichens, mosses, liverworts, fungi and bacteria that live in and on the top few millimetres of soil and bind it together. To the untrained eye it often looks like dark, slightly lumpy dirt. Biocrusts cover an estimated twelve percent of the earth's land surface and are among the oldest forms of life on land.
Is a biological crust the same as the crust that forms after rain?
No, and the difference matters. The crust that forms on bare, tilled soil after heavy rain is a physical crust: raindrops have broken the soil crumbs, fine particles have clogged the pores, and the surface has dried hard. It hinders water and seedlings and is a sign of damage. A biological crust, in contrast, is alive. It is held together by organisms, protects the surface against erosion and supplies the soil with carbon and often nitrogen. One is a wound, the other a protective skin.
Why are biological soil crusts important?
Because they protect open dry ground where plants cannot. The filaments of cyanobacteria and the threads of fungi and mosses glue soil particles together, so wind and rain carry far less soil away. Through photosynthesis, the crust adds carbon to soils poor in organic matter, and many cyanobacteria and lichens fix nitrogen from the air. Biocrusts also trap fine dust and influence how water enters the ground, although whether they increase or reduce infiltration depends on the type of crust and the site.
How quickly does a biological soil crust recover?
Slowly, and that is why it is so vulnerable. A dry crust is brittle, and a single footstep, hoof or tyre can crush it. Depending on climate and type, cyanobacteria can recolonise a disturbed surface within years to decades, while lichens and mosses in very dry deserts may take decades or even centuries to return. In the meantime the unprotected soil erodes more easily. A global study expects that climate change and more intensive land use could reduce the area of biocrusts by a quarter to two fifths by around 2070.
Where do biological soil crusts occur?
Wherever plant cover is sparse and the surface receives light: in deserts and semi-deserts, on dunes, in polar and high mountain regions, and in the open gaps between shrubs in Mediterranean landscapes. Some of the best-studied Mediterranean biocrusts are in south-eastern Spain. They are usually most visible after rain, when mosses turn green and lichens show their colours, and almost invisible in the dry season.
Related terms
See this in practice
Sources
- Bettina Weber, Burkhard Büdel, Jayne Belnap (eds.), Biological Soil Crusts: An Organizing Principle in Drylands, Ecological Studies 226, Springer (2016)
- Jayne Belnap, Otto L. Lange (eds.), Biological Soil Crusts: Structure, Function, and Management, Ecological Studies 150, Springer (2001)
- Jayne Belnap, The world at your feet: desert biological soil crusts, Frontiers in Ecology and the Environment 1 (2003)
- Jayne Belnap, The potential roles of biological soil crusts in dryland hydrologic cycles, Hydrological Processes 20 (2006)
- Emilio Rodriguez-Caballero et al., Dryland photoautotrophic soil surface communities endangered by global change, Nature Geoscience 11 (2018)
Last updated: 2026-10-12