Soil biological crust

Have you ever wondered what that black crusty stuff is that covers the soil here in the Sonoran Desert?

 

It doesn't look like much, but it's actually a very important part of the desert ecosystem. It's called "soil biological crust", which is just a scientific way of saying it's a mix of biological material growing in a crusty form on top of the soil. That means the dark crusty stuff is alive! In that crust, there's a mix of bacteria, algae, lichen, fungi, and moss.

The individuals are of course microscopic, so you can't see them. However, the group of microscopic cells get together to form the crust that you're able to see. In this picture, one of my students shows you the soil crust she's sampling at South Mountain in Phoenix, AZ:

The individual cells make a web of fibers that help hold loose soil particles together. So, the crust has a very important job of holding down soil to prevent erosion from wind and rain.

Another reason they're very important is due to their role in nutrient dynamics. Some of the biological cells in the crust are called cyanobacteria. This group of organisms is able to capture nitrogen from the atmosphere and turn it into a biological form. This process is called "nitrogen fixation". They're "fixing" the di-nitrogen gas from the atmosphere and turning it into cellular compounds. When the cell dies, it releases that nitrogen as minerals into the soil that can be used by plants and animals. Other organisms, like plants and animals, can't use the form of nitrogen gas that's in the atmosphere. They need to use a mineral source from the soil, but they can't fix nitrogen on their own. They rely on the cyanobacteria to do it for them, which is why nitrogen can often limit plant growth. (That's why you have to add it to your garden to make plants grow better.) Without soil crusts, we'd lose that important source of soil nitrogen for desert plants that make up the base of the foodchain!

Soil crusts also provides a habitat for other soil organisms, like the invertebrates I mentioned in my previous post. They provide shade, moisture, and nutrients for those organisms.

So, next time you're walking in the desert, think twice about where you put your feet! The crusts might look dead and uninteresting, but they're very much alive! They're also very fragile and easy to break, so we need to be careful to avoid killing these very important components of desert soil.