Biological Soil Crusts Could Help Reverse Desertification

Biological soil crusts - slender, living layers of organisms that develop across desert ground - can be cultivated artificially to help prevent deserts expanding. They are commonly described as the desert’s “living skin”.

In the natural environment, these crusts usually develop in three principal phases: beginning with basic cyanobacteria, progressing to lichens, and ultimately reaching a mature moss-dominated condition.

Scientists have now created a method intended to reverse desertification completely, converting sand into fertile soil.

They relied on microbes grown in laboratories to join loose desert sand into a fine, durable surface layer that is not readily carried away by wind.

This more robust surface gives restoration crews an opportunity to establish shrubs and grasses before severe winds and desert temperatures destroy young vegetation.

Biological soil crust

Across straw checkerboards installed in north-west China, a dark coating developed on treated sand and remained in place following seasonal dust storms.

By observing these sites through periods of heat and frost, the Chinese Academy of Sciences (CAS) recorded the rate at which the film became firm.

During trials close to the Taklamakan Desert in Xinjiang, north-west China, CAS researchers found that soil crusts stabilised the sand in 10 to 16 months.

Despite that rapid progress, planners concentrated first on creating a stable soil foundation, allowing plants introduced later to survive without continual replanting.

Cyanobacteria and soil crust

Cyanobacteria - bacteria powered by sunlight that can flourish in extreme conditions - probably emerged around 3.5 billion years ago, long before forests appeared.

Drawing on sunlight and air, numerous strains take carbon dioxide into their cells and release the remaining material as simple organic matter.

In desert ground lacking fertiliser, certain species undertake nitrogen fixation, converting nitrogen gas into nutrients that plants can use within the crust’s microbial community.

After becoming established, this living layer secures loose particles and offers the earliest plants a more favourable rooting environment.

Sticky sugars bind sand

Viewed under a microscope, biological soil crusts - fine living layers covering soil surfaces - reveal a network of bacterial filaments encircling individual sand grains.

Cells release sticky sugars between the grains to keep this network intact, with the sugars setting into a slim, cohesive coating.

The crust functions as a glue, binding sand particles together while also helping to stop invasive plants becoming established.

As footsteps, tyres and aggressive raking can fracture the surface, large-scale crust development also requires lasting protection.

Carbon starts building

During the first year, the treated surface started retaining nutrients within the top inch of soil, rather than allowing them to be blown away as dust.

Dead cells and released sugars combined with mineral dust carried in by the wind to create organic matter, which helped retain nitrogen and phosphorus.

As nutrient levels built up, a larger microbial population could feed there and the crust community became more resistant to disruption.

For seedlings, this shift provided a stronger initial environment, although survival still relied on rainfall arriving at an appropriate time.

Soil crust retains water

Following brief rainfall, a crust-covered area held water nearer the surface, whereas adjoining bare sand dried rapidly.

Uneven pores and dark pigments limited evaporation by keeping water shaded and contained beneath the thin layer.

Retaining moisture for only a few additional days can enable grasses and shrubs to establish roots before the heat returns.

In extended dry periods, the living crust may become dormant, meaning outcomes depend on both the climate and careful timing.

Lichens and moss

As time passed, the crust moved beyond a primarily microbial layer and became mixed cover containing lichens and small patches of moss.

Lichens created a more resilient outer layer, while their gradual growth helped the crust withstand strong winds and cold nights.

Moss added height and shade, allowing small reserves of moisture to persist and protecting newly arriving microbes.

When these later organisms became established, the overall system grew more stable, although it also took longer to recover from damage.

Tracking soil change over 59 years

The rapid trials undertaken today are supported by Chinese records documenting crust development across 59 years of desert restoration.

Using crust samples of known ages, researchers compared undisturbed locations with plots that had received laboratory-grown cyanobacteria.

Improvements in nutrients corresponded with the dominant microbes, and cyanobacteria application reduced a process lasting decades to one taking only years.

Even under the most favourable conditions, a mature crust capable of resisting disturbance still required two to three years to form.

Erosion drops fast

Wind is the most demanding test: bare sand fails when stronger gusts lift and remove its grains.

Once cyanobacteria had been sprayed, the connected grains remained in place because the crust held them together, reducing the number of particles carried into the air.

In controlled wind experiments, an engineered crust reduced wind-driven soil loss by more than 90%.

Reduced moving sand could bring fewer sandstorms and roads that last longer, although the crust needs to withstand traffic and grazing pressure.

Limits in the field

Expanding the technique beyond small plots involves difficult decisions over where microbes should be sprayed, as crusts are not needed on every dune.

Locally sourced strains generally tolerate heat, salt and drought more effectively than imported strains, so teams commonly grow microbes collected from nearby deserts.

As desertification - the loss of plant cover that makes land more desert-like - has multiple causes, crusts cannot address overgrazing or misuse of water.

If vehicles and heavy pedestrian use are not kept away, a restored surface may break down and can need years to recover.

Future of biological soil crust

Rapid crust formation makes microbial growth a practical approach, connecting control of desert sand with the slower process of plant-based restoration.

Long-term observation will determine whether the durability, advantages and side effects persist in different deserts and climates.