It has been said that in the Chihuahuan Desert, we don’t have soil. We have dirt. And if you’ve ever tried to grow things out here, you’re likely to agree with that assessment. But in fact, that is not true. Desert soils are often surprisingly complex.
These soils are common throughout the Southwest. You can usually recognize them by their appearance.
These places look sort of “warty” and parts of them are black, or at least significantly darker then the surrounding ground. What you are looking at is an area covered with cryptogamic crust or what is more commonly described today as a biological soil crust.
What Is It?
Biological Soil Crusts are actually a complex community of several plant species, and its existence is vitally important to the ground beneath it. They are made up of an assemblage of species of cyanobacteria, lichens, fungi, green algae and (rarely) mosses.
Cyanobacteria are single cell organisms That have all the machinery they need for photosynthesis. They don’t like living alone – instead they hook up with each other forming long strands – a little like pearls on a string. Every so often, an extra large cell appears, either attached to the strand or floating around by itself. These cells are called heterocytes.
Heterocytes are responsible for “fixing” nitrogen and making it available for plant use. We will talk more about these cells later. The following figure is a microscope image of many cyanobacteria cells. The heterocytes are the cells that look like empty balloons.
Fungi and Lichen
If the algae and the fungi are compatible, the fungi may take in the bacteria, forming symbiotic organisms called lichen. In such an arrangement, fungi protect the bacteria and provide them with nutrients. In return, the bacteria produce food, nitrogen and carbon for the fungus.
As you can see, the organisms in a Biological Soil Crust organize themselves into ordered layers. The following figure illustrates a typical arrangement. You can also see how the filamentous layer is bound together by the tiny threads made by cyanobacteria.
Left alone these crusts can be remarkably stable and long lived. Their key to survival is their ability to tolerate extreme dehydration. Their water content can drop to 5% or less, a condition which terminates all metabolic processes. Nevertheless, during the rare moments when water is present, they awake, and take up living as if there had been no interruption at all. Their colonies are essential in providing stability and fertility to desert soils.
Where Do You See It?
Biological Soil Crusts prefer fine textured soils such as silt, clay, and fine sand. They prefer level or gentle slopes in open areas between plants. In the following picture, the light/almost white soil at the bottom of the frame is perfect habitat for Biological Soil Crusts.
What Good Is It?
Biological Soil Crust both stabilize and fertilize the soil beneath them.
All crust organisms secrete polysaccharides into the soil. Polysaccharides are sticky and long-lasting. They help bind soil particles into aggregates. These aggregated areas increase soil aeration, water infiltration, and resistance to erosion. Desert soils are slow to form and are easily swept away. Biological soil crusts increase the soil’s resistance to erosion, and if undisturbed can protect soil surfaces almost completely. Plants growing in crusted soils generally have greater biomass and higher concentrations of nutrients than plants growing in uncrusted soils. Polysaccharides can bind and prevent loss of positively charged nutrients.
The following picture illustrates just how effective BSC’s are at controlling erosion. The area shown is about 8×12 inches; the gully on the left is about 6 to 8 inches deep. Even in this small time-period, the black Biological Soil Crust has prevented the loss of 6 inches of topsoil.
Enlarging the image to show only about 1 square inch of soil reveals the protective process in detail. This crust is left overhanging the gully.
Biological Soil Crusts are major contributors to to the health of desert soils. Their two greatest contributions are “fixing” nitrogen and carbon from the atmosphere and converting them into forms that plants can use. This is important because lack of nitrogen is often as great a barrier to desert plant life as is the lack of water.
The Trouble with Nitrogen
Nitrogen is required for all forms of life. It is essential for the biosynthesis of molecules such as nucleotides, amino acids and proteins used to create all plants, animals, and other organisms. In the gaseous state, nitrogen occurs in molecules of two atoms, tightly bonded together in an arrangement that is extremely stable and virtually inert. In fact, Nitrogen’s bonds make it one of the tightest bonds in nature. Chemical processes that take the atoms apart are complex, energy intensive, and expensive.
But life has found a way. Cyanobacteria make and use an enzyme called nitrogenase that is capable of converting N2 (atmospheric nitrogen) to NH3 (ammonia). Ammonia is readily utilized by most terrestrial life forms. Biological Soil Crusts are an important source of nitrogen and carbon in deserts where vascular plant coverage is sparse. Cyanobacterial crusts can fix up to 1 kilogram per hectare per year; lichenized crusts can fix up to 10 kg per hectare per year. This makes them the dominant source of these nutrients.
Vascular plants need many nutrients to thrive. Macronutrients such as hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur are needed along with trace elements such as boron, chlorine, manganese, iron, zinc, copper, and molybdenum. As in the case of nitrogen most of these elements, even if present in the environment, are said to be unavailable to the plant because they are already bound to other molecules in the soil. But biological soil crusts create “chelators,” small molecules that bind very tightly to metal ions.
The chelating process secures the desired ions in a sort of organic structural cage that renders them electrically neutral and chemically inert. Plants can break into these cages to get at the goodies inside.
A Soil that Traps Dust
The increased surface roughness and the presence of so many polysaccharide sheaths increases the capture of nutrient rich dust. Incoming dust may originate almost anywhere in the world, and it can often increase the concentration of essential nutrients such as potassium, phosphorous and bioavailable nitrogen up to fourfold.
Studies show that soil crusts reduce or completely eliminate erosion of soil by water and wind. Biological soil crusts also trap soil moisture, and provide sheltered areas for plants to germinate and grow.
Though Biological Soil Crusts can live for literally thousands of years they have one significant weakness – they are physically fragile. If you step on an area of crust, that area dies. Possibly the greatest threat to to Biological Soil Crusts is mechanical disturbance from vehicle traffic and grazing. Loss of these crusts has led to massive invasions of cheat grass throughout the Southwest. The grass makes wildfires more likely and more intense when they happen. Biological Soil Crusts can be killed in very hot fires.
Biological Soil Crusts can recover from mishaps, but the recovery can take decades to millennia, and attempts to artificially restore them have been largely unsuccessful. Currently the only practical way of protecting these delicate assets is to protect them from the physical hazards that destroy them. Removing vehicles and cattle is the best protection we know of to date.
Visitors to parks, preserves, etc should be advised of the value and fragility of these soils so that they can take steps to avoid treading on them. When you visit these areas please try to remember this advise.