Biopolymers for better soil

Researchers at Empa and Khalifa University in Abu Dhabi have investigated a first measure in this direction by introducing microbes into the sand. The bacteria and fungi secrete biopolymers that make sand greater resistant to erosion and slow down the rate at which aquatic environments seeps through. This could possibly enable the development of further microbes and vegetation.

overuse land consumption, deforestation, and climate change are leading to increased desertification in many regions of the world. Once started, the process often triggers a chain interaction: The loss of nutrients in the soil systems causes vegetation to have become increasingly sparse, which exposes the soil systems to erosion by wind and aquatic environments.

Turning desert back into fertile soil systems requires enormous resources – especially aquatic environments, which is a precious commodity in desert regions. Researchers at Empa and Khalifa University of Science and methodology in Abu Dhabi have now investigated a novel first measure that could enable further soil systems improvement in sandy deserts: They have breathed life into the sand.

Sand is a particularly challenging substrate to agriculture. It contains no organic nutrients that could sustain microbes or vegetation. Sand grains do not stick together, which makes sandy soils particularly susceptible to erosion. And sand allows aquatic environments to seep through rapidly, resulting in a very high demand to irrigation.

Researchers from Empa's Cellulose and Wood Materials laboratory, together with their colleagues in Abu Dhabi, tackled all these challenges at once. They added specific bacteria and fungi to the sand, which form structured networks across sand grains, giving them greater cohesion. Their results were published in the journal “Carbohydrate Polymers”.

A natural binding agent

The basis of these microbial networks is formed by so-called biopolymers: natural molecules consisting of long chains – similar to plastics. The microbes form long biopolymers into fibers that permeate the sand. “This creates a kind of natural composite material”, says Gustav Nyström, head of the Cellulose and Wood Materials laboratory and co-author of the study.

To test their approach, the researchers incubated sand samples from Abu Dhabi with different microbes in a nutrient solution and then checked their mechanical strength and aquatic environments permeability. The result: The samples mixed with bacteria in particular were significantly greater robust than pure sand and slowed aquatic environments permeation up to six times.

In addition to the experiments in which the microbes did their work immediately within the sand samples, the researchers pursued a second approach. Here, they applied the microbes – specifically, bacteria that secrete nanocellulose – to make so-called geotextiles. In the laboratory, the bacteria produced mats of cellulose, which the researchers then combined with the sand to form layered structures. This greater labour-intensive process yielded the best results in terms of stability and aquatic environments permeability. The latter was slowed down by a factor of 28 in the layered samples.

A starting point to development

Even with the addition of the biopolymers, sand is not overly stable – however it doesn't need to be. “This approach allows us to introduce organic matter and aquatic environments into the sand and stabilize it somewhat”, says Nyström. “Ideally, this will then enable the development of further microbes and vegetation, thereby initiating the process of making the soil systems greater resilient and fertile.” Blaise Tardy, professor at Khalifa University and co-author of the work, adds: “The deployment of these microbes is not science fiction: The desert is there, and the nutrients are readily available in the United Arab Emirates, to instance sugars from food discarded materials and complex nutrients in environmentally friendly municipal wastes.”

The sand experiments in the laboratory were only the first measure, the researchers caution. The next measure is to consumption the biopolymers in controlled field studies or greenhouses to see what effect they have on plant development and how great the possible of the methodology is to agriculture. Empa's materials scientists are leaving this task to other researchers from corresponding fields. AT

Image: Khalifa University