Biological Soil Crust Restoration?

The biotic soil crust in Red Rock Canyon, Lake Mead National Recreation Area, NV is showing clumps of the dominant moss species, Syntrichia caninervis. Each black dot represents a separate plant. This extreme arid-dwelling moss inhabits the loose sandy soils of the Mojave Desert. [Credit: Alexis Wartelle]

1.  What Biological Soil Crusts (BSC) occur in the desert? Gallery of BSC: from Canyonlands Research Station

Major types (easily observable):
(NB: A misting of water can make crustal organisms more visible...)

Three soil lichens dominate crusts of both the Sonoran and Grea Basin deserts:

Collema: a genus of gelatinous lichen (blackish, jelly-like when moist). First to colonize.

Placidium: a genus of squamulose lichen (discrete rounded flakes, convex or concave). Usually a secondary successional species.

Psora: a genus of squamulose lichen (discrete rounded flakes, convex or concave), a late-successional stage lichen.

Also:
Short moss:  Mosses <10mm and="" b="" bryum="" nbsp="" spp.="">Certodon purpureus 
Heterocystic cyanobacteria: (Notoc, Schizothrix)
Large thalloid liverworts:

Source:  Biological Soil Crusts: Ecology and Management.   Technical Reference 1730-2 2001.

2.  Where do BSC occur?
Soils having high electrical conductivity, high phosphorous, and high salt contents facilitate the
formation of cryptogam crusts. Shrink-swell clays (smectites, montmorillonites dervied from volcanic ash) are the worst.

Sonoran and Chihuahan deserts have more heterocystic cyanobacteria lower lichen, but lichen like Collema, Placidium, and Peltula do occur.  Ecoregions that receive summer monsoons (e.g., the Sonoran Desert) tend to have a greater diversity of heterocystic cyanobacteria (such as Lyngbya, Calothrix, Schizothrix, and Nostoc) and lower lichen abundance. Lichens in these areas generally include the gelatinous genus Collema and squamulose genera Placidium and Peltula.  Large thalloid liverworts are more common in warm deserts than cool deserts.

Sonoran:  heterocystic cyanobacteria (Notoc, Schizothrix), gelatinous (nitrogen-fixing) lichens (e.g. Collema), squamulose lichens, short mosses
Chihuahan:  heterocystic cyanobacteria (Notoc, Schizothrix), short moss


3. Resources for mapping BSC distribution?

Map shrink/swell clays!
Some mosses are on Seinet
Lichen image gallery (from Europe) organized by structure.
Lichens on LichenPortal
Lichens on iNaturalist
USFS Database

Clay minerals determine shrink/swell soils

Geology and Climate Control the Formation of Clay Minerals

Granite is made up of quartz, mica and feldspar. As quartz is resistant to chemical weathering, it may be eroded only as mineral grains of quartz. Feldspars and micas are susceptible to chemical weathering and break down to form clay minerals.

The main group of clay minerals are kaolinite, illite and montmorillonite. The layers in kaolinite are held together by fairly weak bonds, whereas there is strong bonding in illite and montmorillonite because of the presence of positively charged metal ions; potassium in the case of illite, and calcium and sodium in the case of montmorillonite.

Generally, potassium feldspar breaks down to form kaolinite; micas weather to give illite, and ferromagnesian minerals break down to form montmorillonite.  This chart shows that precipitation is also an important factor:

Source (PDF): Clay mineral formation and transformation in rocks and soils.  Eberl.  Phil. Trans. R. Soc. London 1984 

Alternating dry and wet climates can be inferred by the types of clays formed:



The major types of clay
Kaolinite, smectite (montmorillonite) and Illite:



1:1 Kaolinite
The sum of the many hydrogen bonds between micelles results in the micelles being very strongly bonded together and nearly impossible to separate. This bonding of the layers together results in kaolinite being a nonexpanding clay mineral. Since each micelle is constructed of a layer of silicon tetrahedral units and a layer of octahedral units, kaolinite is called a 1:1 clay mineral.

Kaolinite is formed by weathering or hydrothermal alteration of aluminosilicate minerals. Thus, rocks rich in feldspar commonly weather to kaolinite.  In order to form, ions like Na, K, Ca, Mg, and Fe must first be leached away by the weathering or alteration process.  This leaching is favored by acidic conditions (low pH).  Granitic rocks, because they are rich in feldspar, are a common source for kaolinite.



2:1 Smectite (AKA Bentonite) and Vermiculite
Smectites have more Mg2+, Fe2+, or Mn2+ substituted for Al, giving their sheets a permanent negative layer charge, which in turn leads to high CEC values.  Montmorillonite is a type of smectite.  Montmorillinite is the main constituent of Bentonite, derived by weathering of volcanic ash.   Smectites have a high shrink/swell capacity because water molecules can intercalate between the clay sheets, greatly expanding their total volume.

2:1 Illites
Illite clays are non-expanding.  Illites are formed from weathering of K and Al-rich rocks under high pH conditions. Thus, they form by alteration of minerals like muscovite and feldspar. Illite clays are the main constituent of ancient mudrocks and shales.

2:1 Vermiculite
Vermiculite weathers from mica, which often forms at the contact between felsic and mafic rocks.  It swells intermediate amounts.

 Minerology maps of the U.S (PDF)
example:  calcium carbonate map:

 New Mexico shrink/swell clays:

Source:  Swelling clays map of the conterminous United States.  From a good website.

MAP LEGEND

Unit contains abundant clay having high swelling potential Part of unit (generally less than 50%) consists of clay having high swelling potential Unit contains abundant clay having slight to moderate swelling potential Part of unit (generally less than 50%) consists of clay having slight to moderate swelling potential Unit contains little or no swelling clay Data insufficient to indicate clay content of unit and/or swelling potential of clay (Shown in westermost states only)