Biological Soil Crusts Can Help Reverse Desertification

Biological soil crusts are fine, active layers of organisms that form across desert ground. Often called the “living skin” of the desert, they can be cultivated artificially to help prevent deserts expanding.

In the wild, these crusts generally develop in three key phases: first simple cyanobacteria establish themselves, followed by lichens, before a mature, moss-dominated layer emerges.

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

Their approach uses microbes grown in laboratories to fasten loose desert sand into a slim, durable coating that is difficult for wind to remove.

This more resilient surface gives restoration crews an opportunity to establish shrubs and grasses before extreme winds and desert temperatures destroy immature plants.

Biological soil crust

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

The Chinese Academy of Sciences (CAS) monitored these plots through both hot and freezing conditions, recording the rate at which the film became firm.

At test sites close to the Taklamakan Desert in Xinjiang, north-west China, CAS researchers found that biological soil crusts could secure sand in 10 to 16 months.

Despite this rapid progress, planners prioritised creating a sound soil foundation, allowing plants introduced later to persist without repeated replanting.

Cyanobacteria and soil crust

Cyanobacteria are bacteria powered by sunlight that prosper in severe environments, and they are thought to have emerged around 3.5 billion years ago, long before forests appeared.

Many varieties use sunlight and air to draw carbon dioxide into their cells, releasing the remaining material as basic organic matter.

Where desert soils lack fertiliser, certain species carry out nitrogen fixation, transforming nitrogen gas into nutrients available to the crust’s microbial community.

After becoming established, this living cover holds unstable grains in place and provides the earliest plants with improved rooting conditions.

Sticky sugars bind sand

Viewed under a microscope, biological soil crusts - thin, living coatings on soil surfaces - reveal a web of bacterial filaments surrounding individual sand grains.

Cells release sticky sugars into the spaces between grains to maintain this web, and these sugars set into a fine but cohesive surface layer.

By binding grains together, the crust performs a glue-like role and can also discourage invasive plants from becoming established.

The surface may be damaged by feet, tyres or aggressive raking, meaning large-scale crust creation also requires protection over the long term.

Carbon starts building

During the first year, the treated layer started retaining nutrients within the upper 2.5 cm of soil, rather than allowing them to disappear as airborne dust.

Dead cells and released sugars combined with wind-blown mineral dust to create organic matter, which assisted in retaining nitrogen and phosphorus.

As those nutrients built up, they supported additional microbes, making the crust community increasingly resistant to disruption.

This shift gave seedlings a stronger initial environment, although their continued survival still relied on rainfall arriving at a suitable time.

Soil crust retains water

Following brief rainfall, moisture stayed nearer the surface in crust-covered areas, whereas adjoining bare sand dried rapidly.

Its uneven pores and dark pigments limited evaporation by keeping water shaded and held beneath the narrow layer.

Retaining moisture for only a few additional days can give grasses and shrubs sufficient time to develop roots before the heat returns.

In extended droughts, the living crust may become dormant, making outcomes dependent on local climate and careful scheduling.

Lichens and moss

As it developed, the crust shifted from a chiefly microbial layer to a varied cover containing lichens and small areas of moss.

Lichens created a more robust surface, while their gradual growth helped the crust withstand strong winds and cold nights.

Moss added height and shade, enabling minute reservoirs of moisture to remain longer while protecting newly arriving microbes.

When these later organisms became established, the overall system was more stable, although any damage also required more time to repair.

Tracking soil cross over 59 years

Today’s accelerated experiments are supported by Chinese records that tracked crust development throughout 59 years of desert restoration.

Using crust specimens of established ages, researchers compared undisturbed locations with sites treated using laboratory-grown cyanobacteria.

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

Even under the most favourable conditions, a mature crust able to withstand disturbance still required two to three years to form.

Erosion drops fast

Wind is the most demanding test: bare sand fails when gusts intensify and lift grains away.

Once cyanobacteria had been sprayed, the connected crust kept bound grains in place, reducing the number of particles carried into the atmosphere.

In controlled wind conditions, laboratory tests showed that a manufactured crust reduced wind-driven soil loss by more than 90%.

Reduced moving sand could bring fewer sandstorms and roads that last longer, though the crust must withstand pressure from traffic and grazing.

Limits in the field

Applying this technique beyond small plots requires difficult decisions over where microbes should be sprayed, because crusts are not necessary on every dune.

Native strains commonly cope better with heat, salinity and drought than imported alternatives, so teams generally cultivate microbes collected from nearby deserts.

Desertification - the loss of plant cover that makes land increasingly desert-like - has numerous causes, so crusts cannot address overgrazing or poor water use.

If vehicles and heavy pedestrian traffic are not kept away, a restored layer may break down and take years to recover.

Future of biological soil crust

Rapid crust formation makes microbial growth a usable restoration tool, connecting desert sand management with the slower process of restoring vegetation.

Ongoing monitoring will determine whether its durability, advantages and potential side effects remain consistent across different deserts and climates.