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Biological soil crusts

Published 05 Dec 2022

Today is World Soil Day - let’s get crusty!

There are incredibly important communities of living organisms beneath our feet that most people are completely unaware of, but which we should appreciate because they play incredibly important roles in building soil fertility and preventing erosion. They are the biological soil crusts, otherwise known as cryptogamic soil crusts or biocrusts.

Next time you’re in nature, get down close to the ground and you’ll find that there is often a crusty layer of miniature and microscopic life binding the soil surface. It’s made up of fungi, lichens, algaes, cyanobacteria and bryophytes such as mosses and liverworts. Such crusts are especially prevalent in arid and semiarid regions where vegetation is sparse and the soil surface is more exposed to light.

The cyanobacteria and bryophytes photosynthesise, adding carbon to the surface layers of soils. Cyanobacteria also fix nitrogen, taking it from the air and converting it into a form that can be used by plants.

You will have seen stories in the news about dust storms sweeping through inland Australia, sometimes reaching the coastal cities, and even carrying soil out into the Pacific Ocean to New Zealand and beyond. This phenomenon was not as common or severe prior to the European settlement of Australia because biological soil crusts were then much more extensive and intact. The hard hooves of livestock and feral animals, such as goats, break up soil crusts, while agriculture machinery destroys them. This leaves soils exposed and prone to being whipped up by wind. As a result, fertile top soils that have taken centuries to form have been lost to the ocean.

The loss of these crusts also makes soil much more vulnerable to erosion by rain and flowing water. Well-developed soil crusts have a thick, rough surface which prevents rain drops from breaking up the soil surface, soaking up moisture then letting it slowly penetrate.

Biological soil crusts even act to reduce fire intensity. Where they are intact, the seeds of weedy grasses are less able to germinate and as a result, fuel loads are reduced.

Depending on aridity and soil texture, biological soil crusts can take anywhere between a few and thousands of years to reform after being damaged or destroyed. Therefore, it’s incredibly important that land is managed to protect them, especially in arid regions.

So, there has been a wonderous miniature ecosystem below your gaze all this time, benefiting you in ways you may not have imagined! We encourage you to get down on your knees sometime and take a moment to appreciate its complex beauty.

Lichens are an important component of biological soil crusts. Lichens are an important component of biological soil crusts.
A biological soil crust at Naree Reserve in north-western New South Wales. By Rebecca Spindler. A biological soil crust at Naree Reserve in north-western New South Wales. By Rebecca Spindler.
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