When it rains

The desert is normally a very dry place, which makes life in the soil very difficult. Recently we've had a LOT of rain during the monsoon season. This was the 2nd wettest September in Phoenix's history! What does this mean for the organisms living in the soil in the desert? What happens in the soil after it rains?

Chelsey samples the soil crust beneath a creosote.

Most of the rain we get in the Sonoran Desert is in small amounts. Even small rains can make a difference. The microscopic organisms in the soil, particularly the bacteria and fungi, will get active after a sprinkle. Most plants require bigger rains, though. That means that the soil crusts I mentioned in my last post can "wake up" quickly after even a small rain to begin photosynthesizing and respiring, but plants like the creosote in the picture here may not.

Soil microorganisms are responsible for a lot of important processes in the soil. They respire carbon dioxide into the atmosphere, they help recycling nitrogen and phosphorus for plants. If soil microorganisms respond to rain quickly, then so will carbon, nitrogen, and phosphorus cycling. After a rainfall, carbon dioxide release from the soil increases. Microbes take up and release nitrogen more quickly, and nitrogen compounds can be washed away with the water. So rain can make a lot of things happen in the soil!

The predictions for the Sonoran Desert is that climate change will cause the area to become dryer, with larger rain events that will happen more sporadically, delivering less rain overall. This has led many scientists to ask how the Sonoran Desert ecosystem will change in the future. If rain events have this big of an effect, how will the ecosystem respond to changes in the amount and frequency of rainfall?

Kelly uses a watering can to simulate rainfall.

A lot of research has investigated how the plants of desert communities respond to these changes. There may be changes in the amount plants are able to grow, as well as changes in the species that are able to survive. Much less is known about how the microscopic organisms in the soil will respond.

Some of our research investigates how the soil community changes when the timing and amount of precipitation changes. We use watering cans to make "fake" rainfall events that happen in different amounts and at different frequencies, and then look at how the invertebrates living in the soil respond. We will be able to help answer these questions to predict the future for soil communities in the Sonoran Desert.

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.

Get to Know Desert Soils

When you think about a desert landscape, what do you picture? A cactus, perhaps?  Small bushes or tumbleweed? Plants are of course an important part of a desert ecosystem, but when you look at a photo of the desert, you see that it is dominated by one important feature: soil!

Salt River Recreation Area, Mesa AZ, Sonoran Desert

Soil in deserts, such as the Sonoran Desert where I live and work, tends to be very sandy with only small amounts of scattered vegetation. While the soil may look barren, it is not dead! Look carefully at this close-up photo of the soil below. There is a bursage, some grass, even a mouse! There are also clumps of bacteria, lichen, and moss, which are the dark brown "crusts" you see around the plant.

There is a lot of life in this photo, if you look closely!

If you look even closer, with the help of a microscope, you would be able to see that there are a lot of even smaller organisms living in the soil that do many important things to sustain the ecosystem.

http://www.teara.govt.nz/en/photograph/12725/soil-mites

All sorts of organisms live in the soil all over the world. When you walk on the ground, you're walking on thousands and thousands of critters that you can't even see, with every step! Most of the organisms in soils are bacteria and fungi. The fungi living in the soil aren't mushrooms, like you're used to seeing. They're microscopic fungi. Bacteria and fungi live in soils all over the world in every biome, and deserts are no exception! There are also things living in the soil that eat the bacteria and fungi. In many places, like deciduous forests in the U.S., there are tons of different types of organisms living in the soil that eat microscopic bacteria and fungi. On Antarctica, there aren't so many different types, because it's a much harder environment to live in. However, there are some here, but they're all microscopic! There's organisms like protozoa, tardigrades, collembola, nematodes, and many more! You can see some of the diversity of soil organisms in this photo.

These organisms live, eat, breath, and die in the soil, and the result is that they recycle important nutrients through the soil to support plant life. Those plants support herbivores, like the pocket mouse you see here. Soil is the important resource that supports this entire food web of plants and animals!



Want to learn more? You can watch a video that explains the importance of our research in desert soils by clicking here.